GaN films were grown by metal-organic chemical vapor deposition (MOCVD) under various growth conditions. The influences of MOCVD growth parameters, i.e., growth pressure, ammonia (NH3) flux, growth temperature, trimethyl-gallium flux and H-2 flux, on residual carbon concentration ([C]) were systematically investigated. Secondary ion mass spectroscopy measurements show that [C] can be effectively modulated by growth conditions. Especially, it can increase by reducing growth pressure up to two orders of magnitude. High-resistance (HR) GaN epilayer with a resistivity over 1.0 x 10(9) Omega center dot cm is achieved by reducing growth pressure. The mechanism of the formation of HR GaN epilayer is discussed. An AlxGa1-xN/GaN high electron mobility transistor structure with a HR GaN buffer layer and an additional low-carbon GaN channel layer is presented, exhibiting a high two dimensional electron gas mobility of 1815 cm(2)/Vs. (C) 2014 Elsevier B.V. All rights reserved.