Overblog Suivre ce blog
Editer l'article Administration Créer mon blog
9 avril 2011 6 09 /04 /avril /2011 05:35

The primacy of MOSFETs

In 1959, Dawon Kahng and Martin M. (John) Atalla at Bell Labs invented the metal–oxide–semiconductor field-effect transistor (MOSFET). Operationally and structurally different from the bipolar junction transistor, the MOSFET was made by putting an insulating layer on the surface of the semiconductor and then placing a metallic gate electrode on that Ibm ThinkPad G40 Battery .

It used crystalline silicon for the semiconductor and a thermally oxidized layer of silicon dioxide for the insulator. The silicon MOSFET did not generate localized electron traps at the interface between the silicon and its native oxide layer, and thus was inherently free from the trapping and scattering of carriers that had impeded the performance of earlier field-effect transistors

Ibm ThinkPad X200T Battery .

Following the (expensive) development of clean rooms to reduce contamination to levels never before thought necessary, and of photolithography and the planar process to allow circuits to be made in very few steps, the Si–SiO2 system possessed such technical attractions as low cost of production (on a per circuit basis) and ease of integration Apple A1280 battery .

Largely because of these two factors, the MOSFET has become the most widely used type of transistor in integrated circuits.

CMOS circuits

The MOSFET is used in digital CMOS logic, which uses p- and n-channel MOSFETs as building blocks. Overheating is a major concern in integrated circuits since ever more transistors are packed into ever smaller chips Apple A1281 battery .

CMOS logic reduces power consumption because no current flows (ideally), and thus no power is consumed, except when the inputs to logic gates are being switched. CMOS accomplishes this current reduction by complementing every nMOSFET with a pMOSFET and connecting both gates and both drains together Samsung R522 battery .

A high voltage on the gates will cause the nMOSFET to conduct and the pMOSFET not to conduct and a low voltage on the gates causes the reverse. During the switching time as the voltage goes from one state to another, both MOSFETs will conduct briefly. This arrangement greatly reduces power consumption and heat generation  Samsung P467 battery .

Digital and analog CMOS applications are described below.

Digital

The growth of digital technologies like the microprocessor has provided the motivation to advance MOSFET technology faster than any other type of silicon-based transistor  Hp Pavilion dm3 Battery .

A big advantage of MOSFETs for digital switching is that the oxide layer between the gate and the channel prevents DC current from flowing through the gate, further reducing power consumption and giving a very large input impedance  Hp Pavilion dm4 Battery .

The insulating oxide between the gate and channel effectively isolates a MOSFET in one logic stage from earlier and later stages, which allows a single MOSFET output to drive a considerable number of MOSFET inputs. Bipolar transistor-based logic (such as TTL) does not have such a high fanout capacity  Hp Pavilion cq62 Battery .

This isolation also makes it easier for the designers to ignore to some extent loading effects between logic stages independently. That extent is defined by the operating frequency: as frequencies increase, the input impedance of the MOSFETs decreases.

Analog

The MOSFET's advantages in digital circuits do not translate into supremacy in all analog circuits  Hp Pavilion cq42 Battery .

The two types of circuit draw upon different features of transistor behavior. Digital circuits switch, spending most of their time outside the switching region, while analog circuits depend on MOSFET behavior held precisely in the switching region of operationHp Pavilion cq72 Battery .

The bipolar junction transistor (BJT) has traditionally been the analog designer's transistor of choice, due largely to its higher transconductance and its higher output impedance (drain-voltage independence) in the switching region  Hp cq20 Battery .

Nevertheless, MOSFETs are widely used in many types of analog circuits because of certain advantages. The characteristics and performance of many analog circuits can be designed by changing the sizes (length and width) of the MOSFETs used. By comparison, in most bipolar transistors the size of the device does not significantly affect the performance Hp 2230 Battery .

MOSFETs' ideal characteristics regarding gate current (zero) and drain-source offset voltage (zero) also make them nearly ideal switch elements, and also make switched capacitor analog circuits practical. In their linear region, MOSFETs can be used as precision resistors, which can have a much higher controlled resistance than BJTs  Dell Inspiron Mini 12 battery .

In high power circuits, MOSFETs sometimes have the advantage of not suffering from thermal runaway as BJTs do. Also, they can be formed into capacitors and gyrator circuits which allow op-amps made from them to appear as inductors, thereby allowing all of the normal analog devices  Dell XPS M1730 battery ,

except for diodes (which can be made smaller than a MOSFET anyway), to be built entirely out of MOSFETs. This allows for complete analog circuits to be made on a silicon chip in a much smaller space.

Some ICs combine analog and digital MOSFET circuitry on a single mixed-signal integrated circuit, making the needed board space even smaller  Dell XPS M2010 battery .

This creates a need to isolate the analog circuits from the digital circuits on a chip level, leading to the use of isolation rings and Silicon-On-Insulator (SOI). The main advantage of BJTs versus MOSFETs in the analog design process is the ability of BJTs to handle a larger current in a smaller space   Dell Studio XPS 1340 battery .

Fabrication processes exist that incorporate BJTs and MOSFETs into a single device. Mixed-transistor devices are called Bi-FETs (Bipolar-FETs) if they contain just one BJT-FET and BiCMOS (bipolar-CMOS) if they contain complementary BJT-FETs. Such devices have the advantages of both insulated gates and higher current density  Dell Inspiron 9100 battery .

MOSFET scaling

Over the past decades, the MOSFET has continually been scaled down in size; typical MOSFET channel lengths were once several micrometres, but modern integrated circuits are incorporating MOSFETs with channel lengths of tens of nanometers Dell Inspiron N4010 battery .

Intel began production of a process featuring a 32 nm feature size (with the channel being even shorter) in late 2009. The semiconductor industry maintains a "roadmap", the ITRS, which sets the pace for MOSFET development  Dell Inspiron N3610 battery.

Historically, the difficulties with decreasing the size of the MOSFET have been associated with the semiconductor device fabrication process, the need to use very low voltages, and with poorer electrical performance necessitating circuit redesign and innovation (small MOSFETs exhibit higher leakage currents, and lower output resistance, discussed below)  Dell Inspiron N5010 battery .

Reasons for MOSFET scaling

Smaller MOSFETs are desirable for several reasons. The main reason to make transistors smaller is to pack more and more devices in a given chip area. This results in a chip with the same functionality in a smaller area, or chips with more functionality in the same area  Dell Inspiron N7010 battery .

Since fabrication costs for a semiconductor wafer are relatively fixed, the cost per integrated circuits is mainly related to the number of chips that can be produced per wafer. Hence, smaller ICs allow more chips per wafer, reducing the price per chip  Dell Inspiron 1464 battery .

In fact, over the past 30 years the number of transistors per chip has been doubled every 2–3 years once a new technology node is introduced. For example the number of MOSFETs in a microprocessor fabricated in a 45 nm technology is twice as many as in a 65 nmchip   Dell Inspiron 1564 battery .

This doubling of the transistor count was first observed by Gordon Moore in 1965 and is commonly referred to as Moore's law.

It is also expected that smaller transistors switch faster. For example, one approach to size reduction is a scaling of the MOSFET that requires all device dimensions to reduce proportionally  Dell Inspiron 1764 battery .

The main device dimensions are the transistor length, width, and the oxide thickness, each (used to) scale with a factor of 0.7 per node. This way, the transistor channel resistance does not change with scaling, while gate capacitance is cut by a factor of 0.7. Hence, the RC delay of the transistor scales with a factor of 0.7  Dell Inspiron 1320 battery .

While this has been traditionally the case for the older technologies, for the state-of-the-art MOSFETs reduction of the transistor dimensions does not necessarily translate to higher chip speed because the delay due to interconnections is more significant Dell Inspiron 1320n battery .

Difficulties arising due to MOSFET size reduction

Producing MOSFETs with channel lengths much smaller than a micrometer is a challenge, and the difficulties of semiconductor device fabrication are always a limiting factor in advancing integrated circuit technology    Dell Vostro 1710 battery .

In recent years, the small size of the MOSFET, below a few tens of nanometers, has created operational problems.

Higher subthreshold conduction

As MOSFET geometries shrink, the voltage that can be applied to the gate must be reduced to maintain reliability  Dell Vostro 1720 battery .

To maintain performance, the threshold voltage of the MOSFET has to be reduced as well. As threshold voltage is reduced, the transistor cannot be switched from complete turn-off to complete turn-on with the limited voltage swing available; the circuit design is a compromise between strong current in the "on" case and low current in the "off" case  Dell Studio XPS 1640 battery ,

and the application determines whether to favor one over the other. Subthreshold leakage (including subthreshold conduction, gate-oxide leakage and reverse-biased junction leakage), which was ignored in the past, now can consume upwards of half of the total power consumption of modern high-performance VLSI chips   Dell Vostro A840 battery .

Increased gate-oxide leakage

The gate oxide, which serves as insulator between the gate and channel, should be made as thin as possible to increase the channel conductivity and performance when the transistor is on and to reduce subthreshold leakage when the transistor is off  Dell Vostro A860 battery .

