Power BJT (Power Bipolar Junction Transistor)

Basic Structure:

The need for a large blocking voltage in the off state and a high current carrying capability in the on state means that a power BJT must have substantially different structure than its small signal equivalent. The modified structure leads to significant differences in the V-I characteristics.

If we recall the structure of conventional transistor we see a thin p-layer is sandwiched between two n-layers or vice versa to form a three terminal device with the terminals named as Emitter, Base and Collector. The structure of a power transistor is as shown below. 

Figure 1. Basic Structure of NPN Power BJT

A power transistor is a vertically oriented four layer structure of alternating p-type and n-type. The vertical structure is preferred because it maximizes the cross sectional area and through which the current in the device is flowing. This also minimizes on-state resistance and thus power dissipation in the transistor.

The doping of emitter layer and collector layer is quite large typically 10¹⁹ cm^-3. A special layer called the collector drift region (n^-) has a light doping level of 10¹⁴ cm^-3.

The thickness of the drift region determines the breakdown voltage of the transistor. The base

thickness is made as small as possible in order to have good amplification capabilities, however if the base thickness is small the breakdown voltage capability of the transistor is compromised.