Power Semiconductors: The BJT, MOSFET, and IGBT (2)

II. THE BIPOLAR JUNCTION TRANSISTOR (BJT)

A. Physical Characteristics
One of the first types of power semiconductors, the BJT is a three layered semiconductor consisting of a sandwich of p-n-p or n-p-n materials. In addition, it has three terminals: the emitter, the collector, and the base. The base is lightly doped, whereas the emitter is heavily doped and wider.

The emitter-base region is forward biased so that majority carriers will flow across the junction. On the other hand, the collectorbase region is reverse biased, which results in a small minority carrier flow.

B. Operational Advantages and Disadvantages
When used in a common emitter mode, as it is most often, the BJT acts as a current-controlled switch. The base current is in the input and the collector current is the output. Because it is current-controlled, it has a fairly low saturation voltage, which is desirable. In addition, BJTs are able to handle high voltages and currents with few problems.
Of course, there are many drawbacks. The BJT has low gain at high frequencies, so it is not useful for amplification under those conditions. Additionally, it does not have a very high surge rating—the peak current is only about twice the maximum continuous current rating. Unlike MOSFETs, BJTs also have a relatively slow switching speed because it takes time to charge the emitter and collector depletion capacitances, which consequently slows the turn-on time.
There are also two breakdown areas associated with the BJT that reduce its safe operating area. The first is the avalanche breakdown, which causes a rapid rise in current, and a second breakdown can be brought on by inductive loads, which can overheat and destroy the transistor.

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