The effectiveness of Ammonia Molecular Beam Epitaxy (MBE) grown carbon-doped GaN buffer layer as an electrical isolation template was investigated. AlGaN/GaN field effect transistor structures with a product of sheet electron density and mobility (n(s)mu), linearly increasing from 1.5 x 10(16) V-1 s(-1) to 2 x 10(16) V-1 s(-1) with n(s), were grown on 2-mu m-thick carbon-doped GaN buffer layer over sapphire substrates. The measurement of the gate-to-source voltage (V-GS) dependent drain current (I-D) demonstrated excellent dc pinch-off characteristics as revealed by an on-to-off ratio of 10(7) for a drain-source voltage (V-DS) up to 15 V. The gate leakage current was less than 1 mu A/mm at the subthreshold voltage (V-th = -5.2 V). Inter-devices isolation current (I-ISO) measurements demonstrated I-ISO values in the low pico-amperes ranges indicating a complete suppression of the parallel conduction paths. Small-signal rf measurements demonstrated a f(max)/f(t) ratio as high as 2.9 attesting the absence of charge coupling effects. (c) 2005 Elsevier Ltd. All rights reserved.