12.8 : 双极结型晶体管（BJT）的开关

Consider a switching circuit that involves a sudden change in the emitter-base voltage.

Initially, both the base-emitter junction and the collector-emitter junction are reversed-biased.

Here, the current flowing through the base is zero, and the current through the collector-emitter junction is due to minority carriers. This is an off-state of the transistor.

When the base emitter junction becomes forward biased, the base current increases, increasing the collector current. Here, the transistor turns on. So, the transistor operates like an ideal switch.

The transient behavior of the collector current depends on the variation of the total excess minority carrier charge stored in the base.

When this charge is less than Q_S, the base charge at zero collector-base voltage, the transistor operates in active mode.

During the turn-on phase, if the base charge exceeds Q_S, the device enters saturation mode.

During the turn-off phase, the collector current remains nearly constant until the stored charge reduces to Q_S.

Once the stored charge equals Q_S, the device returns to active mode. Afterward, the collector current gradually decreases toward zero.