However, with current gate oxides with a thickness of around 1.2 nm (which in silicon is ~5 atoms thick) the quantum mechanical phenomenon ofelectron tunneling occurs between the gate and channel, leading to increased power consumption  Dell Studio 1535 battery .

Insulators that have a larger dielectric constant than silicon dioxide (referred to as high-k dielectrics), such as group IVb metal silicates e.g. hafnium and zirconium silicates and oxides are being used to reduce the gate leakage from the 45 nanometer technology node onwards   Dell Studio 1735 battery .

Increasing the dielectric constant of the gate dielectric allows a thicker layer while maintaining a high capacitance (capacitance is proportional to dielectric constant and inversely proportional to dielectric thickness). All else equal, a higher dielectric thickness reduces the quantum tunneling current through the dielectric between the gate and the channel  Dell Vostro 1310 battery .

On the other hand, the barrier height of the new gate insulator is an important consideration; the difference in conduction band energy between the semiconductor and the dielectric (and the corresponding difference in valence band energy) also affects leakage current level   Dell Latitude E6400 battery .

For the traditional gate oxide, silicon dioxide, the former barrier is approximately 8 eV. For many alternative dielectrics the value is significantly lower, tending to increase the tunneling current, somewhat negating the advantage of higher dielectric constant  Dell Latitude E6400 Battery .

Increased junction leakage

To make devices smaller, junction design has become more complex, leading to higher doping levels, shallower junctions, "halo" doping and so forth, all to decrease drain-induced barrier lowering (see the section on junction design)  Dell Latitude E6400 Battery .

To keep these complex junctions in place, the annealing steps formerly used to remove damage and electrically active defects must be curtailed increasing junction leakage. Heavier doping is also associated with thinner depletion layers and more recombination centers that result in increased leakage current, even without lattice damage  Dell Latitude E6500 battery .

It is also expected that smaller transistors switch faster. For example, one approach to size reduction is a scaling of the MOSFET that requires all device dimensions to reduce proportionally. The main device dimensions are the transistor length, width, and the oxide thickness, each (used to) scale with a factor of 0.7 per node   Dell Latitude E6500 battery .

This way, the transistor channel resistance does not change with scaling, while gate capacitance is cut by a factor of 0.7. Hence, the RC delay of the transistor scales with a factor of 0.7.

While this has been traditionally the case for the older technologies   Dell Latitude E6500 battery ,

for the state-of-the-art MOSFETs reduction of the transistor dimensions does not necessarily translate to higher chip speed because the delay due to interconnections is more significant.

Difficulties arising due to MOSFET size reduction

Producing MOSFETs with channel lengths much smaller than a micrometer is a challenge   Dell Latitude E6500 battery ,

and the difficulties of semiconductor device fabrication are always a limiting factor in advancing integrated circuit technology. In recent years, the small size of the MOSFET, below a few tens of nanometers, has created operational problems    HP Pavilion DV6-1210SA battery .

Higher subthreshold conduction

As MOSFET geometries shrink, the voltage that can be applied to the gate must be reduced to maintain reliability. To maintain performance, the threshold voltage of the MOSFET has to be reduced as well   HP Pavilion DV7 battery .

As threshold voltage is reduced, the transistor cannot be switched from complete turn-off to complete turn-on with the limited voltage swing available; the circuit design is a compromise between strong current in the "on" case and low current in the "off" case, and the application determines whether to favor one over the other  Ibm THINKPAD X200 Battery .

Subthreshold leakage (including subthreshold conduction, gate-oxide leakage and reverse-biased junction leakage), which was ignored in the past, now can consume upwards of half of the total power consumption of modern high-performance VLSI chips  Ibm THINKPAD X200 Battery .

Increased gate-oxide leakage

The gate oxide, which serves as insulator between the gate and channel, should be made as thin as possible to increase the channel conductivity and performance when the transistor is on and to reduce subthreshold leakage when the transistor is off   Ibm THINKPAD X200 Battery .

However, with current gate oxides with a thickness of around 1.2 nm (which in silicon is ~5 atoms thick) the quantum mechanical phenomenon ofelectron tunneling occurs between the gate and channel, leading to increased power consumption   SONY VAIO VGN-FW21E Battery .

Insulators that have a larger dielectric constant than silicon dioxide (referred to as high-k dielectrics), such as group IVb metal silicates e.g. hafnium and zirconium silicates and oxides are being used to reduce the gate leakage from the 45 nanometer technology node onwards   Asus M52N Battery .

Increasing the dielectric constant of the gate dielectric allows a thicker layer while maintaining a high capacitance (capacitance is proportional to dielectric constant and inversely proportional to dielectric thickness). All else equal, a higher dielectric thickness reduces the quantum tunneling current through the dielectric between the gate and the channel  Dell Latitude E6400 ATG Battery .

On the other hand, the barrier height of the new gate insulator is an important consideration; the difference in conduction band energy between the semiconductor and the dielectric (and the corresponding difference in valence band energy) also affects leakage current level   SONY VAIO VGN-FW21J Battery .

For the traditional gate oxide, silicon dioxide, the former barrier is approximately 8 eV. For many alternative dielectrics the value is significantly lower, tending to increase the tunneling current, somewhat negating the advantage of higher dielectric constant  SONY VAIO VGN-FW21L Battery .

Increased junction leakage

To make devices smaller, junction design has become more complex, leading to higher doping levels, shallower junctions, "halo" doping and so forth, all to decrease drain-induced barrier lowering (see the section on junction design)   SONY VAIO VGN-FW21M Battery .

To keep these complex junctions in place, the annealing steps formerly used to remove damage and electrically active defects must be curtailed increasing junction leakage. Heavier doping is also associated with thinner depletion layers and more recombination centers that result in increased leakage current, even without lattice damage  SONY VAIO VGN-FW21Z Battery .

Lower output resistance

For analog operation, good gain requires a high MOSFET output impedance, which is to say, the MOSFET current should vary only slightly with the applied drain-to-source voltage  SONY VAIO VGN-FW25T/B Battery .

As devices are made smaller, the influence of the drain competes more successfully with that of the gate due to the growing proximity of these two electrodes, increasing the sensitivity of the MOSFET current to the drain voltage. To counteract the resulting decrease in output resistance, circuits are made more complex  SONY VAIO VGN-FW26T/B Battery ,

either by requiring more devices, for example the cascode andcascade amplifiers, or by feedback circuitry using operational amplifiers, for example a circuit like that in the adjacent figure.

Lower transconductance

The transconductance of the MOSFET decides its gain and is proportional to hole or electron mobility (depending on device type), at least for low drain voltages  SONY VAIO VGN-FW27/B Battery .

As MOSFET size is reduced, the fields in the channel increase and the dopant impurity levels increase. Both changes reduce the carrier mobility, and hence the transconductance. As channel lengths are reduced without proportional reduction in drain voltage, raising the electric field in the channel  SONY VAIO VGN-FW27/W Battery ,

the result is velocity saturation of the carriers, limiting the current and the transconductance.

Interconnect capacitance

Traditionally, switching time was roughly proportional to the gate capacitance of gates   SONY VAIO VGN-FW31ZJ Battery .

However, with transistors becoming smaller and more transistors being placed on the chip, interconnect capacitance (the capacitance of the metal-layer connections between different parts of the chip) is becoming a large percentage of capacitance.Signals have to travel through the interconnect, which leads to increased delay and lower performance r SONY VAIO VGN-FW27T/H Battery.

Heat production

The ever-increasing density of MOSFETs on an integrated circuit creates problems of substantial localized heat generation that can impair circuit operation. Circuits operate slower at high temperatures, and have reduced reliability and shorter lifetimes  SONY VAIO VGN-FW29/B Battery .

Heat sinks and other cooling methods are now required for many integrated circuits including microprocessors.

Power MOSFETs are at risk of thermal runaway. As their on-state resistance rises with temperature, if the load is approximately a constant-current load then the power loss rises correspondingly, generating further heat   SONY VAIO VGN-FW290JRB Battery .

When the heatsink is not able to keep the temperature low enough, the junction temperature may rise quickly and uncontrollably, resulting in destruction of the device  SONY VAIO VGN-FW290JTB Battery .

Process variations

With MOSFETS becoming smaller, the number of atoms in the silicon that produce many of the transistor's properties is becoming fewer, with the result that control of dopant numbers and placement is more erratic. During chip manufacturing, random process variations affect all transistor dimensions   SONY VAIO VGN-FW290JTH Battery :

length, width, junction depths, oxide thickness etc., and become a greater percentage of overall transistor size as the transistor shrinks. The transistor characteristics become less certain, more statistical. The random nature of manufacture means we do not know which particular example MOSFETs actually will end up in a particular instance of the circuit SONY VAIO VGN-FW33G/E1 Battery .

This uncertainty forces a less optimal design because the design must work for a great variety of possible component MOSFETs. See process variation, design for manufacturability, reliability engineering, and statistical process control   SONY VAIO VGN-FW33G/W Battery .

Modeling challenges

Modern ICs are computer-simulated with the goal of obtaining working circuits from the very first manufactured lot. As devices are miniaturized, the complexity of the processing makes it difficult to predict exactly what the final devices look like, and modeling of physical processes becomes more challenging as well   SONY VAIO VGN-FW33GW Battery .

