A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current pulse, and continue to conduct while they are forward biased (that is, while the voltage across the device is not reversed) Lenovo S10-2 Battery 5200mAh .
Some sources define silicon controlled rectifiers and thyristors as synonymous.
Other sources define thyristors as a larger set of devices with at least four layers of alternating N and P-type material, includingLenovo Y730 Battery 5200mAh :
- Silicon controlled rectifier (SCR)
- Gate turn-off thyristor (GTO)
- Triode AC switch (TRIAC)
- Static induction thyristor (SITh) Lenovo Y710 Battery 5200mAh
- MOS Controlled Thyristor (MCT)
- Distributed Buffer - Gate Turn-off Thyristor (DB-GTO)
- Integrated gate commutated thyristor (IGCT)
- MOS composite static induction thyristor/CSMT
- Reverse conducting thyristor Lenovo Y530 Battery 5200mAh
The thyristor is a four-layer, three terminal semiconducting device, with each layer consisting of alternately N-type or P-type material, for example P-N-P-N Lenovo Y510 Battery 5200mAh .
The main terminals, labelled anode and cathode, are across the full four layers, and the control terminal, called the gate, is attached to p-type material near to the cathode. (A variant called an SCS—Silicon Controlled Switch—brings all four layers out to terminals.)Lenovo S10-3 Battery 5200mAh
The operation of a thyristor can be understood in terms of a pair of tightly coupled bipolar junction transistors, arranged to cause the self-latching action:
Thyristors have three states Lenovo G450 Battery 5200mAh :
- Reverse blocking mode — Voltage is applied in the direction that would be blocked by a diode
- Forward blocking mode — Voltage is applied in the direction that would cause a diode to conduct, but the thyristor has not yet been triggered into conduction Lenovo E30 Battery 5200mAh
- Forward conducting mode — The thyristor has been triggered into conduction and will remain conducting until the forward current drops below a threshold value known as the "holding current" Lenovo E40 Battery 5200mAh
Function of the gate terminal
The thyristor has three p-n junctions (serially named J1, J2, J3 from the anode).
When the anode is at a positive potential VAK with respect to the cathode with no voltage applied at the gate, junctions J1 and J3 are forward biased, while junction J2 is reverse biased. As J2 is reverse biased, no conduction takes place (Off state) Lenovo E50 Battery 5200mAh .
Now if VAK is increased beyond the breakdown voltage VBO of the thyristor, avalanche breakdown of J2takes place and the thyristor starts conducting (On state).
If a positive potential VG is applied at the gate terminal with respect to the cathode, the breakdown of the junction J2 occurs at a lower value of VAK Lenovo G460 Battery 5200mAh .
By selecting an appropriate value of VG, the thyristor can be switched into the on state suddenly.
Once avalanche breakdown has occurred, the thyristor continues to conduct, irrespective of the gate voltage, until Lenovo Y460 Battery 5200mAh :
(a) the potential VAK is removed or (b) the current through the device (anode−cathode) is less than the holding current specified by the manufacturer. Hence VG can be a voltage pulse, such as the voltage output from a UJT relaxation oscillator.
These gate pulses are characterized in terms of gate trigger voltage (VGT) and gate trigger current (IGT) Lenovo Y450 Battery 5200mAh .
Gate trigger current varies inversely with gate pulse width in such a way that it is evident that there is a minimum gate charge required to trigger the thyristor Lenovo N200 Battery 5200mAh .
In a conventional thyristor, once it has been switched on by the gate terminal, the device remains latched in the on-state (i.e. does not need a continuous supply of gate current to conduct), providing the anode current has exceeded the latching current (IL)Lenovo T410 Battery 5200mAh .
As long as the anode remains positively biased, it cannot be switched off until the anode current falls below the holding current (IH).
A thyristor can be switched off if the external circuit causes the anode to become negatively biased. In some applications this is done by switching a second thyristor to discharge a capacitor into the cathode of the first thyristor Lenovo T510 Battery 5200mAh .
This method is called forced commutation.
After a thyristor has been switched off by forced commutation, a finite time delay must have elapsed before the anode can again be positively biased and retain the thyristor in the off-state Lenovo SL410 Battery 5200mAh .
This minimum delay is called the circuit commutated turn off time (tQ). Attempting to positively bias the anode within this time causes the thyristor to be self-triggered by the remaining charge carriers (holes and electrons) that have not yet recombined Lenovo SL510 Battery 5200mAh .
For applications with frequencies higher than the domestic AC mains supply (e.g. 50 Hz or 60 Hz), thyristors with lower values of tQare required. Such fast thyristors are made by diffusing into the silicon heavy metals ions such as gold or platinum which act as charge combination centres Lenovo C100 Battery 5200mAh .
Alternatively, fast thyristors may be made by neutron irradiation of the silicon.
The Silicon Controlled Rectifier (SCR) or Thyristor proposed by William Shockley in 1950 and championed Lenovo T400S Battery 5200mAh
by Moll and others at Bell Labs was developed in 1956 by power engineers at General Electric(G.E.) led by Gordon Hall and commercialized by G.E.'s Frank W. "Bill" Gutzwiller.
Thyristors are mainly used where high currents and voltages are involved Lenovo Y430 Battery 5200mAh ,
and are often used to controlalternating currents, where the change of polarity of the current causes the device to switch off automatically; referred to as Zero Cross operation. The device can be said to operate synchronously as, once the device is open Ibm ThinkPad G40 Battery 5200mAh ,
it conducts current in phase with the voltage applied over its cathode to anode junction with no further gate modulation being required to replicate; the device is biased fully on. This is not to be confused with symmetrical operation, as the output is unidirectional, flowing only from cathode to anode, and so is asymmetrical in nature Ibm ThinkPad X200T Battery 5200mAh .
Thyristors can be used as the control elements for phase angle triggered controllers, also known as phase fired controllers.
They can also be found in power supplies for digital circuits, where they are used as a sort of "circuit breaker" or "crowbar" to prevent a failure in the power supply from damaging downstream components Apple A1280 battery 5200mAh .
A thyristor is used in conjunction with a zener diode attached to its gate, and when the output voltage of the supply rises above the zener voltage, the thyristor will conduct, then short-circuit the power supply output to ground (and in general blowing an upstream fuse) Apple A1281 battery 5200mAh .
The first large scale application of thyristors, with associated triggering diac, in consumer products related to stabilized power supplies within color television receivers in the early 1970s Samsung R522 battery 5200mAh .
The stabilized high voltage DC supply for the receiver was obtained by moving the switching point of the thyristor device up and down the falling slope of the positive going half of the AC supply input (if the rising slope was used the output voltage would always rise towards the peak input voltage when the device was triggered and thus defeat the aim of regulation) Samsung P467 battery 5200mAh .
The precise switching point was determined by the load on the output DC supply as well fluctuations on the input AC supply.
Thyristors have been used for decades as lighting dimmers in television, motion pictures Hp Pavilion dm3 Battery 5400mAh ,
and theater, where they replaced inferior technologies such as autotransformers and rheostats. They have also been used in photography as a critical part of flashes (strobes).
Thyristors can be triggered by a high rate of rise of off-state voltage Hp Pavilion dm4 Battery 5200mAh .
