AlGaN/GaN heterostructure field-effect transistors (HFETs) were grown by metallorganic chemical vapor deposition on either sapphire or freestanding GaN substrates. The devices in the latter case show consistently lower knee voltage, higher output resistance, less self-heating (the thermal conductivity of GaN is similar to1.5 W mm(-1) compared to 0.5 W mm(-1) for sapphire), higher transconductance, and a much lower degree of current collapse due to surface states in the gate-drain region than do HFETs grown on sapphire. The reduced current collapse suggests that at least some surface states in conventional heteroepitaxial HFETs on sapphire may be associated with the higher dislocation density in this material. The results are consistent with the improved performance of GaN-based photonic devices when grown on bulk GaN. (C) 2003 The Electrochemical Society.