In addition, microscopic variations in structure due simply to the probabilistic nature of atomic processes require statistical (not just deterministic) predictions. These factors combine to make adequate simulation and "right the first time" manufacture difficultDell XPS M1210 Battery .

Gate material

The primary criterion for the gate material is that it is a good conductor. Highly-doped polycrystalline silicon is an acceptable but certainly not ideal conductor, and also suffers from some more technical deficiencies in its role as the standard gate material  http://www.hdd-shop.co.uk .

Nevertheless, there are several reasons favoring use of polysilicon:

  1. The threshold voltage (and consequently the drain to source on-current) is modified by the work function difference between the gate material and channel material  Dell KM958 battery .
  2. Because polysilicon is a semiconductor, its work function can be modulated by adjusting the type and level of doping. Furthermore, because polysilicon has the same bandgap as the underlying silicon channel, it is quite straightforward to tune the work function to achieve low threshold voltages for both NMOS and PMOS devices  Dell Studio 1555 battery .
  3. By contrast, the work functions of metals are not easily modulated, so tuning the work function to obtain low threshold voltages becomes a significant challenge   Sony VGP-BPS13 battery .
  4. Additionally, obtaining low-threshold devices on both PMOS and NMOS devices would likely require the use of different metals for each device type, introducing additional complexity to the fabrication process.
  5. The Silicon-SiO2 interface has been well studied and is known to have relatively few defects  Sony VGP-BPS13/B battery .
  6. By contrast many metal–insulator interfaces contain significant levels of defects which can lead to Fermi-level pinning, charging, or other phenomena that ultimately degrade device performance  Sony VGP-BPS13/S battery .
  7. In the MOSFET IC fabrication process, it is preferable to deposit the gate material prior to certain high-temperature steps in order to make better-performing transistors. Such high temperature steps would melt some metals, limiting the types of metal that can be used in a metal-gate-based process   Sony VGP-BPS13A/B battery .

While polysilicon gates have been the de facto standard for the last twenty years, they do have some disadvantages which have led to their likely future replacement by metal gates. These disadvantages include   Sony VGP-BPS13B/B battery :

  1. Polysilicon is not a great conductor (approximately 1000 times more resistive than metals) which reduces the signal propagation speed through the material. The resistivity can be lowered by increasing the level of doping, but even highly doped polysilicon is not as conductive as most metals  Sony VGP-BPL9 battery .
  2. In order to improve conductivity further, sometimes a high-temperature metal such astungsten, titanium, cobalt, and more recently nickel is alloyed with the top layers of the polysilicon. Such a blended material is called silicide   Sony VGP-BPS13B/B battery .
  3. The silicide-polysilicon combination has better electrical properties than polysilicon alone and still does not melt in subsequent processing. Also the threshold voltage is not significantly higher than with polysilicon alone, because the silicide material is not near the channel   Sony VGP-BPL15 battery .
  4. The process in which silicide is formed on both the gate electrode and the source and drain regions is sometimes called salicide, self-aligned silicide.
  5. When the transistors are extremely scaled down, it is necessary to make the gate dielectric layer very thin, around 1 nm in state-of-the-art technologies   Dell Inspiron E1505 battery .
  6. A phenomenon observed here is the so-called poly depletion, where a depletion layer is formed in the gate polysilicon layer next to the gate dielectric when the transistor is in the inversion. To avoid this problem, a metal gate is desired   HP Pavilion dv6000 Battery .
  7. A variety of metal gates such as tantalum, tungsten, tantalum nitride, and titanium nitride are used, usually in conjunction with high-k dielectrics. An alternative is to use fully-silicided polysilicon gates, a process known as FUSI   HP Pavilion dv8000 battery .

Insulator

As devices are made smaller, insulating layers are made thinner, and at some point tunneling of carriers through the insulator from the channel to the gate electrode takes place. To reduce the resulting leakage current, the insulator can be made thicker by choosing a material with a higher dielectric constant   HP PAVILION DV6000 Battery .

To see how thickness and dielectric constant are related, note that Gauss' law connects field to charge as:

Q=\kappa {\epsilon}_0 \ E,

with Q = charge density, ? = dielectric constant, ?0 = permittivity of empty space and E = electric field   HP PAVILION DV8000 Battery .

From this law it appears the same charge can be maintained in the channel at a lower field provided ? is increased. The voltage on the gate is given by:

V_G = V_{ch} + E \ t_{ins} = V_{ch} + \frac {Q t_{ins}}{\kappa {\epsilon}_0},

with VG = gate voltage, Vch = voltage at channel side of insulator, and tins = insulator thickness   SONY VAIO VGN-FZ Battery .

This equation shows the gate voltage will not increase when the insulator thickness increases, provided ? increases to keep tins /? = constant (see the article on high-? dielectrics for more detail, and the section in this article on gate-oxide leakage)  HP Pavilion DV6000 battery .

The insulator in a MOSFET is a dielectric which can in any event be silicon oxide, but many other dielectric materials are employed. The generic term for the dielectric is gate dielectric since the dielectric lies directly below the gate electrode and above the channel of the MOSFET  HP Pavilion DV8000 battery .

Junction design

The source-to-body and drain-to-body junctions are the object of much attention because of three major factors: their design affects the current-voltage (I-V) characteristics of the device, lowering output resistance, and also the speed of the device through the loading effect of the junction capacitances   HP Pavilion DV6000 battery ,

and finally, the component of stand-by power dissipation due to junction leakage.

The drain induced barrier lowering of the threshold voltage and channel length modulation effects upon I-V curves are reduced by using shallow junction extensions   HP Pavilion DV8000 battery .

In addition, halo doping can be used, that is, the addition of very thin heavily doped regions of the same doping type as the body tight against the junction walls to limit the extent of depletion regions.

The capacitive effects are limited by using raised source and drain geometries that make most of the contact area border thick dielectric instead of silicon   SONY VAIO VGN-FZ18 Battery .

These various features of junction design are shown (with artistic license) in the figure.

Junction leakage is discussed further in the section increased junction leakage  SONY VAIO VGN-FZ21E Battery .

Dual gate MOSFET

The dual gate MOSFET has a tetrode configuration, where both gates control the current in the device. It is commonly used for small signal devices in radio frequency applications where the second gate is normally used for gain control or mixing and frequency conversion   SONY VAIO VGN-FW31J Battery .

FinFET

The Finfet, see figure to right, is a double gate device, one of a number of geometries being introduced to mitigate the effects of short channels and reduce drain-induced barrier lowering  SONY VAIO VGN-FW31J Battery .

Depletion-mode MOSFETs

There are depletion-mode MOSFET devices, which are less commonly used than the standard enhancement-mode devices already described. These are MOSFET devices that are doped so that a channel exists even with zero voltage from gate to source  SONY VAIO VGN-FW21E Battery .

In order to control the channel, a negative voltage is applied to the gate (for an n-channel device), depleting the channel, which reduces the current flow through the device. In essence, the depletion-mode device is equivalent to a normally closed (on) switch, while the enhancement-mode device is equivalent to a normally open (off) switch  SONY VAIO VGN-NR11S/S Battery .

Due to their low noise figure in the RF region, and better gain, these devices are often preferred to bipolars in RF front-ends such as in TV sets. Depletion-mode MOSFET families include BF 960 by Siemens and BF 980 by Philips (dated 1980s), whose derivatives are still used in AGC and RF mixer front-ends  SONY VAIO VGN-NR11M/S Battery .

NMOS logic

n-channel MOSFETs are smaller than p-channel MOSFETs and producing only one type of MOSFET on a silicon substrate is cheaper and technically simpler. These were the driving principles in the design of NMOS logic which uses n-channel MOSFETs exclusivelySONY VAIO VGN-NR11Z/S Battery .

However, unlike CMOS logic, NMOS logic consumes power even when no switching is taking place. With advances in technology, CMOS logic displaced NMOS logic in the mid 1980s to become the preferred process for digital chips  SONY VAIO VGN-NR11Z/T Battery .

Power MOSFET

Power MOSFETs have a different structure than the one presented above. As with all power devices, the structure is vertical and not planar. Using a vertical structure, it is possible for the transistor to sustain both high blocking voltage and high current  Sony VAIO VGN-FZ21E Battery .

The voltage rating of the transistor is a function of the doping and thickness of the N-epitaxial layer (see cross section), while the current rating is a function of the channel width (the wider the channel, the higher the current)   Sony VAIO VGN-FW21E Battery .

In a planar structure, the current and breakdown voltage ratings are both a function of the channel dimensions (respectively width and length of the channel), resulting in inefficient use of the "silicon estate". With the vertical structure, the component area is roughly proportional to the current it can sustain  Sony VAIO VGN-NR11S/S Battery ,

and the component thickness (actually the N-epitaxial layer thickness) is proportional to the breakdown voltage.

Power MOSFETs with lateral structure are mainly used in high-end audio amplifiers and high-power PA systems. Their advantage is a better behaviour in the saturated region (corresponding to the linear region of a bipolar transistor) than the vertical MOSFETs Sony VAIO VGN-NR11Z/S Battery.

Vertical MOSFETs are designed for switching applications.

