MOCVD-grown 0.25 mum gate-length AlGaN/GaN high electron mobility transistors (HEMTs) have been fabricated on sapphire substrates. These 0.25 gm gate-length devices exhibited maximum drain current density as high as 1.43 A/ mm, peak extrinsic transconductance of 354 mS/mm, unity current gain cut-off frequency (f(T)) of 61 GHz, and maximum frequency of oscillation (f(max)) of 89 GHz. At 20 GHz, a continuous-wave output power density of 4.65 W/mm with power-added-efficiency of 29.9% was obtained, yielding the highest reported power performance of AlGaN/GaN HEMT grown on sapphire at 20 GHz. (C) 2003 Elsevier Science Ltd. All rights reserved.