  The intermediate transport regime in nanoscale transistors between the fully
ballistic case and the quasi equilibrium case described by the drift-diffusion
model is still an open modeling issue. Analytical approaches to the problem
have been proposed, based on the introduction of a backscattering coefficient,
or numerical approaches consisting in the MonteCarlo solution of the Boltzmann
transport equation or in the introduction of dissipation in quantum transport
descriptions. In this paper we propose a very simple analytical model to
seamlessly cover the whole range of transport regimes in generic quasi-one
dimensional field-effect transistors, and apply it to silicon nanowire
transistors. The model is based on describing a generic transistor as a chain
of ballistic nanowire transistors in series, or as the series of a ballistic
transistor and a drift-diffusion transistor operating in the triode region. As
an additional result, we find a relation between the mobility and the mean free
path, that has deep consequences on the understanding of transport in nanoscale
devices.
