Modelling of small-signal response and electronic noise in semiconductor high-field transport

We present a survey on the theoretical modelling of the small-signal response and noise associated with velocity fluctuations in semiconductor high-field transport. Because of the high values of the applied electric field, current-voltage characteristics and electrical noise are found to deviate strongly from Ohm's law and Nyquist's relation respectively. Accordingly, in the case of homogeneous (bulk) structures the field and frequency dependence of the differential mobility, diffusivity and electronic noise temperature are investigated within a rigorous microscopic approach which solves exactly the appropriate kinetic equations through analytical and Monte Carte techniques. Spectral functions in the frequency domain are obtained from their correspondent response and correlation functions in the time domain. The subject is also analysed within a balance-equation approach which enables us to obtain simple analytical expressions which can provide a direct microscopic interpretation and can be applied to device modelling. For validation purposes calculations are applied to the relevant case of holes in Si and electrons in GaAs. In the latter material the presence of negative differential conductivity (Gunn effect) leads to interesting behaviour of the small-signal response and noise spectra which are also investigated for the simplest prototype of non-homogeneous structures, that is the n(+)nn(+) diode. The comparison between the different approaches so developed and between calculations and experiments is found to be quite good, thus providing a quantitative microscopic interpretation of the main features associated with small-signal response and fluctuations in semiconductors under high-field conditions.