The effects of Cbc on the linearity of AlGaAs/GaAs power HBTs

It is well known that Cb, is the dominant nonlinear element in heterojunction bipolar transistors (HBTs), To study its behavior, an analytical nonlinear HBT equivalent-circuit model has been developed. The present model includes the effect of the ionized donor charge in the depleted collector region compensated by the injected mobile charge. The model-based simulation shows that, at a small-signal range, the third-order intermodulation (IM3) of the normal HBT has the normal 3:1 gain slope generated by the nonlinearity of Cbc At a large-signal level, the load line passes through some regions with constant Cb, because its collector is fully depleted by the injected free carriers, and the growth rate of the IM3 is decreased. The punch-through collector HBT has constant Cb, during the whole RF cycle, and the IM3, which is generated by g, nonlinearity, has the normal 3:1 gain slope for the all input signal level. Therefore, the IM3 level is significantly lower for the punch-through HBT at a low-power level, but the IM3s of both devices are comparable at a high-power level. The experiment supports our proposed model.