Reduction of low-frequency noise in n-p-n AlGaAs/GaAs HBT's

Low-frequency measurements were made (100 Hz to 10 MHz) of n-p-n AlGaAs/GaAs HBTs with and without AlGaAs ledges as a function of bias current, geometry, aluminum mole fraction in the emitter, and temperature. These measurements show the existence of three distinct regions in the noise spectra: a 1/f shape associated with the base surface, a Lorentzian shape associated with the forward-biased emitter-base junction, and a flat region associated with shot noise. The measurements also indicate that the anomalous noise bump is generated by a trap in the AlGaAs near the surface of the emitter-base junction.