LOW-FREQUENCY NOISE PROPERTIES OF N-P-N ALGAAS GAAS HETEROJUNCTION BIPOLAR-TRANSISTORS

The low-frequency noise characteristics of N-p-n AlxGa1-xAs/GaAs heterojunction bipolar transistors (HBT's) have been investigated as a function of bias current, device geometry, extrinsic-base-surface condition, Al mole fraction in the emitter, and temperature in order to identify the dominant noise mechanisms. These measurements show the existence of three distinct regions in the noise spectra: a 1/f noise line shape, a Lorenztian spectrum (noise "bump"), and a white-noise region. The 1/f noise is attributed to fluctuations in the extrinsic-base surface recombination current. The noise "bump" is generated by an AlGaAs trap in the emitter-base junction. A trap activation energy of about 0.20 eV was determined, and the DX center was identified as a possible candidate for this trap. It is shown that for 4-mu-m x 10-mu-m emitter AlGaAs/GaAs HBT's, the use of a depleted, AlGaAs passivation ledge over the extrinsic-base surface typically reduced the 1/f base noise current by a factor of 10, and the reduction of the Al mole fraction (x) from x = 0.3 to x = 0.2 decreased the magnitude of the noise "bump" by a factor of 3.