This is prevented by connecting a resistor-capacitor (RC) snubber circuit between the anode and cathode terminals in order to limit the dV/dt (i.e., rate of change of voltage versus time).
HVDC electricity transmission
Since modern thyristors can switch power on the scale of megawatts Hp Pavilion cq62 Battery 5200mAh ,
thyristor valves have become the heart of high-voltage direct current (HVDC) conversion either to or from alternating current. In the realm of this and other very high power applications, both electronically switched (ETT) and light switched (LTT) thyristorsare still the primary choice Hp Pavilion cq42 Battery 5200mAh .
The valves are arranged in stacks usually suspended from the ceiling of a transmission building called a valve hall. Thyristors are arranged into a Graetz bridge circuit and to avoid harmonics are connected in series to form a 12 pulse converter. Each thyristor is cooled with deionized water Hp Pavilion cq72 Battery 5200mAh ,
and the entire arrangement becomes one of multiple identical modules forming a layer in a multilayer valve stack called a quadruple valve. Three such stacks are typically hung from the ceiling of the valve building of a long distance transmission facility Hp cq20 Battery 2600mAh .
Comparisons to other devices
The functional drawback of a thyristor is that, like a diode, it only conducts in one direction. A similar self-latching 5-layer device, called a TRIAC, is able to work in both directions. This added capability, though, also can become a shortfall Hp 2230 Battery 2600mAh .
Because the TRIAC can conduct in both directions, reactive loads can cause it to fail to turn off during the zero-voltage instants of the ac power cycle. Because of this, use of TRIACs with (for example) heavily-inductive motor loads usually requires the use of a "snubber" circuit around the TRIAC to assure that it will turn off with each half-cycle of mains power Dell Inspiron Mini 12 battery 7800mAh .
Inverse parallel SCRs can also be used in place of the triac; because each SCR in the pair has an entire half-cycle of reverse polarity applied to it, the SCRs, unlike TRIACs, are sure to turn off. The "price" to be paid for this arrangement, however, is the added complexity of two separate but essentially identical gating circuits Dell XPS M1730 battery 7800mAh .
An earlier gas filled tube device called a Thyratron provided a similar electronic switching capability, where a small control voltage could switch a large current. It is from a combination of "thyratron" and "transistor" that the term "thyristor" is derived Dell XPS M2010 battery 7800mAh .
Although thyristors are heavily used in megawatt scale rectification of AC to DC, in low and medium power (from few tens of watts to few tens of kilowatts) they have almost been replaced by other devices with superior switching characteristics like MOSFETs or IGBTs Dell Studio XPS 1340 battery 5200mAh/7800mAh .
One major problem associated with SCRs is that they are not fully controllable switches. The GTO (Gate Turn-off Thyristor) andIGCT are two related devices which address this problem. In high-frequency applications, thyristors are poor candidates due to large switching times arising from bipolar conduction Dell Inspiron 9100 battery 7800mAh .
MOSFETs, on the other hand, have much faster switching capability because of their unipolar conduction (only majority carriers carry the current).
As well as the usual failure modes due to exceeding voltage, current or power ratings, thyristors have their own particular modes of failure, including Dell Inspiron N4010 battery 5200mAh ;
- Turn on di/dt — in which the rate of rise of on-state current after triggering is higher than can be supported by the spreading speed of the active conduction area (SCRs & triacs) Dell Inspiron N3610 battery 5200mAh .
- Forced commutation — in which the transient peak reverse recovery current causes such a high voltage drop in the sub-cathode region that it exceeds the reverse breakdown voltage of the gate cathode diode junction (SCRs only) Dell Inspiron N5010 battery 5200mAh .
- Switch on dv/dt — the thyristor can be spuriously fired without trigger from the gate if the rate of rise of voltage anode to cathode is too great.
Silicon carbide thyristors
In recent years, some manufacturers have developed thyristors using Silicon carbide (SiC) as the semiconductor material Dell Inspiron N7010 battery 5200mAh .
These have applications in high temperature environments, being capable ofoperating at temperatures up to 350 °C.
A valve hall is a building which contains the valves of the static inverters of a HVDC plant. The valves consist of thyristors, or at older plants, mercury arc rectifiers Dell Inspiron 1464 battery 5200mAh .
Mercury arc rectifiers are usually supported by insulators mounted on the floor, while thyristor valves may be either supported by insulators or hung from the roof of the valve hall. The latter required a stronger ceiling structure, however the hall and the static inverter can better survive earthquakes compared to valve structures standing on the floor Dell Inspiron 1564 battery 5200mAh .
A valve hall is equipped with heating and cooling equipment to control the temperature of the mercury arc rectifiers (which operate best over a narrow temperature range) or thyristors. The valve hall also protects the valves from weather and dust Dell Inspiron 1764 battery 5200mAh .
Several valve assemblies, connected in series for the required terminal voltage, may be installed in the valve hall building.
High voltage bushings are supported through the walls of the valve hall, to allow connections between the converter transformers on the one side and the DC switchyard on the other Dell Inspiron 1320 battery 5200mAh .
Beside the valve hall there is often an additional building, in which are control electronics, equipment for valve cooling and valve monitoring, station service power distribution, and amenities for the plant workers.
Because very high voltages are present while the inverters are in operation Dell Inspiron 1320n battery 5200mAh ,
access to the valve halls is limited while the static inverter is running. The auxiliary control building may have windows to observe the valve hall, but usually the converter is remotely controlled. To protect communication systems from electromagnetic interference, valve hall buildings must have shielding installed to control emission of radio-frequency energy Dell Vostro 1710 battery 5200mAh .
At some HVDC converters such as at Cabora-Bassa, outdoor valves are installed in oil-immersed containers. At such exceptional plants no valve halls or wall bushings are required.
A thyristor drive is a motor drive circuit where AC supply current is regulated by a thyristor phase control to provide variable voltage to a DC motor Dell Vostro 1720 battery 5200mAh .
Thyristor drives are very simple and were first introduced in the 1960s. They remained the predominant type of industrial motor controller until the end of the 1980s when the availability of low cost electronics led to their replacement by chopper drives for high performance systems and inverters for high reliability with AC motors Dell Studio XPS 1640 battery 5200mAh .
They are still employed in very high power applications, such as locomotives, where the high power capability of the thyristors and the simplicity of the design can make them a more attractive proposition than transistor based controllers Dell Vostro A840 battery 5200mAh .
A derivative of the thyristor drive is the simple AC phase controller. This uses a single phase controlled triac to provide a variable voltage AC output for regulating a universal motor. This is the type of motor speed control most commonly used in domestic appliances, such as food mixers, and small AC powered tools, such as electric drills Dell Vostro A860 battery 5200mAh .
Quadracs are a special type of thyristor which combines a "diac" and a "triac" in a single package. The diac is the triggering device for the triac. Thyristors are four-layer (PNPN) semiconductor devices that act as switches, rectifiers or voltage regulators in a variety of applications Dell Studio 1535 battery 5200mAh/7800mAh .
When triggered, thyristors turn on and become low-resistance current paths. They remain so even after the trigger is removed, and until the current is reduced to a certain level (or until they are triggered off). Diacs are bi-directional diodes that switch AC voltages and trigger triacs or silicon-controlled rectifiers (SCRs) Dell Studio 1735 battery 5200mAh/7800mAh .
