Comparison of low-frequency noise in III-V and Si/SiGe HBTs

The low-frequency noise characteristics of double self-aligned InP/InGaAs and two types of Si/SiGe heterojunction bipolar transistors (HBTs) were investigated. Spectral analysis shows no striking differences; the spectra are composed of a 1/f component and the white noise is always reached at low biases. A general trend for all the transistors was the presence of Lorentzian component(s) for the smallest devices. The voltage coherence function was always unity for SiGe transistors; and for the first time, it was found to be close to zero for InP devices. Concerning the 1/f noise level, both types of transistors have approximately a quadratic dependence on base current bias and an inverse dependence on the emitter area. Thus, a comparison of the 1/f noise level has been made using the K_b parameter, and values around 10^-9 μm² for SiGe HBTs and around 10^-8 μm² for InP HBTs were found. These results are of the same order of magnitude as the best published ones. The low-frequency noise results suggest that excess noise sources are mainly located at the intrinsic emitter-base junction for the two types of SiGe devices, and, for the InP HBTs, a correlated noise source is located at the emitter periphery. To compare different devices and technologies, f_c/f_T was studied as a function of collector current density and for some HBT technologies f_c/f_T∝J_c (f_c is corner frequency at which the white noise and 1/f noise are equal and f_T is the unity current gain frequency). The effects of different processing conditions, designs and temperature were also investigated and discussed.