Modern communication systems require high linearity, usually in addition to high output power. High linearity requires a flat device transconductance (g(m)) vs. gate-source voltage (V-gs), 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 g(m) curve characteristic much like other FET devices. In this study, we examine g(m) behavior and conclude that the bell-shaped g(m) is caused by: (1) devices operating in the linear region, generally under low to moderate V-ds bias, (2) nonlinear source and drain resistances, and (3) self-heating thermal effects. When I-ds no longer follows a linear increase with V-gs due to these three causes, g(m) decreases and forms a bell shape. The key to maintain the g(m) flatness is to reduce both parasitic electrical and thermal resistances. (C) 2016 Elsevier Ltd. All rights reserved.