Except for a small leakage current, diacs do not conduct until the breakover voltage is reached. Triacs are three-terminal, silicon devices that function as two SCRs configured in an inverse, parallel arrangement. They provide load current during both halves of the AC supply voltage Dell Vostro 1310 battery 5200mAh .
By combining the functions of diacs and triacs, quadracs eliminate the need to buy and assemble discrete parts.
Quadracs are used in lighting control, speed control, and temperature modulation control applications Dell Latitude E6400 battery,10400mAh/7800mAh .
They carry performance specifications such as peak repetitive off voltage, peak repetitive reverse voltage, root mean square (RMS) on-state current, and temperature junction. Peak repetitive off voltage is the maximum, instantaneous value of the off-state voltage that occurs across a thyristor Dell Latitude E6400 Battery,10400mAh/7800mAh ,
including all repetitive transient voltages and excluding all non-transient voltages. Peak repetitive reverse voltage is the maximum peak reverse voltage hat may be applied continuously to the main terminals (anode and cathode) of quadracs. RMS on-state current is the maximum RMS current allowed for the specified use-case temperature Dell Latitude E6400 Battery,10400mAh/7800mAh .
Temperature junction for quadracs is expressed as a full-required range.
Quadracs are available in a variety of integrated circuit (IC) package types with different numbers of pins. Basic IC packages types for quadracs include discrete packaging (DPAK), power packaging (PPAK), and in-line packaging (IPAK) Dell Latitude E6500 battery,10400mAh/7800mAh .
Other IC package types include diode outline (DO), transistor outline (TO), and small outline transistor (SOT). Quadracs that use metal electrode leadless face (MELF) packaging have metallized terminals at each end of a cylindrical body. Other available package types for quadracs include thin small outline package (TSOP) Dell Latitude E6500 battery,10400mAh/7800mAh ,
thin shrink small outline L-leaded package (TSSOP), and thin small outline J-lead (TSOJ) package.
Latchup is a term used in the realm of integrated circuits (ICs) to describe a particular type of short circuit which can occur in an improperly designed circuit Dell Latitude E6500 battery,10400mAh/7800mAh .
More specifically it is the inadvertent creation of a low-impedance path between the power supply rails of a MOSFET circuit, triggering a parasitic structure which disrupts proper functioning of the part and possibly even leading to its destruction due to overcurrent Dell Latitude E6500 battery,10400mAh/7800mAh .
A power cycle is required to correct this situation.
The parasitic structure is usually equivalent to a thyristor (or SCR), a PNPN structure which acts as a PNP and an NPN transistor stacked next to each other. During a latchup when one of the transistors is conducting, the other one begins conducting too HP Pavilion DV6-1210SA battery - 5200mAh/8800mAh .
They both keep each other in saturation for as long as the structure is forward-biased and some current flows through it - which usually means until a power-down. The SCR parasitic structure is formed as a part of the totem-pole PMOS and NMOS transistor pair on the output drivers of the gates HP Pavilion DV7 battery - 7200mAh .
The latchup does not have to happen between the power rails; it can happen at any place where the required parasitic structure exists. A spike of positive or negative voltage on an input or output pin of a digital chip, exceeding the rail voltage by more than a diode drop, is a common cause of latchup Ibm THINKPAD X200 Battery 8800mAh .
Another cause is the supply voltage exceeding the absolute maximum rating, often from a transientspike in the power supply, leading to a breakdown of some internal junction. This frequently happens in circuits which use multiple supply voltages that do not come up in the proper order after a power-up Ibm THINKPAD X200 Battery 8800mAh ,
leading to voltages on data lines exceeding the input rating of parts that have not yet reached a nominal supply voltage.
Yet another common cause of latchups is ionizing radiation which makes this a significant issue in electronic products designed for space (or very high-altitude) applications Ibm THINKPAD X200 Battery 8800mAh .
It is possible to design chips that are latchup-resistant, where a layer of insulating oxide (called a trench) surrounds both the NMOS and the PMOS transistors SONY VAIO VGN-FW21E Battery .
This breaks the parasitic SCR structure between these transistors. Such parts are important in the cases where the proper sequencing of power and signals cannot be guaranteed (e.g., in hot swap devices).
Devices fabricated in lightly doped epitaxial layers grown on heavily doped substrates are also less susceptible to latchup Asus M52N Battery .
The heavily doped layer acts as a current sink where excess minority carriers can quickly recombine.
Another possibility for a latchup prevention is the Latchup Protection Technology circuit. When a latchup is detected, the LPT circuit shuts down the chip and holds it powered-down for a preset time Dell Latitude E6400 ATG Battery .
Most silicon-on-insulator devices are inherently latchup-resistant. Latchup is the low resistance connection between tub and power supply rails.
A silicon-controlled rectifier (or semiconductor-controlled rectifier) is a four-layer solid state device that controls current SONY VAIO VGN-FW21J Battery .
The name "silicon controlled rectifier" or SCR is General Electric's trade name for a type of thyristor. The SCR was developed by a team of power engineers led by Gordon Hall and commercialized by Frank W. "Bill" Gutzwiller in 1957.
Construction of SCR
An SCR consists of four layers of alternating P and N type semiconductor materials SONY VAIO VGN-FW21L Battery .
Silicon is used as the intrinsic semiconductor, to which the properdopants are added. The junctions are either diffused or alloyed. The planar construction is used for low power SCRs (and all the junctions are diffused). Themesa type construction is used for high power SCRs SONY VAIO VGN-FW21M Battery .
In this case, junction J2 is obtained by the diffusion method and then the outer two layers are alloyed to it, since the PNPN pellet is required to handle large currents. It is properly braced with tungsten or molybdenum plates to provide greater mechanical strength. One of these plates is hard soldered to a copper stud, which is threaded for attachment of heat sink SONY VAIO VGN-FW21Z Battery .
The doping of PNPN will depend on the application of SCR, since its characteristics are similar to those of the thyratron. Today, the term thyristor applies to the larger family of multilayer devices that exhibit bistable state-change behaviour, that is, switching either ON or OFF SONY VAIO VGN-FW25T/B Battery .
Modes of operation
In the normal "off" state, the device restricts current to the leakage current. When the gate-to-cathode voltage exceeds a certain threshold, the device turns "on" and conducts current SONY VAIO VGN-FW26T/B Battery .
The device will remain in the "on" state even after gate current is removed so long as current through the device remains above the holding current. Once current falls below the holding current for an appropriate period of time, the device will switch "off" SONY VAIO VGN-FW27/B Battery .
If the gate is pulsed and the current through the device is below the holding current, the device will remain in the "off" state.
If the applied voltage increases rapidly enough, capacitive coupling may induce enough charge into the gate to trigger the device into the "on" state; this is referred to as "dv/dt triggering. SONY VAIO VGN-FW27/W Battery "
This is usually prevented by limiting the rate of voltage rise across the device, perhaps by using a snubber. "dv/dt triggering" may not switch the SCR into full conduction rapidly and the partially-triggered SCR may dissipate more power than is usual, possibly harming the device SONY VAIO VGN-FW31ZJ Battery .
SCRs can also be triggered by increasing the forward voltage beyond their rated breakdown voltage (also called as break over voltage), but again, this does not rapidly switch the entire device into conduction and so may be harmful so this mode of operation is also usually avoided SONY VAIO VGN-FW27T/H Battery .
