Low-frequency noise characterization of Npn AlGaAs/GaAs heterojunction bipolar transistors

We report low-frequency noise measurements of Npn, Al_xGa_1−xAs/GaAs heterojunction bipolar transistors (HBTs) as a function of bias current, device geometry, extrinsic-base-surface condition, aluminum mole fraction, and temperature. The 1/f noise is mainly associated with fluctuations in the base-surface recombination velocity. The noise “bump” (observed at intermediate frequencies) is generated by a trap in the AlGaAs, possibly the DX center, near the surface of the emitter-base junction. It is shown that the use of an AlGaAs surface passivation ledge and the reduction of the Al mole fraction from x = 0.3 to x = 0.2 significantly improves the low-frequency noise performance of our HBTs.