Bipolar Junction Transistor (BJT) is a three terminal, three layer, two junction semiconductor device. Emitter(E), Base(B) and Collector(C) are the three terminals of the device.
Symbol: The symbol of the Power BJT is same as signal level transistor.
Figure 5.17 Symbol of power BJT
The construction of the Power Transistor is different from the signal transistor as shown in the following figure. The n- layer is added in the power BJT which is known as drift region.
· A Power BJT has a four layer structure of alternating P and N type doping as shown in above npn transistor.
· It has three terminals labeled as Collector, Base, Emitter.
· In most of Power Electronic applications, the Power Transistor works in Common Emitter configuration.
· ie, Base is the input terminal, the Collector is the output terminal and the Emitter is common between input and output.
· In power switches npn transistors are most widely used than pnp transistors.
· The thickness of the dirft region determines the breakdown voltage of the Power transistor.
Figure 5.18 Structure of Power BJT
· The characteristics of the device is determined by the doping level in each of the layers and the thickness of the layers.
Figure 5.18 VI Characteristics Power BJT
· The VI characteristics of the Power BJT is different from signal level transistor.
· The major differences are Quasi saturation region & secondary breakdown region.
· The Quasi saturation region is available only in Power transistor characteristic not in signal transistors. It is because of the lightly doped collector drift region present in Power BJT.
· The primary breakdown is similar to the signal transistor's avalanche breakdown
· Operation of device at primary and secondary breakdown regions should be avoided as it will lead to the catastrophic failure of the device.