Also, the actual breakdown voltage may be substantially higher than the rated breakdown voltage, so the exact trigger point will vary from device to device. This device is generally used in switching applications.
SCR are available with or without reverse blocking capability SONY VAIO VGN-FW29/B Battery .
Reverse blocking capability adds to the forward voltage drop because of the need to have a long, low doped P1 region. Usually, the reverse blocking voltage rating and forward blocking voltage rating are the same. The typical application for reverse blocking SCR is in current source inverters SONY VAIO VGN-FW290JRB Battery .
SCR incapable of blocking reverse voltage are known as asymmetrical SCR, abbreviated ASCR. They typically have a reverse breakdown rating in the 10's of volts. ASCR are used where either a reverse conducting diode is applied in parallel (for example, in voltage source inverters) SONY VAIO VGN-FW290JTB Battery .
or where reverse voltage would never occur (for example, in switching power supplies or DC traction choppers).
Asymmetrical SCR can be fabricated with a reverse conducting diode in the same package. These are known as RCT, for reverse conducting thyristor.
Application of SCRs
SCRs are mainly used in devices where the control of high power, possibly coupled with high voltage, is demanded SONY VAIO VGN-FW290JTH Battery .
Their operation makes them suitable for use in medium to high-voltage AC power control applications, such as lamp dimming, regulators and motor control.
The insulated gate bipolar transistor or IGBT is a three-terminal power semiconductor device, noted for high efficiency and fast switching SONY VAIO VGN-FW33G/E1 Battery .
It switches electric power in many modern appliances: electric cars, trains, variable speed refrigerators, air-conditioners and even stereo systems with switching amplifiers. Since it is designed to rapidly turn on and off, amplifiers that use it often synthesize complex waveforms with pulse width modulation and low-pass filters SONY VAIO VGN-FW33G/W Battery .
The IGBT combines the simple gate-drive characteristics of the MOSFETs with the high-current and low–saturation-voltage capability of bipolar transistors by combining an isolated gate FET for the control input, and a bipolar power transistor as a switch, in a single device SONY VAIO VGN-FW33GW Battery .
The IGBT is used in medium- to high-power applications such as switched-mode power supply, traction motor control and induction heating. Large IGBT modules typically consist of many devices in parallel and can have very high current handling capabilities in the order of hundreds of amperes with blocking voltages of 6000 V, equating to hundreds ofkilowatts Dell XPS M1210 Battery) http://www.hdd-shop.co.uk .
The IGBT is a fairly recent invention. The first-generation devices of the 1980s and early 1990s were relatively slow in switching, and prone to failure through such modes as latchup (in which the device won't turn off as long as current is flowing) and secondary breakdown Dell KM958 battery
(in which a localized hotspot in the device goes into thermal runaway and burns the device out at high currents). Second-generation devices were much improved, and the current third-generation ones are even better, with speed rivaling MOSFETs, and excellent ruggedness and tolerance of overloads Dell Studio 1555 battery .
The extremely high pulse ratings of second- and third-generation devices also make them useful for generating large power pulses in areas like particle andplasma physics, where they are starting to supersede older devices like thyratrons and triggered spark gapsSony VGP-BPS13 battery .
Their high pulse ratings, and low prices on the surplus market, also make them attractive to the high-voltage hobbyist for controlling large amounts of power to drive devices such as solid-state Tesla coils and coilguns.
Availability of affordable, reliable IGBTs is a important enabler for electric vehicles and hybrid cars Sony VGP-BPS13/B battery .
The insulated gate bipolar transistor or IGBT is a three-terminal power semiconductor device, noted for high efficiency and fast switching. It switches electric power in many modern appliances: electric cars, trains, variable speed refrigerators, air-conditioners and even stereo systems with switching amplifiers Sony VGP-BPS13/S battery .
Since it is designed to rapidly turn on and off, amplifiers that use it often synthesize complex waveforms with pulse width modulation and low-pass filters.
The IGBT combines the simple gate-drive characteristics of the MOSFETs with the high-current and low–saturation-voltage capability of bipolar transistors by combining an isolated gate FET for the control input Sony VGP-BPS13A/B battery ,
and a bipolar power transistor as a switch, in a single device. The IGBT is used in medium- to high-power applications such as switched-mode power supply, traction motor control and induction heating. Large IGBT modules typically consist of many devices in parallel and can have very high current handling Sony VGP-BPS13B/B battery
capabilities in the order of hundreds of amperes with blocking voltages of 6000 V, equating to hundreds ofkilowatts.
The IGBT is a fairly recent invention. The first-generation devices of the 1980s and early 1990s were relatively slow in switching Sony VGP-BPL9 battery ,
and prone to failure through such modes as latchup (in which the device won't turn off as long as current is flowing) and secondary breakdown (in which a localized hotspot in the device goes into thermal runaway and burns the device out at high currents) Sony VGP-BPS13B/B battery .
Second-generation devices were much improved, and the current third-generation ones are even better, with speed rivaling MOSFETs, and excellent ruggedness and tolerance of overloads.
The extremely high pulse ratings of second- and third-generation devices also make them useful for generating large power pulses in areas like particle andplasma physics Sony VGP-BPL15 battery ,
where they are starting to supersede older devices like thyratrons and triggered spark gaps.
Their high pulse ratings, and low prices on the surplus market, also make them attractive to the high-voltage hobbyist for controlling large amounts of power to drive devices such as solid-state Tesla coils and coilguns Dell Inspiron E1505 battery .
Availability of affordable, reliable IGBTs is a important enabler for electric vehicles and hybrid cars.
An IGBT cell is constructed similarly to a n-channel vertical construction power MOSFET except the n+ drain is replaced with a p+ collector layer, thus forming a vertical PNP bipolar junction transistor HP Pavilion dv6000 Battery .
This additional p+ region creates a cascade connection of a PNP bipolar junction transistor with the surface n-channel MOSFET.
Comparison With Power MOSFETS
An IGBT has a significantly lower forward voltage drop compared to a conventional MOSFET in higher blocking voltage rated devices HP Pavilion dv8000 battery .
As the blocking voltage rating of both MOSFET and IGBT devices increases, the depth of the n- drift region must increase and the doping must decrease, resulting in roughly square relationship increase in forward conduction loss compared to blocking voltage capability of the device HP PAVILION DV6000 Battery, 8800mAh .
By injecting minority carriers (holes) from the collector p+ region into the n- drift region during forward conduction, the resistance of the n- drift region is considerably reduced. However, this resultant reduction in on-state forward voltage comes with several penalties HP PAVILION DV8000 Battery, 8800mAh :
- The additional PN junction blocks reverse current flow. This means that unlike a MOSFET, IGBTs cannot conduct in the reverse direction. In bridge circuits where reverse current flow is needed an additional diode (called a freewheeling diode) is placed in parallel with the IGBT to conduct current in the opposite direction SONY VAIO VGN-FZ Battery .
- The penalty isn't as severe as first assumed though, because at the higher voltages where IGBT usage dominates, discrete diodes are of significantly higher performance than the body diode of a MOSFET HP Pavilion DV6000 battery - 8800mAh .