DMOS

DMOS stands for double-diffused metal–oxide–semiconductor. Most power MOSFETs are made using this technology  Sony VAIO VGN-NR11M/S Battery .

RHBD MOSFETs

Semiconductor sub-micrometer and nanometer electronic circuits are the primary concern for operating within the normal tolerance in harsh radiation environments like space. One of the design approaches for making a radiation-hardened-by-design (RHBD) device is Enclosed-Layout-Transistor (ELT)  Sony VAIO VGN-NR11Z/T Battery .

Normally, the gate of the MOSFET surrounds the drain, which is placed in the center of the ELT. The source of the MOSFET surrounds the gate. Another RHBD MOSFET is called H-Gate. Both of these transistors have very low leakage current with respect to radiation  SONY VAIO VGN-FZ180E Battery .

However, they are large in size and take more space on silicon than a standard MOSFET .

Newer technologies are emerging for smaller devices for cost saving, low power and increased operating speed. The standard MOSFET is also becoming extremely sensitive to radiation for the newer technologies   SONY VAIO VGN-FZ220E Battery .

A lot more research works should be completed before space electronics can safely use RHBD MOSFET circuits of nanotechnology.

When radiation strikes near the silicon oxide region (STI) of the MOSFET, the channel inversion occurs at the corners of the standard MOSFET due to accumulation of radiation induced trapped charges  SONY VAIO VGN-FZ340E Battery .

If the charges are large enough, the accumulated charges affect STI surface edges along the channel near the channel interface (gate) of the standard MOSFET. Thus the device channel inversion occurs along the channel edges and the device creates off-state leakage path, causing device to turn on  SONY VAIO VGN-FZ460E Battery .

So the reliability of circuits degrades severely. The ELT offers many advantages. These advantages include improvement of reliability by reducing unwanted surface inversion at the gate edges that occurs in the standard MOSFET. Since the gate edges are enclosed in ELT  SONY VAIO VGN-FZ480E Battery ,

there is no gate oxide edge (STI at gate interface), and thus the transistor off-state leakage is reduced very much.

Low-power microelectronic circuits including computers, communication devices and monitoring systems in space shuttle and satellites are very different than what we use on earth   SONY VAIO VGN-FZ4000 Battery .

They are radiation (high-speed atomic particles like proton and neutron, solar flare magnetic energy dissipation in earth's space, energetic cosmic rays like X-ray, Gamma-ray etc.) tolerant circuits. These special electronics are designed by applying very different techniques using RHBD MOSFETs to ensure the safe space journey and also space-walk of astronauts SONY VAIO VGN-FZ31E Battery.

MOSFET analog switch

MOSFET analog switches use the MOSFET channel as a low–on-resistance switch to pass analog signals when on, and as a high impedance when off. Signals flow in both directions across a MOSFET switch  SONY VAIO VGN-FZ31B Battery .

In this application the drain and source of a MOSFET exchange places depending on the voltages of each electrode compared to that of the gate. For a simple MOSFET without an integrated diode, the source is the more negative side for an N-MOS or the more positive side for a P-MOS   SONY VAIO VGN-FZ31J Battery .

All of these switches are limited on what signals they can pass or stop by their gate-source, gate-drain and source-drain voltages, and source-to-drain currents; exceeding the voltage limits will potentially damage the switch  SONY VAIO VGN-FZ31M Battery .

Single-type MOSFET switch

This analog switch uses a four-terminal simple MOSFET of either P or N type. In the case of an N-type switch, the body is connected to the most negative supply (usually GND) and the gate is used as the switch control   SONY VAIO VGN-FZ31Z Battery .

Whenever the gate voltage exceeds the source voltage by at least a threshold voltage, the MOSFET conducts. The higher the voltage, the more the MOSFET can conduct. An N-MOS switch passes all voltages less than (Vgate–Vtn). When the switch is conducting, it typically operates in the linear (or Ohmic) mode of operation, since the source and drain voltages will typically be nearly equal SONY VAIO VGN-FZ38M Battery .

In the case of a P-MOS, the body is connected to the most positive voltage, and the gate is brought to a lower potential to turn the switch on. The P-MOS switch passes all voltages higher than (Vgate+|Vtp|). Threshold voltage (Vtp) is typically negative in the case of P-MOS  SONY VGP-BPS8 Battery .

A P-MOS switch will have about three times the resistance of an N-MOS device of equal dimensions because electrons have about three times the mobility of holes in silicon.

Dual-type (CMOS) MOSFET switch

This "complementary" or CMOS type of switch uses one P-MOS and one N-MOS FET to counteract the limitations of the single-type switch  SONY VGP-BPS13/S Battery .

The FETs have their drains and sources connected in parallel, the body of the P-MOS is connected to the high potential (VDD) and the body of the N-MOS is connected to the low potential (Gnd). To turn the switch on the gate of the P-MOS is driven to the low potential and the gate of the N-MOS is driven to the high potential  SONY VGP-BPS13A/B Battery .

For voltages between (VDD–Vtn) and (Gnd+Vtp) both FETs conduct the signal, for voltages less than (Gnd+Vtp) the N-MOS conducts alone and for voltages greater than (VDD–Vtn) the P-MOS conducts alone.

The only limits for this switch are the gate-source, gate-drain and source-drain voltage limits for both FETs   SONY VGP-BPS13B/B Battery .

Also, the P-MOS is typically three times the width of the N-MOS so the switch will be balanced.

Tri-state circuitry sometimes incorporates a CMOS MOSFET switch on its output to provide for a low ohmic, full range output when on and a high ohmic, mid level signal when off  SONY VGP-BPS13A/S Battery .

BSIM (Berkeley Short-channel IGFET Model)[1] refers to a family of MOSFET transistor models for integrated circuit design. Accurate transistor models are needed for electronic circuit simulation, which in turn is needed for integrated circuit design SONY VGP-BPS13AS Battery .

As the devices become smaller each process generation (see Moore's law), new models are needed to accurately reflect the transistor's behavior.

Commercial and industrial analog simulators (such as SPICE) have added many other device models as technology advanced and earlier models became inaccurate  Dell Inspiron 1320n Battery .

To attempt standardization of these models so that a set of model parameters may be used in different simulators, an industry working group was formed, the Compact Model Council , to choose, maintain and promote the use of standard models . BSIM models, developed at UC Berkeley are one of these standards  Dell Studio 1450 Battery .

They include BSIM3 , BSIM4 , and BSIMSOI .

Other models supported by the council are PSP, HICUM, and MEXTRAM.

Electronic circuit simulation uses mathematical models to replicate the behavior of an actual electronic device or circuit Dell Studio 1457 Battery .

Simulation software allows for modeling of circuit operation and is an invaluable analysis tool. Due to its highly accurate modeling capability, many Colleges and Universities use this type of software for the teaching of electronics technician and electronics engineeringprograms   Dell Latitude D610 Battery .

Electronics simulation software engages the user by integrating them into the learning experience. These kinds of interactions actively engage learners to analyze, synthesize, organize, and evaluate content and result in learners constructing their own knowledge  Toshiba NB100 Battery .

Simulating a circuit’s behavior before actually building it can greatly improve design efficiency by making faulty designs known as such, and providing insight into the behavior of electronics circuit designs. In particular, for integrated circuits, the tooling (photomasks) is expensive  Toshiba Satellite M65 battery ,

breadboards are impractical, and probing the behavior of internal signals is extremely difficult. Therefore almost all IC design relies heavily on simulation. The most well known analog simulator is SPICE. Probably the best known digital simulators are those based on Verilog and VHDL  Toshiba Satellite M60 battery .

Some electronics simulators integrate a schematic editor, a simulation engine, and on-screen waveforms (see Figure 1), and make “what-if” scenarios easy and instant. They also typically contain extensive model and device libraries   Dell Latitude D830 Battery .

These models typically include IC specific transistor models such as BSIM, generic components such as resistors, capacitors, inductors and transformers, user defined models (such as controlled current and voltage sources, or models in Verilog-A or VHDL-AMS)  Dell Latitude D620 Battery .

Printed circuit board (PCB) design requires specific models as well, such as transmission linesfor the traces and IBIS models for driving and receiving electronics.

Types

While there are strictly analog   electronics circuit simulators, popular simulators often include both analog and event-driven digital simulation  capabilities, and are known as mixed-mode simulators  Dell Inspiron Mini 10 Battery .

This means that any simulation may contain components that are analog, event driven (digital or sampled-data), or a combination of both. An entire mixedsignal analysis can be driven from one integrated schematic. All the digital models in mixed-mode simulators provide accurate specification of propagation time and rise/fall time delays  Sony VGN-FW11S Battery .

The event driven algorithm provided by mixed-mode simulators is general purpose and supports non-digital types of data. For example, elements can use real or integer values to simulate DSP functions or sampled data filters  Sony VGN-FW11M Battery .

Because the event driven algorithm is faster than the standard SPICE matrix solution, simulation time is greatly reduced for circuits that use event driven models in place of analog models  .

Mixed-mode simulation is handled on three levels   Sony VGN-FW139E/H battery ;

(a) with primitive digital elements that use timing models and the built-in 12 or 16 state digital logic simulator, (b) with subcircuit models that use the actual transistor topology of the integrated circuit, and finally, (c) with In-line Boolean logic expressions. An example of a mixed-mode simulator is shown in Figure 2  Dell Latitude E5400 Battery .

