SiGe, GaAs, and InP Heterojunction Bipolar Transistors

CHAPTER FOUR

RF and Transient Performance

4.1   GENERAL DEVICE BEHAVIOR

4.1.1   Output Conductance

The output conductance describes the base width modulation effect on the collector current in the small-signal equivalent circuit. The common-emitter output conductance is known by

It is clear from (4.1) that the larger dW_B/dV_CB, the larger the output conductance. For a bipolar transistor with low base doping and narrow base width, W_B is very sensitive to the change in the collector–base voltage. This gives a large output conductance (or a small output resistance) for the bipolar transistor.

4.1.2   Transconductance

The transconductance of the bipolar device is defined as the change of the collector current with respect to the base–emitter voltage:

The common-emitter small-signal voltage gain is proportional to the transconductance. The transconductance depends linearly on the collector current if I_C is given by I_S exp(q V_BE/kT).

4.1.3   Heterojunction Junction Capacitance

Heterojunction capacitances play an important role in determining device switching speed and high-frequency response. Because of heavy doping in the quasi-neutral base and emitter regions, minority-carrier charge storage in the emitter and base is small. The junction capacitance can be much larger than the diffusion ...