12.7 : 双极结型晶体管（BJT）的截止频率

The common base configuration's current gain is constant at low frequencies of the input signal but attenuates after a specific frequency known as the cut-off frequency.

The cut-off frequency for alpha is the frequency at which the current gain is 0.707 times the low-frequency current gain.

Similarly, the beta cut-off frequency is defined for the common emitter configuration. This cut-off frequency is much smaller than the alpha cut-off frequency.

The frequency at which the magnitude of the common emitter gain becomes unity is known as the transition cut-off frequency. The transition cut-off frequency is slightly lower than the alpha cut-off frequency.

The transition frequency can also be expressed in terms of the total time a carrier takes to transit from the emitter to the collector. This includes the emitter delay time, base transit time, and collector transit time.

The most critical delay time is the base transit time, the duration required for minority carriers to cross the base.

To reduce this transit time, high-frequency transistors are designed with a narrow base width.