Exact representations are used mainly in the analysis of transmission line and signal integrity problems where a close inspection of an IC’s I/O characteristics is needed. Boolean logic expressions are delay-less functions that are used to provide efficient logic signal processing in an analog environment  Dell Latitude E4200 Battery .

These two modeling techniques use SPICE to solve a problem while the third method, digital primitives, use mixed mode capability. Each of these methods has its merits and target applications. In fact, many simulations (particularly those which use A/D technology) call for the combination of all three approaches  Dell Inspiron 300M Battery .

No one approach alone is sufficient.

Another type of simulation used mainly for power electronics represent piecewise linear algorithms. These algorithms use an analog (linear) simulation until a power electronic switch changes its state  Dell Studio 1737 battery .

At this time a new analog model is calculated to be used for the next simulation period. This methodology both enhances simulation speed and stability significantly.

Complexities

Often circuit simulators do not take into account the process variations that occur when the design is fabricated into silicon Dell Inspiron E1505 battery .

These variations can be small, but taken together can change the output of a chip significantly.

Process variations occur in the manufacture of circuits in silicon.

Temperature variation can also be modeled to simulate the circuit's performance through temperature ranges Dell RM791 battery .

SPICE (Simulation Program with Integrated Circuit Emphasis) is a general-purpose open source analog electronic circuit simulator. It is a powerful program that is used in integrated circuit and board-level design to check the integrity of circuit designs and to predict circuit behavior  p10-series-laptop-battery.htm">Toshiba Satellite P10 Battery .

Introduction

Integrated circuits, unlike board-level designs composed of discrete parts, are impossible to breadboard before manufacture. Further, the high costs ofphotolithographic masks and other manufacturing prerequisites make it essential to design the circuit to be as close to perfect as possible before the integrated circuit is first built  Acer Aspire One battery 10400mAh .

Simulating the circuit with SPICE is the industry-standard way to verify circuit operation at the transistor level before committing to manufacturing an integrated circuit.

Board-level circuit designs can often be breadboarded for testing. Even with a breadboard, some circuit properties may not be accurate compared to the final printed wiring board, such as parasitic resistances and capacitances Dell Precision M70 Battery .

These parasitic components can often be estimated more accurately using SPICE simulation. Also, designers may want more information about the circuit than is available from a single mock-up. For instance, circuit performance is affected by component manufacturing tolerances   SONY VGN -FZ210CE Battery .

In these cases it is common to use SPICE to perform Monte Carlo simulations of the effect of component variations on performance, a task which is impractical using calculations by hand for a circuit of any appreciable complexity  Toshiba Satellite L305 Battery .

Circuit simulation programs, of which SPICE and derivatives are the most prominent, take a text netlist describing the circuit elements (transistors, resistors, capacitors, etc.) and their connections, and translate this description into equations to be solved  Toshiba Satellite T4900 Battery .

The general equations produced are nonlinear differential algebraic equations which are solved using implicit integration methods, Newton's method and sparse matrix techniques.

Origins

SPICE was developed at the Electronics Research Laboratory of the University of California, Berkeley by Laurence Nagel with direction from his research advisor, Prof. Donald Pederson  Toshiba PA3399U-2BRS battery .

SPICE1 was largely a derivative of the CANCER program,  which Nagel had worked on under Prof. Ronald Rohrer. CANCER was an acronym for "Computer Analysis of Nonlinear Circuits, Excluding Radiation," a hint to Berkeley's liberalism of 1960s  Sony VGP-BPS10 battery :

at these times many circuit simulators were developed under the United States Department of Defense contracts that required the capability to evaluate theradiation hardness of a circuit. When Nagel's original advisor, Prof. Rohrer, left Berkeley, Prof. Pederson became his advisor  Sony VGP-BPS9 battery .

Pederson insisted that CANCER, a proprietary program, be rewritten enough that restrictions could be removed and the program could be put in the public domain.

SPICE1 was first presented at a conference in 1973.  SPICE1 was coded in FORTRAN and used nodal analysis to construct the circuit equations  Sony VGP-BPS11 battery .

Nodal analysis has limitations in representing inductors, floating voltage sources and the various forms of controlled sources. SPICE1 had relatively few circuit elements available and used a fixed-timestep transient analysis. The real popularity of SPICE started with SPICE2  in 1975   Sony VGP-BPS22 battery .

SPICE2, also coded in FORTRAN, was a much-improved program with more circuit elements, variable timestep transient analysis using either the trapezoidal (second order Adams-Moulton method) or the Gear integration method (also known as BDF) Sony VGP-BPL8 battery ,

equation formulation via modified nodal analysis  (avoiding the limitations of nodal analysis), and an innovative FORTRAN-based memory allocation system developed by another graduate student, Ellis Cohen. The last FORTRAN version of SPICE was 2G.6 in 1983   Sony VGP-BPS14 battery .

SPICE3 was developed by Thomas Quarles (with A. Richard Newton as advisor) in 1989. It is written in C, uses the same netlist syntax, and added X Window System plotting.

As an early open source program, SPICE was widely distributed and used   Sony VGP-BPS21 battery .

Its ubiquity became such that "to SPICE a circuit" remains synonymous with circuit simulation. SPICE source code was from the beginning distributed by UC Berkeley for a nominal charge (to cover the cost of magnetic tape). The license originally included distribution restrictions for countries not considered friendly to the USA  Toshiba Satellite A200 Battery ,

but the source code is currently covered by the BSD license.

SPICE inspired and served as a basis for many other circuit simulation programs, in academia, in industry, and in commercial products   Toshiba Satellite 1200 Battery .

The first commercial version of SPICE was ISPICE, an interactive version on a timeshare service, National CSS. The most prominent commercial versions of SPICE include HSPICE (originally commercialized by Shawn and Kim Hailey of Meta Software, but now owned by Synopsys) and PSPICE (now owned by Cadence Design Systems)   Toshiba Satellite M300 Battery .

The academic spinoffs of SPICE include XSPICE, developed at Georgia Tech, which added mixed analog/digital "code models" for behavioral simulation, and Cider (previously CODECS, from UC Berkeley/Oregon State Univ.) which added semiconductor device simulation   SONY VGP-BPS13A/Q Battery .

The integrated circuit industry adopted SPICE quickly, and until commercial versions became well developed many IC design houses had proprietary versions of SPICE. Today a few IC manufacturers, typically the larger companies, have groups continuing to develop SPICE-based circuit simulation programs  SONY VGP-BPS13A/Q Battery .

Among these are ADICE at Analog Devices, LTspice at Linear Technology, Mica at Freescale Semiconductor, and TISPICE atTexas Instruments. (Other companies maintain internal circuit simulators which are not directly based upon SPICE, among them PowerSpice at IBM, Titan at Qimonda, Lynx at Intel Corporation, and Pstar at NXP Semiconductor.)SONY VGP-BPS13B/Q Battery

Program features and structure

SPICE became popular because it contained the analyses and models needed to design integrated circuits of the time, and was robust enough and fast enough to be practical to use  SONY VGP-BPS13B/Q Battery .

Precursors to SPICE often had a single purpose: The BIAS  program, for example, did simulation of bipolar transistor circuit operating points; the SLIC program did only small-signal analyses. SPICE combined operating point solutions, transient analysis, and various small-signal analyses with the circuit elements and device models needed to successfully simulate many circuits SONY VGP-BPS13/Q Battery .

Analyses

SPICE2 included these analyses:

  • AC analysis (linear small-signal frequency domain analysis)
  • DC analysis (nonlinear quiescent point calculation) SONY VGP-BPS13/Q Battery
  • DC transfer curve analysis (a sequence of nonlinear operating points calculated while sweeping an input voltage or current, or a circuit parameter)
  • Noise analysis (a small signal analysis done using an adjoint matrix technique which sums uncorrelated noise currents at a chosen output point)  SONY VGP-BPS21A/B Battery
  • Transfer function analysis (a small-signal input/output gain and impedance calculation)
  • Transient analysis (time-domain large-signal solution of nonlinear differential algebraic equations)  SONY VGP-BPS21/S Battery

Since SPICE is generally used to model nonlinear circuits, the small signal analyses are necessarily preceded by a quiescent point calculation at which the circuit is linearized. SPICE2 also contained code for other small-signal analyses: sensitivity analysis, pole-zero analysis, and small-signal distortion analysis  SONY VGP-BPS21B Battery .

Analysis at various temperatures was done by automatically updating semiconductor model parameters for temperature, allowing the circuit to be simulated at temperature extremes.

Other circuit simulators have since added many analyses beyond those in SPICE2 to address changing industry requirements SONY VGP-BPS21A Battery .

Parametric sweeps were added to analyze circuit performance with changing manufacturing tolerances or operating conditions. Loop gain and stability calculations were added for analog circuits. Harmonic balance or time-domain steady state analyses were added for RF and switched-capacitor circuit design  SONY VGP-BPS21 Battery .

However, a public-domain circuit simulator containing the modern analyses and features needed to become a successor in popularity to SPICE has not yet emerged.

