Patent ID: 7700973

Claim:
A high electron mobility transistor (HEMT), comprising: a substrate; a semi-insulating buffer layer, comprised of gallium nitride (GaN) or aluminum gallium nitride (AlGaN), deposited on the substrate; an AlGaN barrier layer, with an aluminum (Al) mole fraction larger than that of the semi-insulating buffer layer, deposited on the semi-insulating buffer layer; an n-type doped graded AlGaN layer deposited on the AlGaN barrier layer, wherein an Al mole fraction is decreased from a bottom of the n-type doped graded AlGaN layer to a top of the n-type doped graded AlGaN layer and the n-type doped graded AlGaN layer includes a recess in a gate region of the HEMT; a source contact and a drain contact formed on the n-type doped graded AlGaN layer on opposite sides of the gate region; a gate contact formed in the recess in the gate region of the n-type doped graded AlGaN layer; and a cap layer, comprised of GaN or AlGaN with an Al mole fraction smaller than that of the AlGaN barrier layer, deposited on the n-type doped graded AlGaN layer in an access region between the source contact and the gate contact and between the drain contact and the gate contact; wherein a thickness of the cap layer as compared to a distance between a channel and the gate contact is such that dispersion is less than 10%.