Conditions for enhanced shot noise in field-effect transistors

We demonstrate that it is possible to observe enhanced shot noise in field-effect transistors, i.e., a current noise spectral density $S>S_{\mathrm{Poisson}}$, where $S_{\mathrm{Poisson}}=2qI$ is the so-called “shot noise” spectral density associated to a Poissonian process of electrons traversing the channel. Whereas the effects responsible for shot-noise suppression have been broadly investigated, here we unveil the mechanism and the conditions leading to an enhancement of shot noise in field-effect transistors biased in the subthreshold or weak inversion regime, that have particular relevance in the case of short-channel metal-oxide-semiconductor field-effect transistors. The effect is due to the interplay between carrier backscattering in the channel and Coulomb repulsion among carriers. We evaluate quantitatively the effect with a semianalytical model for different types of transistors, and find a characteristic shape of the Fano factor $F=S/S_{\mathrm{Poisson}}$ as a function of gate bias, that enables us to look for the signature of this effect in experiments.