Device models

SPICE2 included many semiconductor device compact models  Sony VGP-BPS21A/B Battery :

three levels of MOSFET model, a combined Ebers–Moll and Gummel-Poon bipolar model, a JFET model, and a model for a junction diode. In addition, it had many other elements: resistors, capacitors, inductors (including coupling), independent voltage and current sources, ideal transmission lines, and voltage and current controlled sources Sony VGP-BPS21/S Battery .

SPICE3 added more sophisticated MOSFET models, which were required due to advances in semiconductor technology. In particular, the BSIM family of models were added, which were also developed at UC Berkeley   Sony VGP-BPS21A Battery .

Commercial and industrial SPICE simulators have added many other device models as technology advanced and earlier models became inaccurate. To attempt standardization of these models so that a set of model parameters may be used in different simulators  Sony VGP-BPS21 Battery ,

an industry working group was formed, the Compact Model Council, to choose, maintain and promote the use of standard models. The standard models today include BSIM3, BSIM4, BSIMSOI, PSP, HICUM, and MEXTRAM  Sony Vaio PCG-5G2L Battery .

Input and output: Netlists, schematic capture and plotting

SPICE2 took a text netlist as input and produced line-printer listings as output, which fit with the computing environment in 1975Sony Vaio PCG-5G3L Battery .

These listings were either columns of numbers corresponding to calculated outputs (typically voltages or currents), or line-printer character "plots". SPICE3 retained the netlist for circuit description, but allowed analyses to be controlled from a command-line interface similar to the C shell  Sony Vaio PCG-5J1L Battery .

SPICE3 also added basic X-Window plotting, as UNIX and engineering workstations became common.

Vendors and various free software projects have added schematic capture front-ends to SPICE, allowing a schematic diagram of the circuit to be drawn and the netlist to be automatically generated  Sony Vaio PCG-5K2L Battery .

Also, graphical user interfaces were added for selecting the simulations to be done and manipulating the voltage and current output vectors. In addition, very capable graphing utilities have been added to see waveforms and graphs of parametric dependencies  Sony Vaio PCG-5J2L Battery .

Several free versions of these extended programs are available, some as introductory limited packages, and some without restrictions.

Transistors are simple devices with complicated behavior   Sony Vaio PCG-5K1L Battery .

In order to ensure the reliable operation of circuits employing transistors, it is necessary to scientifically model the physical phenomena observed in their operation using transistor models. There exists a variety of different models that range in complexity and in purpose  Sony Vaio PCG-6S2L Battery .

Transistor models divide into two major groups: models for device design and models for circuit design.

Models for device design

The modern transistor has an internal structure that exploits complex physical mechanisms   Sony Vaio PCG-6S3L Battery .

Device design requires a detailed understanding of how device manufacturing processes such as ion implantation, impurity diffusion, oxide growth, annealing, and etching affect device behavior. Process models simulate the manufacturing steps and provide a microscopic description of device "geometry" to the device simulator  Sony Vaio PCG-6V1L Battery .

By "geometry" is meant not only readily identified geometrical features such as whether the gate is planar or wrap-around, or whether the source and drain are raised or recessed (see Figure 1 for a memory device with some unusual modeling challenges related to charging the floating gate by an avalanche process) Sony Vaio PCG-6W1L Battery ,

but also details inside the structure, such as the doping profiles after completion of device processing.

With this information about what the device looks like, the device simulator models the physical processes taking place in the device to determine its electrical behavior in a variety of circumstances  Sony Vaio PCG-6W2L Battery :

DC current-voltage behavior, transient behavior (both large-signal and small-signal), dependence on device layout (long and narrow versus short and wide, or interdigitated versus rectangular, or isolated versus proximate to other devices)  Sony Vaio PCG-6W3L Battery .

These simulations tell the device designer whether the device process will produce devices with the electrical behavior needed by the circuit designer, and is used to inform the process designer about any necessary process improvements  Sony Vaio PCG-7111L Battery .

Once the process gets close to manufacture, the predicted device characteristics are compared with measurement on test devices to check that the process and device models are working adequately.

Although long ago the device behavior modeled in this way was very simpleSony Vaio PCG-7112L Battery

- mainly drift plus diffusion in simple geometries - today many more processes must be modeled at a microscopic level; for example, leakage currents in junctions and oxides, complex transport of carriers including velocity saturation and ballistic transport, quantum mechanical effects Sony Vaio PCG-7113L Battery ,

use of multiple materials (for example, Si-SiGe devices, and stacks of different dielectrics) and even the statistical effects due to the probabilistic nature of ion placement and carrier transport inside the device. Several times a year the technology changes and simulations have to be repeated Sony Vaio PCG-7133L Battery .

The models may require change to reflect new physical effects, or to provide greater accuracy. The maintenance and improvement of these models is a business in itself.

These models are very computer intensive, involving detailed spatial and temporal solutions of coupled partial differential equations on three-dimensional grids inside the device  Sony Vaio PCG-7Z2L Battery .

Such models are slow to run and provide detail not needed for circuit design. Therefore, faster transistor models oriented toward circuit parameters are used for circuit design.

Models for circuit design (compact models)

Transistor models are used for almost all modern electronic design work  Sony Vaio PCG-8Y1L Battery .

Analog circuit simulators such as SPICE use models to predict the behavior of a design. Most design work is related tointegrated circuit designs which have a very large tooling cost, primarily for the photomasks used to create the devices, and there is a large economic incentive to get the design working without any iterations  Sony Vaio PCG-8Y2L Battery .

Complete and accurate models allow a large percentage of designs to work the first time.

Modern circuits are usually very complex. The performance of such circuits is difficult to predict without accurate computer models, including but not limited to models of the devices used    Sony Vaio PCG-8Z1L Battery .

The device models include effects of transistor layout: width, length, interdigitation, proximity to other devices; transient and DC current-voltage characteristics; parasitic device capacitance, resistance, and inductance; time delays; and temperature effects; to name a few items  Sony Vaio PCG-8Z2L Battery .

Large-signal nonlinear models

Nonlinear, or large signal transistor models fall into three main types:

Physical models

These are models based upon device physics, based upon approximate modeling of physical phenomena within a transistorSONY VAIO VGN-FW31J Battery .
Parameters within these models are based upon physical properties such as oxide thicknesses, substrate doping concentrations, carrier mobility, etc. In the past these models were used extensively, but the complexity of modern devices makes them inadequate for quantitative design  SONY VAIO VGN-FW31E Battery .
Nonetheless, they find a place in hand analysis (that is, at the conceptual stage of circuit design), for example, for simplified estimates of signal-swing limitations.

Empirical models

This type of model is entirely based upon curve fitting  SONY VAIO VGN-FW31B Battery ,
using whatever functions and parameter values most adequately fit measured data to enable simulation of transistor operation. Unlike a physical model, the parameters in an empirical model need have no fundamental basis, and will depend on the fitting procedure used to find them   SONY VAIO VGN-FW31M Battery .
The fitting procedure is key to success of these models if they are to be used to extrapolate to designs lying outside the range of data to which the models were originally fitted. Such extrapolation is a hope of such models, but is not fully realized so far  SONY VAIO VGN-FW31Z Battery .

Tabular models

The third type of model is a form of look-up table containing a large number of values for common device parameters such as drain current and device parasitics. These values are indexed in reference to their corresponding bias voltage combinationsSONY VAIO VGN-FW38M Battery .
Thus, model accuracy is increased by inclusion of additional data points within the table. The chief advantage of this type of model is decreased simulation time (see article look-up table for discussion of the computational advantages of look-up tables)   SONY VAIO VGN-AW41JF Battery .
A limitation of these models is that they work best for designs that use devices within the table (interpolation) and are unreliable for devices outside the table (extrapolation)   SONY VAIO VGN-AW41JF/H Battery .

The use of nonlinear models, which describe the entire operating area of a transistor, is required for digital designs, for circuits that operate in a large-signal regime such as power amplifiers and mixers, and for the large-signal simulation of any circuit, for example, for stability or distortion analysis  SONY VAIO VGN-AW41MF Battery .

Nonlinear models are used with a computer simulation program, such as SPICE. The models in SPICE are a hybrid of physical and empirical models, and such models are incomplete unless they include specification of how parameter values are to be extractedSONY VAIO VGN-AW41MF/H Battery ,

especially as "unrealistic" (that is, unphysical) values can be made to fit the measured data without such a prescription. An incorrect set of fitting parameters results in wild predictions for devices that were not part of the originally fitted data set   SONY VAIO VGN-FW30B Battery .

Large-signal computer models for devices continually evolve to keep up with changes in technology. To attempt standardization of model parameters used in different simulators, an industry working group was formed, the Compact Model Council, to choose, maintain and promote the use of standard models   SONY VAIO VGN-FW50B Battery .

An elusive goal in such modeling is prediction of how circuits using the next generation of devices should work, to identify before the next step which direction the technology should take, and have models ready beforehand.

Small-signal linear models

Small-signal or linear models are used to evaluate stability, gain, noise and bandwidth  SONY VAIO VGN-FW51B/W Battery ,

both in the conceptual stages of circuit design (to decide between alternative design ideas before computer simulation is warranted) and using computers. A small-signal model is generated by taking derivatives of the current-voltage curves about a bias point or Q-point   SONY VAIO VGN-FW51MF Battery .

