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Power semiconductor device - Wikipedia, the free encyclopediaPower semiconductor deviceFrom Wikipedia, the free encyclopediaJump to: navigation, searchPower semiconductor devices are semiconductor devices used as switches orrectifiers in power electronic circuits (switch mode power supplies forexample). They are also called power devices or when used in integratedcircuits, called power ICs.

Most power semiconductor devices are only used in commutation mode (i.e they are either on or off), and are therefore optimized for this. Most of them should not be used in linear operation.

Contents1 History2 Common power devices3 Common power semiconductor devices3.1 Diodes3.2 Switches

4 Parameters of power semiconductor devices

5 Research and development5.1 Packaging5.2 Improvement of structures5.3 Wide band-gap semiconductors

6 See also7 Notes and references7.1 Notes7.2 References

[edit] HistoryPower semiconductor devices first appeared in 1952 with theintroduction of the power diode by R.N. Hall. It was made of Germanium and had a voltage capability of 200 volts and a current rating of 35 amperes.

The thyristor appeared in 1957. Thyristors are able to withstand very highreverse breakdown voltage and are also capable of carrying high current. Onedisadvantage of the thyristor for switching circuits is that once it is'latched-on' in the conducting state it cannot be turned off by externalcontrol. The thyristor turn-off is passive, i.e., the power must be disconnected from the device.The first bipolar transistors devices with substantial power handlingcapabilities were introduced in the 1960s. These components overcame somelimitations of the thyristors because they can be turned on or off with anapplied signal.With the improvements of the Metal Oxide Semiconductor technology (initially

developed to produce integrated circuits), power MOSFETs became available in the late 1970s. International Rectifier introduced a 25 A, 400 V power MOSFET in1978.[1] These devices allow operation at higher frequency than bipolartransistors, but are limited to the low voltage applications.The Insulated Gate Bipolar Transistor (IGBT) developed in the 1980s becamewidely available in the 1990s. This component has the power handling capabilityof the bipolar transistor, with the advantages of the isolated gate drive of the power MOSFET.[edit] Common power devicesSome common power devices are the power diode,thyristor, power MOSFET and IGBT. A power diode or MOSFET operates on similarprinciples to its low-power counterpart, but is able to carry a larger amount of

 current and typically is able to support a larger reverse-bias voltage in theoff-state.

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Structural changes are often made in power devices to accommodate the highercurrent density, higher power dissipation and/or higher reverse breakdownvoltage. The vast majority of the discrete (i.e non integrated) power devicesare built using a vertical structure, whereas small-signal devices employ alateral structure. With the vertical structure, the current rating of the device is proportional to its area, and the voltage blocking capability is achieved in

the height of the die. With this structure, one of the connections of the device is located on the bottom of the semiconductor die.[edit] Common power semiconductor devicesFig. 1: The power devices family, showing the principal power switches.The realm of power devices is divided into two main categories (see figure 1):The two-terminal devices (diodes), whose state is completely dependent on theexternal power circuit they are connected to;The three-terminal devices, whose state is not only dependent on theirexternal power circuit, but also on the signal on their driving terminal (gate

 

or base). Transistors and thyristors belong to that category.A second classification is less obvious, but has a strong influence on deviceperformance: Some devices are majority carrier devices (Schottky diode, MOSFET), while the others are minority carrier devices (Thyristor, bipolar transistor,IGBT). The former use only one type of charge carriers, while the latter useboth (i.e electrons and holes). The majority carrier devices are faster, but the charge injection of minority carrier devices allows for better On-stateperformance.[edit] DiodesAn ideal diode should have the following characteristics:When forward-biased, the voltage across the end terminals of the diode shouldbe zero, whatever the current that flows through it (on-state);

When reverse-biased, the leakage current should be zero, whatever the voltage(off-state).The transition between on and off states should be instantaneous.

In reality, the design of a diode is a trade-off between performance inon-state, off-state and commutation. Indeed, the same area of the device mustsustain the blocking voltage in the off-state and allow current flow in theon-state. As the requirements for the two states are completely opposite, adiode has to be either optimised for one of them, or time must be allowed toswitch from one state to the other (i.e slow down the commutation speed).This trade-off between on-state/off-state and switching speed is the same forall power devices. A Schottky diode has excellent switching speed and on-stateperformance, but a high level of leakage current in off-state. On the other

hand, PIN diodes are commercially available in different commutation speeds(so-called "fast" and "ultrafast" rectifiers), but any increase in speed is paid for by a lower performance in the on-state.[edit] SwitchesFig.2 : Current/Voltage/switching frequency domains of the main powerelectronics switches.The trade-off between voltage, current and frequencyratings also exists for the switches. Actually, all power semiconductors rely on a PIN diode structure to sustain voltage. This can be seen in figure 2. Thepower MOSFET has the advantages of the majority carrier devices, so it canachieve very high operating frequency, but can't be used with high voltages. Asit is a physical limit, no improvement is expected from silicon MOSFETs

concerning their maximum voltage ratings. However, its excellent performance inlow voltage make it the device of choice (actually the only choice) forapplications below 200 V. By paralleling several devices, it is possible to