- The reverse bias rating of the N- drift region to collector P+ diode is usually only of tens of volts, so if the circuit application applies a reverse voltage to the IGBT, an additional series diode must be used.
- The minority carriers injected into the n- drift region take time to enter and exit or recombine at turn on and turn off HP Pavilion DV8000 battery - 7800mAh .
- This results in longer switching time and hence higher switching loss compared to a power MOSFET.
- The on-state forward voltage drop in IGBTs behaves very differently to that in power MOSFETS. The MOSFET voltage drop can be modeled as a resistance, with the voltage drop proportional to current HP Pavilion DV6000 battery - 8800mAh .
- By contrast, IGBT has a diode like voltage drop (typically of the order of 2V) increasing only with the log of the current. Additionally, MOSFET resistance is typically lower for smaller blocking voltages meaning that the choice between IGBTs and power MOSFETS depend HP Pavilion DV8000 battery - 6600mAh
- on both the blocking voltage and current involved in a particular application, as well as the different switching characteristics mentioned above.
In general high voltage, high current and low switching frequencies favor IGBTs while low voltage, low current and high switching frequencies are the domain of the MOSFET SONY VAIO VGN-FZ18 Battery .
Rather than using a device physics-based model, SPICE simulates IGBTs using Macromodels, a method that combines an ensemble of components such as FETs and BJTs in a Darlington configuration. An alternative physics-based model is the Hefner model, introduced by Allen Hefner of the NIST SONY VAIO VGN-FZ21E Battery .
It is a fairly complex model that has shown very good results. Hefner's model is described in a 1988 paper and was later extended to a thermo-electrical model and a version using SABER.
A unijunction transistor (UJT) is an electronic semiconductor device that has only one junction SONY VAIO VGN-FW31J Battery 10400mAh .
The UJT has three terminals: an emitter (E) and two bases (B1 and B2). The base is formed by lightly doped n-type bar of silicon. Two ohmic contacts B1 and B2 are attached at its ends. The emitter is of p-type and it is heavily doped. The resistance between B1 and B2, when the emitter is open-circuit is called interbase resistance SONY VAIO VGN-FW31J Battery 10400mAh .
There are two types of unijunction transistor:
- The original unijunction transistor, or UJT, is a simple device that is essentially a bar of N type semiconductor material into which P type material has been diffused somewhere along its length, defining the device parameter η SONY VAIO VGN-FW21E Battery .
- The 2N2646 is the most commonly used version of the UJT.
- The programmable unijunction transistor, or PUT, is a close cousin to the thyristor. Like the thyristor it consists of four P-N layers and has an anodeand a cathode connected to the first and the last layer, and a gate connected to one of the inner layers SONY VAIO VGN-NR11S/S Battery.
- They are not directly interchangeable with conventional UJTs but perform a similar function. In a proper circuit configuration with two "programming" resistors for setting the parameter η, they behave like a conventional UJT. The 2N6027 is an example of such a device SONY VAIO VGN-NR11M/S Battery .
The UJT is biased with a positive voltage between the two bases. This causes a potential drop along the length of the device. When the emitter voltage is driven approximately one diode voltage above the voltage at the point where the P diffusion (emitter) is, current will begin to flow from the emitter into the base region SONY VAIO VGN-NR11Z/S Battery .
Because the base region is very lightly doped, the additional current (actually charges in the base region) causes conductivity modulation which reduces the resistance of the portion of the base between the emitter junction and the B2 terminal SONY VAIO VGN-NR11Z/T Battery .
This reduction in resistance means that the emitter junction is more forward biased, and so even more current is injected. Overall, the effect is a negative resistance at the emitter terminal. This is what makes the UJT useful, especially in simple oscillator circuitsSony VAIO VGN-FZ21E Battery .
Unijunction transistor circuits were popular in hobbyist electronics circuits in the 1960s and 1970s because they allowed simple oscillators to be built using just one active device. For example, they were used for relaxation oscillators in variable-rate strobe lights Sony VAIO VGN-FW21E Battery .
Later, as integrated circuits became more popular, oscillators such as the 555 timer IC became more commonly used.
In addition to its use as the active device in relaxation oscillators, one of the most important applications of UJTs or PUTs is to trigger thyristors (SCR, TRIAC, etc.) Sony VAIO VGN-NR11S/S Battery .
In fact, a DC voltage can be used to control a UJT or PUT circuit such that the "on-period" increases with an increase in the DC control voltage. This application is important for large AC current control.
A phase-shift oscillator is a simple electronic oscillator Sony VAIO VGN-NR11Z/S Battery .
It contains an inverting amplifier, and a feedback filter which 'shifts' the phase of the amplifier output by 180 degrees at the oscillation frequency.
The filter produces a phase shift that increases with frequency Sony VAIO VGN-NR11M/S Battery .
It must have a maximum phase shift of considerably greater than 180° at high frequencies, so that the phase shift at the desired oscillation frequency is 180°.
The most common way of achieving this kind of filter is using three identical cascaded resistor-capacitor filters Sony VAIO VGN-NR11Z/T Battery ,
which together produce a phase shift of zero at low frequencies, and 270 degrees at high frequencies. At the oscillation frequency each filter produces a phase shift of 60 degrees and the whole filter circuit produces a phase shift of 180 degrees SONY VAIO VGN-FZ180E Battery .
High electron mobility transistor (HEMT), also known as heterostructure FET (HFET) or modulation-doped FET(MODFET), is afield effect transistor incorporating a junction between two materials with different band gaps (i.e., a heterojunction) as the channel instead of a doped region, as is generally the case for MOSFET SONY VAIO VGN-FZ220E Battery .
A commonly used material combination is GaAs with AlGaAs, though there is wide variation, dependent on the application of the device. Devices incorporating more indium generally show better high-frequency performance, while in recent years, gallium nitride HEMTs have attracted attention due to their high-power performance SONY VAIO VGN-FZ340E Battery .
To allow conduction, semiconductors are doped with impurities which donate mobile electrons (or holes). However, these electrons are slowed down through collisions with the impurities (dopants) used to generate them in the first place SONY VAIO VGN-FZ430E Battery .
HEMTs avoid this through the use of high mobility electrons generated using the heterojunction of a highly-doped wide-bandgap n-type donor-supply layer (AlGaAs in our example) and a non-doped narrow-bandgap channel layer with no dopant impurities (GaAs in this case) SONY VAIO VGN-FZ460E Battery .
The electrons generated in the thin n-type AlGaAs layer drop completely into the GaAs layer to form a depleted AlGaAs layer, because the heterojunction created by different band-gap materials forms a quantum well (a steep canyon) in the conduction band on the SONY VAIO VGN-FZ480E Battery
GaAs side where the electrons can move quickly without colliding with any impurities because the GaAs layer is undoped, and from which they cannot escape. The effect of this is to create a very thin layer of highly mobile conducting electrons with very high concentration, giving the channel very low resistivity (or to put it another way, "high electron mobility") SONY VAIO VGN-FZ4000 Battery .
This layer is called a two-dimensional electron gas. As with all the other types of FETs, a voltage applied to the gate alters the conductivity of this layer.
Ordinarily, the two different materials used for a heterojunction must have the same lattice constant (spacing between the atoms)SONY VAIO VGN-FZ31E Battery .
