The causes of GaN HEMT bell-shaped transconductance degradation

Modern communication systems require high linearity, usually in addition to high output power. High linearity requires a flat device transconductance (gm) vs. gate-source voltage (Vgs), while at the same time, transconductance must be high for high gain. In spite of much research to investigate device/system linearity, GaN high electron mobility transistor (HEMT) devices generally show a bell-shaped gm curve characteristic much like other FET devices. In this study, we examine gm behavior and conclude that the bell-shaped gm is caused by: (1) devices operating in the linear region, generally under low to moderate Vds bias, (2) nonlinear source and drain resistances, and (3) self-heating thermal effects. When Ids no longer follows a linear increase with Vgs due to these three causes, gm decreases and forms a bell shape. The key to maintain the gm flatness is to reduce both parasitic electrical and thermal resistances.