Quantum Transport in the Channel of a Field Effect Transistor

Abstract

The channel region of a GaAs FET is a complex structure and in conventional device modelling a numerical solution of the two-dimensional Poisson equation together with the transport equations is required to provide a satisfactory picture of the potential distribution within the device. It is at present outside the scope of quantum transport calculations to handle an arbitrary two-dimensional potential and so it is necessary to look for a simpler structure which retains important aspects of the FET channel, yet represents a tractable problem which can be tackled within a quantum transport framework. The approach adopted here is to suggest that in short gate structures some features it is important to describe are: high field effects such as ballistic transport and velocity overshoot, and the quantum mechanical confinement arising from the potential well formed transverse to the electron motion. In a depletion mode GaAs MESFET the potential well is formed by the band bending between the gate and the substrate, and it has been demonstrated by Poole et al1 that at very low temperatures a two-dimensional sub-band structure can be observed. Even at room temperature close to pinch-off the sub-band separation should be sufficiently large as to restrict scattering. This of course depends both on the shape of the potential well and the temperature.