Effects of Space-Charge Layer Widening in Junction Transistors

Some effects of the dependence of collector barrier (space-charge layer) thickness on collector voltage are analyzed. Transistor base thickness is shown to decrease as collector voltage is increased, resulting in an increase of the current-gain factor (α) and a decrease in the emitter potential required to maintain any fixed emitter current. These effects are shown to lead to two new elements in the theoretical small-signal equivalent circuit. One, the collector conductance (gc´), is proportional to emitter current and varies inversely with collector voltage. This term is the dominant component of collector conductance in high-quality junction transistors. The other element, the voltage feedback factor (μ
_ec
), is independent of emitter current, but varies inversely with collector voltage. The latter element is shown to modify the elements of the conventional equivalent tee network.