Non-Linearity of Transconductance and Source-Gate Resistance of HEMTs

Experimental results from GaN-based high electron mobility transistors (HEMTs) show a pronounced decrease of the transconductance gm at higher gate bias, due to increasing source-gate and gate-drain resistances. In this work we show through simulations that the electric field distribution and the resulting carrier velocity quasi-saturation are the main source for the transconductance collapse, consequently also for the resistance rise. A shorter source-gate distance leads to a higher gm peak value, but a more abrupt collapse at high gate bias. These effects are further discussed with respect to device linearity.