As an analogy, imagine pushing together two plastic combs with a slightly different spacing. At regular intervals, you'll see two teeth clump together. In semiconductors, these discontinuities form deep-level traps, and greatly reduce device performance SONY VAIO VGN-FZ31B Battery .
A HEMT where this rule is violated is called a pHEMT or pseudomorphic HEMT. This is achieved by using an extremely thin layer of one of the materials – so thin that the crystal lattice simply stretches to fit the other material. This technique allows the construction of transistors with larger bandgap differences than otherwise possible, giving them better performance SONY VAIO VGN-FZ31J Battery .
Another way to use materials of different lattice constants is to place a buffer layer between them. This is done in the mHEMTormetamorphic HEMT, an advancement of the pHEMT. The buffer layer is made of AlInAs, with the indium concentration graded so that it can match the lattice constant of both the GaAs substrate and the GaInAs channel SONY VAIO VGN-FZ31M Battery .
This brings the advantage that practically any Indium concentration in the channel can be realized, so the devices can be optimized for different applications (low indium concentration provides low noise; high indium concentration gives high gain) SONY VAIO VGN-FZ31Z Battery .
Applications are similar to those of MESFETs – microwave and millimeter wave communications, imaging, radar, and radio astronomy – any application where high gain and low noise at high frequencies are required. HEMTs have shown current gain to frequencies greater than 600 GHz and power gain to frequencies greater than 1 THz SONY VAIO VGN-FZ38M Battery .
(Heterojunction bipolar transistors were demonstrated at current gain frequencies over 600 GHz in April 2005.) Numerous companies worldwide develop and manufacture HEMT-based devices. These can be discrete transistors but are more usually in the form of a 'monolithic microwave integrated circuit' (MMIC) SONY VGP-BPS8 Battery .
HEMTs are found in many types of equipment ranging from cellphones and DBS receivers to electronic warfare systems such as radar and forradio astronomy.
The invention of the HEMT is usually attributed to Takashi Mimura (Fujitsu, Japan) SONY VGP-BPS13/S Battery .
In America, Ray Dingle and his co-workers in Bell Laboratories also played an important role in the invention of the HEMT. In Europe, Daniel Delagebeaudeuf and Trong Linh Nuyen from Thomson_CSF (France) filed for a patent of this device on the 28th of March 1979 SONY VGP-BPS13A/B Battery .
Since GaAs has higher electron affinity, free electrons in the AlGaAs layer are transferred to the undoped GaAs layer where they form a two dimensional high mobility electron gas within 100 angstromof the interface. The n-type AlGaAs layer of the HEMT is depleted completely through two depletion mechanisms SONY VGP-BPS13B/B Battery :
- Trapping of free electrons by surface states causes the surface depletion.
- Transfer of electrons into the undoped GaAs layer brings about the interface depletion SONY VGP-BPS13A/S Battery .
The Fermi energy level of the gate metal is matched to the pinning point, which is 1.2 eV below the conduction band. with the reduced AlGaAs layer thickness, the electrons supplied by donors in the AlGaAs layer are insufficient to pin the layer. As a result, band bending is moving upward and the two-dimensional electrons gas does not appear SONY VGP-BPS13AS Battery .
When a positive voltage than the threshold voltage is applied to the gate, electrons accumulate at the interface and form a two-dimensional electron gas.
MESFET stands for metal semiconductor field effect transistor. It is quite similar to a JFET in construction and terminologyDell Inspiron 1320n Battery .
The difference is that instead of using a p-n junction for a gate, aSchottky (metal-semiconductor) junction is used. MESFETs are usually constructed in compound semiconductor technologies lacking high quality surface passivation such as GaAs, InP, or SiC, and are
Production MESFETs are operated up to approximately 45 GHz, and are commonly used for microwave frequencycommunications and radar. From a digital circuit design perspective, it is increasingly difficult to use MESFETs as the basis for digital integrated circuits as the scale of integration goes up, compared to CMOS silicon based fabrication Dell Studio 1457 Battery .
The MESFET differs from the common insulated gate FET in that there is no insulator under the gate over the active switching region. This implies that the MESFET gate should, in transistor mode, be biased such that one does not have a forward conducting metal semiconductor diode instead of a reversed biased depletion zone controlling the underlying channel Dell Latitude D610 Battery .
While this restriction inhibits certain circuit possibilities, MESFET analog and digital devices work reasonably well if kept within the confines of design limits. The most critical aspect of the design is the gate metal extent over the switching region Toshiba NB100 Battery .
Generally the narrower the gate modulated carrier channel the better the frequency handling abilities, overall. Spacing of the source and drain with respect to the gate, and the lateral extent of the gate are important though somewhat less critical design parametersToshiba Satellite M65 battery .
MESFET current handling ability improves as the gate is elongated laterally, keeping the active region constant, however is limited by phase shift along the gate due to the transmission line effect. As a result most production MESFETs use a built up top layer of low resistance metal on the gate, often producing a mushroom-like profile in cross section Toshiba Satellite M60 battery .
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a transistor used for amplifying or switching electronicsignals. The basic principle of this kind of transistor was first proposed by Julius Edgar Lilienfeld in 1925 Dell Latitude D830 Battery .
In MOSFETs, a voltage on the oxide-insulated gate electrode can induce a conducting channel between the two other contacts called source and drain. The channel can be of n-type or p-type (see article onsemiconductor devices), and is accordingly called an nMOSFET or a pMOSFET (also commonly nMOS, pMOS) Dell Latitude D620 Battery .
It is by far the most common transistor in both digital and analog circuits, though the bipolar junction transistor was at one time much more common.
The 'metal' in the name is now often a misnomer because the previously metal gate material is now often a layer of polysilicon (polycrystalline silicon) Dell Inspiron Mini 10 Battery .
Aluminium had been the gate material until the mid 1970s, when polysilicon became dominant, due to its capability to form self-aligned gates. Metallic gates are regaining popularity, since it is difficult to increase the speed of operation of transistors without metal gates Sony VGN-FW11S Battery .
IGFET is a related term meaning insulated-gate field-effect transistor, and is almost synonymous with MOSFET, though it can refer to FETs with a gate insulator that is not oxide. Another synonym is MISFET for metal–insulator–semiconductor FET Sony VGN-FW11M Battery .
Usually the semiconductor of choice is silicon, but some chip manufacturers, most notably IBM and Intel, recently started using a chemical compound of silicon and germanium (SiGe) in MOSFET channels. Unfortunately, many semiconductors with better electrical properties than silicon, such as gallium arsenide Sony VGN-FW139E/H battery ,
do not form good semiconductor-to-insulator interfaces, thus are not suitable for MOSFETs. Research continues on creating insulators with acceptable electrical characteristics on other semiconductor material Dell Latitude E5400 Battery .
In order to overcome power consumption increase due to gate current leakage, high-κ dielectric replaces silicon dioxide for the gate insulator, while metal gates return by replacing polysilicon (see Intel announcement ).
The gate is separated from the channel by a thin insulating layer, traditionally of silicon dioxide and later of silicon oxynitride Dell Latitude E4200 Battery .
Some companies have started to introduce a high-κ dielectric + metal gate combination in the 45 nanometer node.