As long as the signal is small relative to the nonlinearity of the device, the derivatives do not vary significantly, and can be treated as standard linear circuit elements. A big advantage of small signal models is they can be solved directly, while large signal nonlinear models are generally solved iteratively, with possible convergence or stability issues  SONY VAIO VGN-FW51MF/H Battery .

By simplification to a linear model, the whole apparatus for solving linear equations becomes available, for example, simultaneous equations, determinants, and matrix theory (often studied as part of linear algebra), especially Cramer's rule. Another advantage is that a linear model is easier to think about, and helps to organize thought  SONY VAIO VGN-FW51ZF Battery .

Small-signal parameters

A transistor’s parameters represent its electrical properties. Engineers employ transistor parameters in production-line testing and in circuit design. A group of a transistor’s parameters sufficient to predict circuit gain, input impedance, and output impedance are components in its small-signal model  SONY VAIO VGN-FW51ZF/H Battery .

Parameters used in small-signal circuits (two ports) adopt names related to the names of these circuits such as

  • Transmission parameters (T-parameters),
  • Hybrid-parameters (h-parameters),
  • Impedance parameters (z-parameters),  SONY VAIO VGN-FW52JB Battery
  • Admittance parameters (y-parameters), and
  • Scattering parameters (S-parameters).

These parameters all can be evaluated using measured scattering parameter data. Scattering parameters, or S parameters, can be measured for a transistor at a given bias point with a vector network analyzer  SONY VAIO VGN-FW54FB Battery .

VHDL (VHSIC hardware description language) is a hardware description language used in electronic design automation to describe digital and mixed-signal systems such as field-programmable gate arrays and integrated circuits  SONY VAIO VGN-FW70DB Battery .

History

VHDL was originally developed at the behest of the U.S Department of Defense in order to document the behavior of the ASICs that supplier companies were including in equipment. That is to say, VHDL was developed as an alternative to huge, complex manuals which were subject to implementation-specific details  SONY VAIO VGN-FW71DB/W Battery .

The idea of being able to simulate this documentation was so obviously attractive that logic simulators were developed that could read the VHDL files. The next step was the development of logic synthesis tools that read the VHDL, and output a definition of the physical implementation of the circuit  SONY VAIO VGN-FW72JGB Battery .

Due to the Department of Defense requiring as much of the syntax as possible to be based on Ada, in order to avoid re-inventing concepts that had already been thoroughly tested in the development of Ada,[citation needed] VHDL borrows heavily from the Ada programming language in both concepts and syntax  SONY VAIO VGN-FW73JGB Battery .

The initial version of VHDL, designed to IEEE standard 1076-1987, included a wide range of data types, including numerical (integer and real), logical (bit and boolean), character and time, plus arrays ofbit called bit_vector and of character called string  SONY VAIO VGN-FW74FB Battery .

A problem not solved by this edition, however, was "multi-valued logic", where a signal's drive strength (none, weak or strong) and unknown values are also considered. This required IEEE standard 1164, which defined the 9-value logic types: scalar std_ulogicand its vector version std_ulogic_vector  SONY VAIO VGN-FW81HS Battery .

The updated IEEE 1076, in 1993, made the syntax more consistent, allowed more flexibility in naming, extended the charactertype to allow ISO-8859-1 printable characters, added the xnoroperator, etc.

Minor changes in the standard (2000 and 2002) added the idea of protected types (similar to the concept of class in C++) and removed some restrictions from port mapping rules  SONY VAIO VGN-FW81NS Battery .

In addition to IEEE standard 1164, several child standards were introduced to extend functionality of the language. IEEE standard 1076.2 added better handling of real and complex data types. IEEE standard 1076.3 introduced signed and unsigned types to facilitate arithmetical operations on vectors  SONY VAIO VGN-FW81S Battery .

IEEE standard 1076.1 (known as VHDL-AMS) provided analog and mixed-signal circuit design extensions.

Some other standards support wider use of VHDL, notably VITAL (VHDL Initiative Towards ASIC Libraries) and microwave circuit design extensions  SONY VAIO VGN-FW82DS Battery .

In June 2006, the VHDL Technical Committee of Accellera (delegated by IEEE to work on the next update of the standard) approved so called Draft 3.0 of VHDL-2006. While maintaining full compatibility with older versions, this proposed standard provides numerous extensions that make writing and managing VHDL code easier  SONY VAIO VGN-FW82JS Battery .

Key changes include incorporation of child standards (1164, 1076.2, 1076.3) into the main 1076 standard, an extended set of operators, more flexible syntax of case and generate statements, incorporation of VHPI (interface to C/C++ languages) and a subset of PSL (Property Specification Language)  SONY VAIO VGN-FW82XS Battery .

These changes should improve quality of synthesizable VHDL code, make testbenches more flexible, and allow wider use of VHDL for system-level descriptions.

In February 2008, Accellera approved VHDL 4.0 also informally known as VHDL 2008, which addressed more than 90 issues discovered during the trial period for version 3.0 and includes enhanced generic types  SONY VAIO VGN-FW83DS Battery .

In 2008, Accellera released VHDL 4.0 to the IEEE for balloting for inclusion in IEEE 1076-2008. The VHDL standard IEEE 1076-2008 was published in January 2009.

Design

VHDL is commonly used to write text models that describe a logic circuit. Such a model is processed by a synthesis program, only if it is part of the logic design  SONY VAIO VGN-FW83JS Battery .

A simulation program is used to test the logic design using simulation models to represent the logic circuits that interface to the design. This collection of simulation models is commonly called a testbench.

VHDL has constructs to handle the parallelism inherent in hardware designs, but these constructs (processes) differ in syntax from the parallel constructs in Ada (tasksSONY VAIO VGN-FW83XS Battery .

Like Ada, VHDL is strongly typed and is not case sensitive. In order to directly represent operations which are common in hardware, there are many features of VHDL which are not found in Ada, such as an extended set of Boolean operators including nand and nor  SONY VAIO VGN-FW90NS Battery .

VHDL also allows arrays to be indexed in either ascending or descending direction; both conventions are used in hardware, whereas in Ada and most programming languages only ascending indexing is available   SONY VAIO VGN-FW37J/B Battery .

VHDL has file input and output capabilities, and can be used as a general-purpose language for text processing, but files are more commonly used by a simulation testbench for stimulus or verification data. There are some VHDL compilers which build executable binaries   SONY VAIO VGN-FW37J Battery .

In this case, it might be possible to use VHDL to write a testbench to verify the functionality of the design using files on the host computer to define stimuli, to interact with the user, and to compare results with those expected. However, most designers leave this job to the simulator  SONY VAIO VGN-FW378J/B Battery .

It is relatively easy for an inexperienced developer to produce code that simulates successfully but that cannot be synthesized into a real device, or is too large to be practical. One particular pitfall is the accidental production of transparent latches rather than D-type flip-flops as storage elements  SONY VAIO VGN-FW378DH Battery .

One can design hardware in a VHDL IDE (for FPGA implementation such as Xilinx ISE, Altera Quartus, Synopsys Synplify or Mentor Graphics HDL Designer) to produce the RTL schematic of the desired circuit   SONY VAIO VGN-FW378J/B Battery .

After that, the generated schematic can be verified using simulation software which shows the waveforms of inputs and outputs of the circuit after generating the appropriate testbench. To generate an appropriate testbench for a particular circuit or VHDL code, the inputs have to be defined correctly   SONY VAIO VGN-FW37J Battery .

For example, for clock input, a loop process or an iterative statement is required.

A final point is that when a VHDL model is translated into the "gates and wires" that are mapped onto a programmable logic device such as a CPLD or FPGA, then it is the actual hardware being configured, rather than the VHDL code being "executed" as if on some form of a processor chip  SONY VAIO VGN-FW41E/H Battery .

Advantages

The key advantage of VHDL, when used for systems design, is that it allows the behavior of the required system to be described (modeled) and verified (simulated) before synthesis tools translate the design into real hardware (gates and wires)  SONY VAIO VGN-FW41J/H Battery .

Another benefit is that VHDL allows the description of a concurrent system. VHDL is a dataflow language, unlike procedural computing languages such as BASIC, C, and assembly code, which all run sequentially, one instruction at a time  SONY VAIO VGN-FW41M/H Battery .

VHDL project is multipurpose. Being created once, a calculation block can be used in many other projects. However, many formational and functional block parameters can be tuned (capacity parameters, memory size, element base, block composition and interconnection structure)  SONY VAIO VGN-FWZJ/H Battery .

VHDL project is portable. Being created for one element base, a computing device project can be ported on another element base, for example VLSI with various technologies   Dell Inspiron 6400 AC Adapter .

Design examples

In VHDL, a design consists at a minimum of an entity which describes the interface and an architecture which contains the actual implementation. In addition, most designs import library modules. Some designs also contain multiple architectures and configurations  Dell LATITUDE D800 battery .

A simple AND gate in VHDL would look something like this:

While the example above may seem very verbose to HDL beginners, many parts are either optional or need to be written only onceDell Inspiron Mini 12 battery .