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increase the current rating of a switch. The MOSFET is particularly suited tothis configuration because its positive thermal coefficient of resistance tendsto balance current between individual devices.The IGBT is a recent component, so its performance improves regularly astechnology evolves. It has already completely replaced the bipolar transistor in power applications, and the availability of power modules (in which several IGBT

 dice are connected in parallel) makes it attractive for power levels up toseveral megawatts, pushing further the limit where thyristors and GTOs becomethe only option. Basically, an IGBT is a bipolar transistor driven by a powerMOSFET: it has the advantages of being a minority carrier device (goodperformance in on-state, even for high voltage devices), with the high inputimpedance of a MOSFET (it can be driven on or off with a very low amount ofpower).Its major limitation for low voltage applications is the high voltage drop itexhibits in on-state (2 to 4 V). Compared to the MOSFET, the operating frequency of the IGBT is relatively low (few devices are rated over 50 kHz), mainly

because of a so-called 'current-tail' problem during turn-off. This problem iscaused by the slow decay of the conduction current during turn-off resultingfrom slow recombination of large number of carriers, which flood the thick'drift' region of the IGBT during conduction. The net result is that theturn-off switching loss of an IGBT is considerably higher than its turn-on loss. Generally, in datasheets, turn-off energy is mentioned as a measured parameterand one has to multiply that number with the switching frequency of the intended application to estimate the turn-off loss.At very high power levels, thyristor-based devices (SCRs, GTOs, MCTs) are stillthe only choice. Though driving a thyristor is somewhat complicated, as thisdevice can only be turned on. It turns off by itself as soon as no more current

flows through it. This requires a circuit with the means to divert current, orspecific applications where current is known to drop to zero regularly (i.e.Alternating Current). MCTs and GTOs have been developed to overcome thislimitation; these components are widely used in power distribution applications.[edit] Parameters of power semiconductor devicesA power device is usually attached to a heatsink to remove the heat caused byoperation losses.The power semiconductor die of a three-terminal device (IGBT, MOSFET or BJT).Two contacts are on top of the die, the remaining one is on the back.Breakdownvoltage: Often there is a trade-off between breakdown voltage rating andon-resistance, because increasing the breakdown voltage by incorporating athicker and lower doped drift region leads to higher on-resistance.

On-resistance: Higher current rating lowers the on-resistance due to greaternumbers of parallel cells. This increases overall capacitance and slows downthe speed.Rise and fall times for switching between on and off states.Safe-operating area (from thermal dissipation and "latch-up" consideration)Thermal resistance: This is an often ignored but extremely important parameter

 from practical design point of view. Semiconductors do not perform well atelevated temperature but due to large current conduction, all powersemiconductor devices heat up. Therefore they need to be cooled by removingthat heat continuously. Packaging and heatsinks provide a means of removingheat from the semiconductor device by conducting it to the externalenvironment. Generally, large current devices have large die and packaging

surface areas and lower thermal resistance.[edit] Research and development[edit] PackagingThe role of packaging is to:connect a die to the external circuit;

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provide a way to remove the heat generated by the device;protect the die from the external environment (moisture, dust);

Many of the reliability issues of power device are either related to excessivetemperature of fatigue due to thermal cycling. Research is currently carried out on the following topics:improve the cooling performance.

improve the resistance to thermal cycling by closely matching the Coefficientof thermal expansion of the packaging to that of the silicon.increase the maximum operating temperature of the packaging material.

Research is also ongoing on electrical issues such as reducing the parasiticinductance of packaging. This inductance limits the operating frequency as itgenerates losses in the devices during commutation.Low-voltage MOSFETs are also limited by the parasitic resistance of thepackages, as their intrinsic on-state resistance can be as low as one or twomilliohms.Some of the most common type of power semiconductor packages include TO-220,TO-247, TO-262, TO-3, D2Pak, etc.[edit] Improvement of structuresIGBTs are still under development and we can

expect increased operating voltages in the future. At the high-power end of therange, MOS-Controlled Thyristor are promising devices. A major improvement overconventional MOSFET structure is achieved by employing superjunctioncharge-balance principle to the design. Essentially, it allows the thick driftregion of a power MOSFET to be heavily doped (thereby reducing the electricalresistance for electron flow) without compromising the breakdown voltage. Anadjacent region of similarly doped (but of opposite carrier polarity - holes) is created within the structure. These two similar but opposite doped regionseffectively cancel out their mobile charge and develop a 'depleted region' which supports the high voltage during off-state. On the other hand, during conducting 

state, the higher doping of the drift region allows easier flow of carrierthereby reducing on-resistance. Commercial devices, based on this principle,have been developed by International Rectifier and Infineon in the name ofCoolMOSTM.[edit] Wide band-gap semiconductorsThe major breakthrough in power semiconductor devices is expected from the replacement of silicon by a wide band-gapsemiconductor. At the moment, silicon carbide (SiC) is considered to be the most promising. SiC Schottky diodes with a breakdown voltage of 1200 V arecommercially available, as are 1200 V JFETs. As both are majority carrierdevices, they can operate at high speed. Bipolar devices are being developed for

 higher voltages, up to 20 kV. Among its advantages, silicon carbide can operateat higher temperature (up to 400°C) and has a lower thermal resistance thansilicon, allowing better cooling.[edit] See alsoBipolar junction transistorBootstrappingFGMOSPower electronicsPower MOSFETDimmerGate turn-off thyristorIGBTIntegrated gate-commutated thyristor

ThyristorTriacVoltage regulator

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[edit] Notes and references[edit] Notes^ Jacques Arnould, Pierre MerleDispositifs de l'électronique de puissance, Ãditions Hermès, ISBN2-86601-306-9 (in French)

[edit] ReferencesBaliga, B. Jayant. Power Semiconductor Devices. Boston: PWSpublishing Company. ISBN 0-534-94098-6.Jain, Alok. Power Electronics and Its Applications. Mumbai: PenramInternational Publishing. ISBN 81-879-7222-X.

Semikron: Application Manual IGBT and MOSFET Power Modules, 1. Edition, ISLEPress, 1998, ISBN 3-932633-24-5 PDF-Version

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