An improved SPICE model for high-frequency noise of BJTs and HBTs

An improved compact model for the high-frequency noise of bipolar junction transistors (BJTs) and heterojunction bipolar transistors (HBTs) is presented and implemented in SPICE3. It properly takes into account the frequency function of thermal noise in the input circuit, which is related to the real part of the input admittance. This quantity is the result of an interplay between the base resistance, the internal emitter-base capacitance, the small-signal input conductance of the intrinsic transistor, and the emitter series resistance. This requires the replacement of the noisy base resistance in SPICE by a frequency-dependent expression consisting of the appropriate SPICE parameters. A similar substitution is needed in the output circuit. For a Si/SiGe HBT these improvements lead to excellent agreement between the noise figure and device simulation results.