12.4 : 双极结型晶体管（BJT） 的特性

Consider the common-emitter configuration of the BJT.

The voltage and current dependencies are measured experimentally to understand the current-voltage characteristics.

For the input characteristics, the base-emitter voltage is varied while keeping the collector-emitter voltage constant.

The resulting graph shows the Shockley-type dependence of the collector current on the base-emitter voltage.

The output characteristics represent the collector current as a function of the collector-emitter voltage, keeping the base current constant.

The saturation region corresponds to a small collector-emitter voltage where the collector current drops towards zero. Here, the transistor's resistance between the collector and emitter terminals is small.

When the collector-emitter voltage increases, the transistor enters the active region. Here, the transistor's collector-emitter resistance becomes infinitely high while the collector current remains independent of the collector-emitter voltage.

However, in practice, a slight increase in the collector current is noticed, which is due to the widening of the depletion region and shortening of the base length.

This is known as the Early effect, leading to a finite output resistance of the transistor.