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

It is well known that C_bc 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 C_bc. At a large-signal level, the load line passes through some regions with constant C_bc 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 C_bc during the whole RF cycle, and the IM3, which is generated by g_m 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