Focus on BASICS: How IGBTs work

Isolated gate bipolar transistors (IGBTs) lie at the heart of today’s high-power motor drives.

12/04/2008


Recent technology advances in power electronics have arisen primarily from improvements in semiconductor power devices, with insulated gate bipolar transistors (IGBTs) leading the market today for

low-voltage

variable-speed drive (VSD) applications. IGBTs feature many desirable properties including a metal-oxide-semiconductor (MOS) input gate, high switching speed, low conduction voltage drop, high current carrying capability, and a high degree of robustness. Devices have drawn closer to the "ideal switch," with typical voltage ratings of 600-1,700 volts, on-state voltage of 1.7-2.0 volts at currents of up to 1,000 amperes, and switching speeds of 200-500 ns. The availability of IGBTs has lowered the cost of systems and enhanced the number of economically viable applications
First-generation IGBTs of the 1980s and early 1990s were relatively slow in switching, and prone to failure through such modes as latchup and secondary breakdown. Current fifth-generation IGBTs feature speed rivaling MOSFETs, and excellent ruggedness and tolerance of overloads. The IGBT is used in medium-to-high-power applications such as switched-mode power supply, traction motor control and induction heating, as well as being an enabling technology for variable speed motor drives. The extremely high pulse ratings of fifth-generation devices also make them useful for generating large power pulses in areas like particle and plasma physics, where they are starting to supersede older devices like thyratrons and triggered spark gaps. Large IGBT modules typically consist of many devices in parallel and can have very high current handling capabilities in the order of hundreds of amps with blocking voltages of 6,000 V.
The IGBT combines the positive attributes of bipolar junction transistors (BJTs) and MOSFETs, showing the simple gate-drive characteristics of the MOSFETs, and the high-current and low–saturation-voltage capability of bipolar transistors. BJTs have lower conduction losses in the on-state, especially in devices with larger blocking voltages, but have longer switching times, especially at turn-off, while MOSFETs can be turned on and off much faster. MOSFET on-state conduction losses are larger, however, especially in devices rated for higher blocking voltages. Hence, IGBTs have lower on-state voltage drop with high blocking voltage capabilities in addition to fast switching speeds.
Device structure
IGBTs have a vertical structure as shown in Figure 1. This structure is quite similar to that of the vertical diffused MOSFET except the N+ drain is replaced with a P+ collector layer that forms the IGBT drain. This layer forms a PN junction that injects minority carriers into the drain drift region. This additional P+ region creates a cascade connection of a PNP bipolar junction transistor with the surface N-channel MOSFET, resulting in a significantly lower forward voltage drop compared to a conventional MOSFET in higher blocking voltage rated devices.

IGBT device structure
Figure 1: IGBTs’ structure is similar to a vertical diffused MOSFETwith the N+ drain replaced by a P+ collector.


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. 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:
The additional PN junction blocks reverse current flow. This means that IGBTs cannot conduct in the reverse direction, unlike a MOSFET. 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. 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.
A parasitic thyristor that could latch up in IGBTs if it is turned on. The N+ buffer layer between the N+ drain contact and the N+ drift layer, with proper doping density and thickness, can significantly improve the operation of the IGBT in two important respects: it lowers the on-state voltage drop, and shortens the turn-off time. On the other hand, the presence of this layer greatly reduces the IGBT’s reverse-blocking capability.
IGBT Characteristics
The IGBT is a three-terminal power semiconductor device, noted for high efficiency and fast switching. One of the main important performance features of any semiconductor switching device is its switching characteristics (mainly turn-on and turn-off switching transients, and the device’s safe operating area). The reverse bias rating of the N- drift region to collector P+ diode is usually only tens of volts, so, if the external circuit 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. This results in longer switching time and hence higher switching loss compared to a power MOSFET.
IGBTs are voltage-controlled devices and require gate voltage to establish collector-to-emitter conduction. Due to the large input gate-to-emitter capacitance of IGBTs, MOSFET drive techniques can be used. However, the off biasing needs to be stronger. A 15 V positive gate drive is normally recommended to guarantee full saturation and limit short circuit current. A negative voltage bias is used to improve the IGBT immunity to collector-to-emitter dv/dt injected noise and reduce turn-off losses.
The additional PN junction adds a diode-like voltage drop to the device. At lower blocking voltage ratings, this additional drop means that an IGBT would have a higher on-state voltage drop. As the voltage rating of the device increases, the advantage of the reduced N- drift region resistance overcomes the penalty of this diode drop and the overall on-state voltage drop is lower (the crossover is around 400 V blocking rating). Thus IGBTs are rarely used where the blocking voltage requirement is below 600 V.
References:
G. Ledwich, IGBTs Basics , PowerDesigners, LLC , 1998.
" Insulated-gate bipolar transistor ," Wikipedia , Nov. 14, 2008.





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