Transport and noise in GaAs/AlGaAs heterojunction bipolar transistors. II. Noise and gain at low frequencies

For pt.I see ibid., vol.36, no.6, p.1015-19 (June 1989). Low-frequency noise measured in high-current-gain GaAs/AlGaAs double-heterojunction transistors is shown to originate from noise processes in the base. High base resistance associated with high current gain causes Johnson noise to be dominant at high frequencies and low bias, while at low frequencies interface 1/f and generation-recombination noise exceed Johnson noise over a bandwidth that increases with base current. At high forward bias, this 1/f noise saturates, but by then can extend over megahertz bandwidths. A low-frequency decrease in devices gain and an excess base voltage noise in this saturation region is explained by punchthrough and the mechanism for high gain in these devices