When a voltage is applied between the gate and body terminals, the electric field generated penetrates through the oxide and creates an "inversion layer" or "channel" at the semiconductor-insulator interface Dell Inspiron 300M Battery .
The inversion channel is of the same type, P-type or N-type, as the source and drain, thus it provides a channel through which current can pass. Varying the voltage between the gate and body modulates theconductivity of this layer and allows to control the current flow between drain and source Dell Studio 1737 battery .
A variety of symbols are used for the MOSFET. The basic design is generally a line for the channel with the source and drain leaving it at right angles and then bending back at right angles into the same direction as the channel Dell Inspiron E1505 battery .
Sometimes three line segments are used for enhancement mode and a solid line for depletion mode. Another line is drawn parallel to the channel for the gate.
The bulk connection, if shown, is shown connected to the back of the channel with an arrow indicating PMOS or NMOS Dell RM791 battery .
Arrows always point from P to N, so an NMOS (N-channel in P-well or P-substrate) has the arrow pointing in (from the bulk to the channel). If the bulk is connected to the source (as is generally the case with discrete devices) it is sometimes angled to meet up with the source leaving the transistor Toshiba Satellite P10 Battery .
If the bulk is not shown (as is often the case in IC design as they are generally common bulk) an inversion symbol is sometimes used to indicate PMOS, alternatively an arrow on the source may be used in the same way as for bipolar transistors (out for nMOS, in for pMOS) Acer Aspire One battery 10400mAh .
Comparison of enhancement-mode and depletion-mode MOSFET symbols, along with JFET symbols (drawn with source and drain ordered such that higher voltages appear higher on the page than lower voltages):
For the symbols in which the bulk, or body, terminal is shown, it is here shown internally connected to the source SONY VGN-FZ210CE Battery .
This is a typical configuration, but by no means the only important configuration. In general, the MOSFET is a four-terminal device, and in integrated circuits many of the MOSFETs share a body connection, not necessarily connected to the source terminals of all the transistors Dell Precision M70 Battery .
A traditional metal–oxide–semiconductor (MOS) structure is obtained by growing a layer of silicon dioxide (SiO2) on top of a silicon substrate and depositing a layer of metal or polycrystalline silicon (the latter is commonly used) Toshiba Satellite L305 Battery .
As the silicon dioxide is a dielectric material, its structure is equivalent to a planarcapacitor, with one of the electrodes replaced by a semiconductor.
When a voltage is applied across a MOS structure, it modifies the distribution of charges in the semiconductor Toshiba Satellite T4900 Battery .
If we consider a P-type semiconductor (withNA the density of acceptors, p the density of holes; p = NA in neutral bulk), a positive voltage, VGB, from gate to body (see figure) creates a depletion layer by forcing the positively charged holes away from the gate-insulator/semiconductor interface Toshiba PA3399U-2BRS battery ,
leaving exposed a carrier-free region of immobile, negatively charged acceptor ions (see doping (semiconductor)). If VGB is high enough, a high concentration of negative charge carriers forms in an inversion layerlocated in a thin layer next to the interface between the semiconductor and the insulator Sony VGP-BPS10 battery 10400mAh .
Unlike the MOSFET, where the inversion layer electrons are supplied rapidly from the source/drain electrodes, in the MOS capacitor they are produced much more slowly by thermal generation through carrier generation and recombination centers in the depletion region Sony VGP-BPS9 battery 10400mAh .
Conventionally, the gate voltage at which the volume density of electrons in the inversion layer is the same as the volume density of holes in the body is called the threshold voltage.
This structure with p-type body is the basis of the N-type MOSFET, which requires the addition of an N-type source and drain regionsSony VGP-BPS11 battery 10400mAh .
MOSFET structure and channel formation
A metal–oxide–semiconductor field-effect transistor (MOSFET) is based on the modulation of charge concentration by a MOS capacitance between a bodyelectrode and a gate electrode located above the body and insulated from all other device regions by a gate dielectric layer which in the case of a MOSFET is an oxide, such as silicon dioxide Sony VGP-BPS22 battery 10400mAh .
If dielectrics other than an oxide such as silicon dioxide (often referred to as oxide) are employed the device may be referred to as a metal–insulator–semiconductor FET (MISFET). Compared to the MOS capacitor, the MOSFET includes two additional terminals (source and drain) Sony VGP-BPL8 battery 10400mAh ,
each connected to individual highly doped regions that are separated by the body region. These regions can be either p or n type, but they must both be of the same type, and of opposite type to the body region. The source and drain (unlike the body) are highly doped as signified by a '+' sign after the type of doping Sony VGP-BPS14 battery 10400mAh .
If the MOSFET is an n-channel or nMOS FET, then the source and drain are 'n+' regions and the body is a 'p' region. As described above, with sufficient gate voltage, holes from the body are driven away from the gate, forming an inversion layer or n-channel at the interface between the p region and the oxide Sony VGP-BPS21 battery 10400mAh .
This conducting channel extends between the source and the drain, and current is conducted through it when a voltage is applied between source and drain. Increasing the voltage on the gate leads to a higher electron density in the inversion layer and therefore increases the current flow between the source and drain Toshiba Satellite A200 Battery .
For gate voltages below the threshold value, the channel is lightly populated, and only a very small subthreshold leakage current can flow between the source and the drain.
If the MOSFET is a p-channel or pMOS FET, then the source and drain are 'p+' regions and the body is a 'n' region Toshiba Satellite 1200 Battery .
When a negative gate-source voltage (positive source-gate) is applied, it creates a p-channel at the surface of the n region, analogous to the n-channel case, but with opposite polarities of charges and voltages. When a voltage less negative than the threshold value (a negative voltage for p-channel) is applied between gate and source Toshiba Satellite M300 Battery ,
the channel disappears and only a very small subthreshold current can flow between the source and the drain.
The source is so named because it is the source of the charge carriers (electrons for n-channel, holes for p-channel) that flow through the channel; similarly, the drain is where the charge carriers leave the channel SONY VGP-BPS13A/Q Battery .
The device may comprise a Silicon On Insulator (SOI) device in which a Buried OXide (BOX) is formed below a thin semiconductor layer. If the channel region between the gate dielectric and a Buried Oxide (BOX) region is very thin SONY VGP-BPS13A/Q Battery ,
the very thin channel region is referred to as an Ultra Thin Channel (UTC) region with the source and drain regions formed on either side thereof in and/or above the thin semiconductor layer. Alternatively, the device may comprise a SEMiconductor On Insulator (SEMOI) device in which semiconductors other than silicon are employed SONY VGP-BPS13B/Q Battery .
Many alternative semiconductor materials may be employed.
When the source and drain regions are formed above the channel in whole or in part, they are referred to as Raised Source/Drain (RSD) regions SONY VGP-BPS13B/Q Battery .
Modes of operation
The operation of a MOSFET can be separated into three different modes, depending on the voltages at the terminals. In the following discussion, a simplified algebraic model is used that is accurate only for old technology SONY VGP-BPS13/Q Battery .
Modern MOSFET characteristics require computer models that have rather more complex behavior.
For an enhancement-mode, n-channel MOSFET, the three operational modes are:
- Cutoff, subthreshold, or weak-inversion mode SONY VGP-BPS13/Q Battery
- When VGS < Vth:
- where Vth is the threshold voltage of the device.
