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

This paper addresses scaling issues in AIGaN/GaN heterojunction field-effect transistors (HFETs) using ensemble Monte Carlo techniques. For gate lengths below 0.25 μm, 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 AIGaN/GaN are compared with In_0.52Al_0.48-In_0.53Ga_0.47As-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 f_T of ∼ 220 GHz compared to ∼ 500 GHz for InGaAs devices.