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SiGe, GaAs, and InP Heterojunction Bipolar Transistors by Jiann S. Yuan

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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

images

It is clear from (4.1) that the larger dWB/dVCB, the larger the output conductance. For a bipolar transistor with low base doping and narrow base width, WB 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:

images

The common-emitter small-signal voltage gain is proportional to the transconductance. The transconductance depends linearly on the collector current if IC is given by IS exp(q VBE/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 ...

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