Device scaling physics and channel velocities in AlGaN/GaN HFETs: Velocities and effective gate length

This paper addresses scaling issues in AlGaN/GaN heterojunction field-effect transistors (HFETs) using ensemble Monte Carlo techniques. For gate lengths below 0.25 mu m, f(T) values are known not to scale linearly with the inverse gate length. The authors' simulations show this to be due to an increasing difference between the lithographic gate length and the effective gate length as the devices shrink. The results for AlGaN/GaN are compared with In0.52Al0.48-In0.53Ga0.47As-InP devices, and the authors found that the limiting role of velocity overshoot and depletion region spread causes the GaN HFETs to have a peak fT of similar to 220 GHz compared to similar to 500 GHz for InGaAs devices.