Generally simple functions like this are part of a larger behavioral module, instead of having a separate module for something so simple. In addition, use of elements such as the std_logic type might at first seem to be an overkill. One could easily use the built-inbit type and avoid the library import in the beginning   Dell Inspiron 1720 battery .

However, using this 9-valued logic (U,X,0,1,Z,W,H,L,-) instead of simple bits (0,1) offers a very powerful simulation and debugging tool to the designer which currently does not exist in any other HDL.

In the examples that follow, you will see that VHDL code can be written in a very compact form   Dell Inspiron 1564 battery .

However, the experienced designers usually avoid these compact forms and use a more verbose coding style for the sake of readability and maintainability. Another advantage to the verbose coding style is the smaller amount of resources used when programming to a Programmable Logic Device such as a CPLD  Dell Inspiron 1764 battery .

Synthesizeable constructs and VHDL templates

VHDL is frequently used for two different goals: simulation of electronic designs and synthesis of such designs. Synthesis is a process where a VHDL is compiled and mapped into an implementation technology such as an FPGA or an ASIC  Dell Inspiron 1464 battery .

Many FPGA vendors have free (or inexpensive) tools to synthesize VHDL for use with their chips, where ASIC tools are often very expensive.

Not all constructs in VHDL are suitable for synthesis   Dell XPS 1340 battery .

For example, most constructs that explicitly deal with timing such as wait for 10 ns; are not synthesizable despite being valid for simulation. While different synthesis tools have different capabilities, there exists a common synthesizable subset of VHDL that defines what language constructs and idioms map into common hardware for many synthesis tools Dell INSPIRON 9100 battery .

IEEE 1076.6 defines a subset of the language that is considered the official synthesis subset. It is generally considered a "best practice" to write very idiomatic code for synthesis as results can be incorrect or suboptimal for non-standard constructs.

Some examples of synthesizable code follow below  Dell Inspiron N4010 battery :

MUX template

The multiplexer, or 'MUX' as it is usually called, is a simple construct very common in hardware design. The example below demonstrates a simple two to one MUX, with inputs A and B, selector S and output X. Note that there are many other ways to express the same MUX in VHDL  Dell N3010 battery .

In the semiconductor and electronic design industry, Verilog is a hardware description language (HDL) used to model electronic systems. Verilog HDL, not to be confused with VHDL, is most commonly used in the design, verification, and implementation of digital logic chips at the register transfer level (RTL) of abstraction  Dell D6400 battery .

It is also used in the verification of analog and mixed-signal circuits.

Overview

Hardware description languages such as Verilog, differ from software programming languages because they include ways of describing the propagation of time and signal dependencies (sensitivity)  HP Pavilion DV6-1210SA battery .

There are two assignment operators, a blocking assignment (=), and a non-blocking (<=) assignment. The non-blocking assignment allows designers to describe a state-machine update without needing to declare and use temporary storage variables (in any general programming language we need to define some temporary storage spaces for the operands to be operated on subsequently HP Pavilion DM4 battery ;

those are temporary storage variables). Since these concepts are part of Verilog's language semantics, designers could quickly write descriptions of large circuits, in a relatively compact and concise form. At the time of Verilog's introduction (1984)  HP Pavilion DM3 battery ,

Verilog represented a tremendous productivity improvement for circuit designers who were already using graphical schematic capture software and specially-written software programs to document and simulate electronic circuits  Sony Vaio VGN-CR23/W Battery .

The designers of Verilog wanted a language with syntax similar to the C programming language, which was already widely used in engineering software development. Verilog is case-sensitive, has a basic preprocessor (though less sophisticated than that of ANSI C/C++), and equivalent control flow keywords (if/else, for, while, case, etc.), and compatible operator precedence Sony Vaio VGN-CR23/N Battery .

Syntactic differences include variable declaration (Verilog requires bit-widths on net/reg types , demarcation of procedural blocks (begin/end instead of curly braces {}), and many other minor differences.

A Verilog design consists of a hierarchy of modules   Sony Vaio VGN-CR23/L Battery .

Modules encapsulate design hierarchy, and communicate with other modules through a set of declared input, output, and bidirectional ports. Internally, a module can contain any combination of the following: net/variable declarations (wire, reg, integer, etc.), concurrent and sequential statement blocks, and instances of other modules (sub-hierarchies)  Sony Vaio VGN-CR23/R Battery.

Sequential statements are placed inside a begin/end block and executed in sequential order within the block. But the blocks themselves are executed concurrently, qualifying Verilog as adataflow language.

Verilog's concept of 'wire' consists of both signal values (4-state: "1, 0, floating, undefined")  Sony Vaio VGN-CR23/P Battery ,

and strengths (strong, weak, etc.) This system allows abstract modeling of shared signal-lines, where multiple sources drive a common net. When a wire has multiple drivers, the wire's (readable) value is resolved by a function of the source drivers and their strengths  Sony Vaio VGN-CR23/B Battery .

A subset of statements in the Verilog language are synthesizable. Verilog modules that conform to a synthesizable coding-style, known as RTL (register transfer level), can be physically realized by synthesis software. Synthesis-software algorithmically transforms the (abstract) Verilog source into a netlist  Sony Vaio VGN-CR220E/R Battery ,

a logically-equivalent description consisting only of elementary logic primitives (AND, OR, NOT, flipflops, etc.) that are available in a specific FPGA or VLSI technology. Further manipulations to the netlist ultimately lead to a circuit fabrication blueprint (such as a photo mask set for an ASIC, or a bitstream file for an FPGA)  Sony Vaio VGN-CR21Z/R Battery .

Beginning

Verilog was invented by Phil Moorby and Prabhu Goel during the winter of 1983/1984 at Automated Integrated Design Systems (renamed to Gateway Design Automation in 1985) as a hardware modeling language   Sony Vaio VGN-CR21Z/N Battery .

Gateway Design Automation was purchased by Cadence Design Systems in 1990. Cadence now has full proprietary rights to Gateway's Verilog and the Verilog-XL simulator logic simulators. Originally, Verilog was intended to describe and allow simulation; only afterwards was support for synthesis added  Sony Vaio VGN-CR21S/W Battery .

Verilog-95

With the increasing success of VHDL at the time, Cadence decided to make the language available for open standardization. Cadence transferred Verilog into the public domain under the Open Verilog International (OVI) (now known as Accellera) organization  Sony Vaio VGN-CR21S/P Battery .

Verilog was later submitted to IEEE and became IEEE Standard 1364-1995, commonly referred to as Verilog-95.

In the same time frame Cadence initiated the creation of Verilog-A to put standards support behind its analog simulator Spectre. Verilog-A was never intended to be a standalone language and is a subset of Verilog-AMS which encompassed Verilog-95 Sony Vaio VGN-CR21S/L Battery .

Verilog 2001

Extensions to Verilog-95 were submitted back to IEEE to cover the deficiencies that users had found in the original Verilog standard. These extensions became IEEE Standard 1364-2001 known as Verilog-2001  Sony Vaio VGN-CR21E/W Battery .

Verilog-2001 is a significant upgrade from Verilog-95. First, it adds explicit support for (2's complement) signed nets and variables. Previously, code authors had to perform signed-operations using awkward bit-level manipulations (for example, the carry-out bit of a simple 8-bit addition required an explicit description of the boolean-algebra to determine its correct value) Sony Vaio VGN-CR21E/P Battery .

The same function under Verilog-2001 can be more succinctly described by one of the built-in operators: +, -, /, *, >>>. A generate/endgenerate construct (similar to VHDL's generate/endgenerate) allows Verilog-2001 to control instance and statement instantiation through normal decision-operators (case/if/else)   Sony Vaio VGN-CR21E/L Battery .

Using generate/endgenerate, Verilog-2001 can instantiate an array of instances, with control over the connectivity of the individual instances. File I/O has been improved by several new system-tasks. And finally, a few syntax additions were introduced to improve code-readability Sony Vaio VGN-CR21/B Battery .

Verilog-2001 is the dominant flavor of Verilog supported by the majority of commercial EDA software packages.

Verilog 2005

Not to be confused with SystemVerilog, Verilog 2005 (IEEE Standard 1364-2005) consists of minor corrections, spec clarifications, and a few new language features (such as the uwire keyword)  Sony Vaio VGN-CR190E/W Battery .

A separate part of the Verilog standard, Verilog-AMS, attempts to integrate analog and mixed signal modeling with traditional Verilog.

SystemVerilog

SystemVerilog is a superset of Verilog-2005, with many new features and capabilities to aid design-verification and design-modelingSony Vaio VGN-CR190E/R Battery .

As of 2009, the SystemVerilog and Verilog language standards were merged into SystemVerilog 2009 (IEEE Standard 1800-2009).

The advent of hardware verification languages such as OpenVera, and Verisity's e language encouraged the development of Superlog by Co-Design Automation Inc  Sony Vaio VGN-CR190E/P Battery .

Co-Design Automation Inc was later purchased by Synopsys. The foundations of Superlog and Vera were donated to Accellera, which later became the IEEE standard P1800-2005: SystemVerilog  Sony Vaio VGN-CR190E/L Battery .

Partager cet article

Repost 0
Published by batterys - dans Laptop Battery
commenter cet article

commentaires