Power BJT

Introduction:
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.
Note:
(1) This page discuss about the Power BJT.
(2) Signal level Transistor configurations, operation, characteristics are not the scope of this page.

Symbol: The symbol of the Power BJT is same as signal level transistor.
Structure:
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 characteristics of the device is determined by the doping level in each of the layers and the thickness of the layers.
  • The thickness of the dirft region determines the breakdown voltage of the Power transistor.

VI Characteristics:
Power Transistor VI Characteristics

  • 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.

Read More:
Comparison of MOSFET with BJT
Base Drive Circuit of Power BJT
Darlington Pair Circuit
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