Power Bipolar Transistors
Power bipolar transistors are semiconductors in which a base n-type or p–type layer is sandwiched between emitter and collector layers of the opposite type. There are two polarities available: PNP and NPN. PNP devices consist of an n-type layer sandwiched between two p-type layers. NPN devices consist of a p-type layer sandwiched between two n-type layers. With both arrangements, the junctions between semiconductor sections amplify weak incoming signals. In addition, the thick and low-doped collector region results in a large blocking voltage. Typically, power bipolar transistors are operated at lower current densities to improve the power dissipation per unit of area. Larger devices are used with larger currents. Silicon is the most commonly used material because of its high thermal conductivity and relatively low cost. Silicon carbide offers performance advantages, but is a more expensive material.
Performance specifications for power bipolar transistors include collector-to-emitter breakdown voltage, collector-to-base breakdown voltage, maximum collector current, and current gain bandwidth. Static forward current transfer ratio, which is also known as common-emitter current gain, is the ratio of the input DC current and the output DC current. Power dissipation, the total power consumption of the device, is usually measured in watts (W) or milliwatts (mW).