Small-signal model and high-frequency performance of the BICFET

The small-signal model for the bipolar inversion channel field-effect transistor (BICFET) is presented. Expressions are obtained for all the common-emitter small-signal parameters in terms of the physical parameters of the device. The unity current gain bandwidth product frequency fTis derived and calculated for the device and found to be 10 000 GHz. However, frequency limitations imposed by transit time effects are shown to be much lower than this, of the order 530 GHz. The unity power transfer ratio frequency ωPT, which is considered to be a much more practical figure of merit than fT, is derived for the BICFET. Unlike fT, which is simply the ratio of the transconductance and base diffusion capacitance this frequency incorporates all the small-signal parameters including internodal capacitance and source resistance. Assuming internodal capacitance of the same order as the input capacitance and using the source resistance of 1000 Ω, fPTis found to be approximately 400 GHz. It is also shown that due to the high current gain, current crowding and, hence, the channel resistance in the BICFET is insignificant for any practical operating conditions, and for current densities up to 10⁶A/ cm².