Transconductance Overshoot, a New Trap-Related Effect in AlGaN/GaN HEMTs

DC characteristics of AlGaN/GaN HEMTs with different thickness values of the undoped GaN channel layer were compared. An abnormal transconductance (${g}_{m}{)}$overshoot accompanied by a negative threshold voltage (${V}_{\text {TH}}{)}$shift was observed during${I}_{\text {DS}}$–${V}_{\text {GS}}$sweep in devices with thinner GaN layer. At the same time, a non-monotonic increase in gate current was observed. In OFF-state, electron trapping occurs in the undoped GaN layer or at the GaN/AlN interface, leading to a positive${V}_{\text {TH}}$shift. When the device is turning on at a sufficiently high${V}_{\text {DS}}$, electron de-trapping occurs due to trap impact-ionization; consequently,${V}_{\text {TH}}$and therefore${I}_{\text {D}}$suddenly recovers, leading to the${g}_{m}$overshoot effect. These effects are attributed to electron trap impact-ionization and consequent modulation of the device’s electric field.