Will the velocity of GaN HEMTs ever overshoot? [mm-wave frequencies]

Summary form only given. Traditionally, the electron velocity of III-V FETs overshoots as the gate length reduces to sub-micron scale, which overcomes the higher percentage of parasitic charging times as the intrinsic input capacitance reduces. This overshoot is not yet seen in GaN HEMTs. What is worse, the f/sub t/ of these devices peaks at only 25-30% of the full-channel current and deteriorates severely at higher current levels. These problems are responsible for the measly 2-5 W/mm and 15-40% efficiency produced by GaN HEMTs at >30 GHz. This paper discusses the issues limiting the speed of GaN HEMTs at mm-wave frequencies, focusing on electron velocity overshoot effects.