Velocity overshoot effects and scaling issues in III-V nitrides

Empirical evidence from submicrometer technology in GaAs- and InGaAs-based field-effect transistors (FETs) has led to an expectation that velocities exceeding the steady state values would be observed in III-V nitride devices. However, scaling of devices down to 0.7 and 0.25 μm has so far not yielded any performance enhancement that may suggest an overshoot. In this paper, we examine transport in AlGaN-GaN heterojunction FETs (HFETs) to examine whether velocity overshoot effects occur. Our findings show that very high scattering rates when combined with unusual field profiles, result in a change in the local transport mechanism, and, in the source-gate region, combine to reduce/ify velocity overshoot effects. We also find that the effect of nonequilibrium phonons on transport in the channel is minimal, with the peak nonequilibrium phonon occupation being smaller than the equilibrium phonon occupation.