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Decananometre-T-shaped-Ni/Pt/Au-gate AlGaN/GaN HEMTs on sapphire substrates are fabricated and their DC characteristics measured. The negative shifts of threshold voltages occur below 5-6 of channel aspect ratio Lg/d, where Lg is the gate length and d is the AlGaN barrier layer thickness. This is a similar trend as observed in AlGaAs/GaAs HEMTs. The negative threshold voltage shifts are enhanced with increasing d, which results from the increase of the two-dimensional electron gas thickness.