Question

Transistor operation may be explained in terms of three different currents: injection, diffusion, and collection. Describe each of these currents and discuss how it influences amplification.

Answer 1

“Injection” current consists of those majority charge carriers (either electrons or holes, depending on the transistor type) that are “injected” from the emitter region into the base region of a transistor.

“Diffusion” current is the current through the base terminal of the transistor resulting from recombination of electrons and holes in the emitter-base junction.

Most of the injection current, however, becomes “collection” current and goes through the collector terminal of the transistor.

The base is doped much lighter than the emitter so that the collection current will greatly exceed the diffusion current. If the two layers were equally doped, the base-emitter junction would tend to act as a normal diode, passing nearly all the injection current through the base terminal. As it is, only a small portion of the injected charge carriers diffuse into the base, the rest being “collected” by the collector layer. The collector is not as heavily doped as the emitter, in order that the collector-base junction will have a greater reverse breakdown voltage. Since this junction typically operates in reverse-bias mode with a substantial voltage drop, the breakdown voltage rating is important.

Amplification is the process of increasing the strength of a SIGNAL. A signal is just a general term used to refer to any particular current, voltage, or power in a circuit. The small current travels from the voltage source into the base of the transistor. A current at the base turns on the transistor. The current is then amplified and travels from the emitter of the transistor to the collector.