The contact resistance of Ti/Al/Pt/Au metallization on AlGaN/GaN high electron mobility transistors was measured as a function of plasma exposure conditions prior to metal deposition. Inductively coupled plasma N-2 discharges were used to create nitrogen-deficient near-surface regions in the AlGaN/GaN structures. At modest ion fluxes (similar to 4 x 10(16) cm(2) s(-1)) and low ion energies (125 eV), there was no detectable surface roughening of the AlGaN. Under optimized conditions, the plasma treatment reduces the ohmic contact resistance by a factor of approximately 3.