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.