Chapter 9Bipolar Power Switching Devices
9.1 Bipolar Junction Transistors (BJTs)
Bipolar transistors were the first power transistors developed in silicon, but they have now been largely displaced by silicon insulated-gate bipolar transistors (IGBTs) and thyristors. This occurred because silicon bipolar junction transistors (BJTs) suffer from a phenomenon known as “second breakdown” that limits the safe operating area (SOA) of the device. As will be shown below, the higher critical field for avalanche breakdown in SiC essentially eliminates second breakdown, making high-performance power BJTs practical. BJTs are particularly attractive for high-temperature applications, since they are not dependent on a gate oxide for their operation, and are not subject to the oxide reliability limitations of metal-oxide-semiconductor field effect transistors (MOSFETs) and IGBTs.
As is our custom when considering a new device, we begin our discussion with a review of the basics of BJT operation. We then consider a number of special effects that occur during high-current operation of power BJTs. Figure 9.1 shows a basic BJT, along with standard current and voltage definitions. In these devices the emitter is more heavily doped than the base, and the base is more heavily doped than the collector, so . We can identify four internal junction currents, the hole and electron currents crossing the ...
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