Bipolar beginnings

The bipolar transistor was the first important commercial device of the 1950s. Figure A-1.1.1 is a schematic of an n-p-n bipolar transistor. It consists of two p-n junctions back to back. The two n-type regions, called the collector and the emitter, are separated by a very thin p-type semiconductor layer, referred to as the base.

This structure is the successor to the original point-contact transistor invented at Bell Labs. The p-n junctions make it far more robust and practical than that first pioneering device.

We can get an appreciation of the bipolar transistor's principles of operation by applying a negative voltage to the n-type top side (emitter region) and a positive voltage to the n-type bottom side of the structure (collector region), where we see a resistor in series with the battery.

Because of the negative (forward) bias on the emitter-to-base p-n junction, electrons are injected into the base region. Since the base is very thin (a fraction of a micron), the injected electrons traverse it with little loss. They are collected by the reverse-biased base-to-collector p-n junction.

The result is that a large current flows through the collector circuit with the large resistor. On the other hand, the current through the other circuit (the emitter-to-base circuit) is very small, because few electrons flow through that circuit. In effect we have built an amplifier, in which a small current in the emitter-to-base circuit is translated into a large current in the collector-to-base circuit.