Has SiGe lowered the noise in transistors?

It is shown that low-frequency noise (LFN) in SiGe-base HBTs and SiGe-channel pMOSFETs can be made significantly lower than it is in their all-Si counterparts. The noise reduction in the HBTs results principally from the elimination of the emitter/base interfacial oxide, which, in the Si bipolar junction transistors, serves to boost the current gain. A reduction in the LFN in the SiGe channel pMOSFETs is also possible, but requires a careful design of the structure, as the power spectral density (PSD) of drain current fluctuations is often a non-monotonic function of their structure parameters. Optimisation of the LFN requires a simulation of the PSD. This can be done using an analytical model assuming (i) carrier-number fluctuation noise generation by trapping/release of charges at the SiO₂/Si interface, resulting in correlated fluctuations in the retrograded SiGe channel and in the SiO₂/Si interface channel, and (ii) mobility fluctuations in each channel. The quality of the SiO₂/Si interface is crucial for the overall LFN properties of the MOSFETs, as it is responsible for the number fluctuation noise which is predominant in the range of currents commonly used