- According to the basic threshold model, the transistor is turned off, and there is no conduction between drain and sourceSONY VGP-BPS21A/B Battery .
- In reality, the Boltzmann distribution of electron energies allows some of the more energetic electrons at the source to enter the channel and flow to the drain, resulting in a subthreshold current that is an exponential function of gate–source voltageSONY VGP-BPS21/S Battery .
- While the current between drain and source should ideally be zero when the transistor is being used as a turned-off switch, there is a weak-inversion current, sometimes called subthreshold leakage.
- In weak inversion the current varies exponentially with gate-to-source bias VGS as given approximately by SONY VGP-BPS21B Battery :
- where ID0 = current at VGS = Vth, the thermal voltage VT = kT / q and the slope factor n is given by
- n = 1 + CD / COX, SONY VGP-BPS21A Battery
- with CD = capacitance of the depletion layer and COX = capacitance of the oxide layer. In a long-channel device, there is no drain voltage dependence of the current once VDS > >SONY VGP-BPS21 Battery
- VT, but as channel length is reduced drain-induced barrier lowering introduces drain voltage dependence that depends in a complex way upon the device geometry (for example, the channel doping, the junction doping and so on) Sony VGP-BPS21A/B Battery .
- Frequently, threshold voltage Vth for this mode is defined as the gate voltage at which a selected value of current ID0 occurs, for example, ID0 = 1 μA, which may not be the same Vth-value used in the equations for the following modes Sony VGP-BPS21/S Battery .
- Some micropower analog circuits are designed to take advantage of subthreshold conduction. By working in the weak-inversion region, the MOSFETs in these circuits deliver the highest possible transconductance-to-current ratio, namely: gm/ ID = 1 / (nVT), almost that of a bipolar transistor Sony VGP-BPS21A Battery .
- The subthreshold I–V curve depends exponentially upon threshold voltage, introducing a strong dependence on any manufacturing variation that affects threshold voltage; for example: variations in oxide thickness, junction depth, or body doping that change the degree of drain-induced barrier lowering Sony VGP-BPS21 Battery .
- The resulting sensitivity to fabricational variations complicates optimization for leakage and performance.
- Triode mode or linear region (also known as the ohmic mode )
- When VGS > Vth and VDS < ( VGS – Vth )
- The transistor is turned on, and a channel has been created which allows current to flow between the drain and the source Sony Vaio PCG-5G2L Battery .
- The MOSFET operates like a resistor, controlled by the gate voltage relative to both the source and drain voltages. The current from drain to source is modeled as:
- Sony Vaio PCG-5G3L Battery
- where μn is the charge-carrier effective mobility, W is the gate width, L is the gate length and Cox is the gate oxide capacitance per unit area. The transition from the exponential subthreshold region to the triode region is not as sharp as the equations suggest Sony Vaio PCG-5J1L Battery .
- Saturation or active mode
- When VGS > Vth and VDS > ( VGS – Vth )
- The switch is turned on, and a channel has been created, which allows current to flow between the drain and sourceSony Vaio PCG-5K2L Battery .
- Since the drain voltage is higher than the gate voltage, the electrons spread out, and conduction is not through a narrow channel but through a broader, two- or three-dimensional current distribution extending away from the interface and deeper in the substrate Sony Vaio PCG-5J2L Battery .
- The onset of this region is also known as pinch-off to indicate the lack of channel region near the drain. The drain current is now weakly dependent upon drain voltage and controlled primarily by the gate–source voltage, and modeled approximately as Sony Vaio PCG-5K1L Battery
- The additional factor involving λ, the channel-length modulation parameter, models current dependence on drain voltage due to the Early effect, or channel length modulation Sony Vaio PCG-6S2L Battery .
- According to this equation, a key design parameter, the MOSFET transconductance is:
- where the combination Vov = VGS – Vth is called the overdrive voltage, Sony Vaio PCG-6S3L Battery
- and where VDSsat = VGS - Vth (which Sedra neglects) accounts for a small discontinuity in ID which would otherwise appear at the transition between the triode and saturation regions.
- Another key design parameter is the MOSFET output resistance rout given by Sony Vaio PCG-6V1L Battery :
- rout is the inverse of gDS where . VDS is the expression in saturation region Sony Vaio PCG-6W1L Battery .
- If λ is taken as zero, an infinite output resistance of the device results that leads to unrealistic circuit predictions, particularly in analog circuits.
- As the channel length becomes very short, these equations become quite inaccurate Sony Vaio PCG-6W2L Battery .
- New physical effects arise. For example, carrier transport in the active mode may become limited by velocity saturation. When velocity saturation dominates, the saturation drain current is more nearly linear than quadratic in VGS Sony Vaio PCG-6W3L Battery .
- At even shorter lengths, carriers transport with near zero scattering, known as quasi-ballistic transport. In addition, the output current is affected by drain-induced barrier lowering of the threshold voltage Sony Vaio PCG-7111L Battery .
The body effect describes the changes in the threshold voltage by the change in the source-bulk voltage, approximated by the following equation Sony Vaio PCG-7112L Battery :
where VTN is the threshold voltage with substrate bias present, and VTO is the zero-VSB value of threshold voltage, γ is the body effect parameter, and 2φ is the surface potential parameter.
The body can be operated as a second gate, and is sometimes referred to as the "back gate" Sony Vaio PCG-7113L Battery ;
the body effect is sometimes called the "back-gate effect".
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) Sony Vaio PCG-7133L Battery .
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. It used crystalline silicon for the semiconductor and a thermally oxidized layer of silicon dioxide for the insulator Sony Vaio PCG-7Z2L Battery .
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 Sony Vaio PCG-8Y1L 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 Sony Vaio PCG-8Y2L Battery .
Largely because of these two factors, the MOSFET has become the most widely used type of transistor in integrated 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 Sony Vaio PCG-8Z1L 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 Sony Vaio PCG-8Z2L 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 SONY VAIO VGN-FW31J Battery .
This arrangement greatly reduces power consumption and heat generation. Digital and analog CMOS applications are described below.
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 SONY VAIO VGN-FW31E 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 SONY VAIO VGN-FW31B 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 SONY VAIO VGN-FW31M 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 SONY VAIO VGN-FW31Z Battery .
The MOSFET's advantages in digital circuits do not translate into supremacy in all analog circuits. 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 operation SONY VAIO VGN-FW38M 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.
Nevertheless, MOSFETs are widely used in many types of analog circuits because of certain advantages SONY VAIO VGN-AW41JF Battery .
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 performanceSONY VAIO VGN-AW41JF/H 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 SONY VAIO VGN-AW41MF 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 SONY VAIO VGN-AW41MF/H 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 SONY VAIO VGN-FW30B Battery ,
making the needed board space even smaller. 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 SONY VAIO VGN-FW50B 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 SONY VAIO VGN-FW51B/W Battery .
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 nanometersSONY VAIO VGN-FW51MF 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 SONY VAIO VGN-FW51MF/H 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 SONY VAIO VGN-FW51ZF 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 SONY VAIO VGN-FW51ZF/H 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 chipSONY VAIO VGN-FW52JB 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 SONY VAIO VGN-FW54FB Battery .
This doubling of the transistor count was first observed by Gordon Moore in 1965 and is commonly referred to as Moore's law.