Drain current collapse in GaN metal–semiconductor field-effect transistors due to surface band-bending effects

Abstract

The small gate length of high-frequency field-effect transistors (FETs) leads to situations where the role played by the parasitic access regions in their circuit properties becomes dominant. Due to surface band-bending effects present in wide bandgap semiconductors such as GaAs or GaN, two parasitic FETs having ungated surfaces as floating gates appear in series with the targeted one. They have to be considered properly under any electrical or optical stimulation applied. From the electrical point of view, the drain current set-up process itself leads to voltage drops along the channel that modify the surface charge of the above floating gates. By this mechanism, some surface states beside the drain terminal are charged, thus building a negatively-charged floating gate that strongly degrades the current transfer ability of the drain contact. These degraded output features are responsible for the drain current collapse observed in GaN metal–semiconductor field-effect transistors. Their recovery from this collapsed state by both electrical and optical excitations is also explained by the model.