Unbiased Bipolar Junction Transistor (BJT)

UNBIASED BIPOLAR JUNCTION TRANSISTOR

A transistor three terminals (emitter, base and collector) left open, is called an unbiased transistor (or) an open-circuited transistor. Under these conditions, the diffusion of free electrons across the junction produces two depletion layers as shown in fig.3.35(a). The barrier potential, for each of these layers at 25°C is approximately 0.7 volts for silicon transistor and 0.3 volts for germanium transistor. Since the three regions have different doping levels, therefore the depletion layers do not have the same width.

It has been observed that an emitter-base depletion layer penetrates slightly into the emitter as it is a heavily doped region, whereas it penetrates deeply into the base as it's a lightly doped region. Similarly, the collector-base depletion layer penetrates more into the base region and less into the collector region. Both these depletion layers are as shown in fig.3.35 (b). It may be noted that emitter-base depletion layer width is smaller than that of collector base depletion layer.