We investigated the C concentration in GaN as a function of the V/III ratio and growth rate for a p-n junction diode structure on a bulk GaN substrate by metalorganic chemical vapor deposition (MOCVD). The C concentration was independent of the growth rate for growth at atmospheric pressure. Moreover, the C concentration in GaN was 3.3 x 10(15) cm(-3) at a V/III ratio of 5000 with a growth rate of 2.3 mu m/h and 4 x 10(15) cm(-3) at a Will ratio of 3700 with a growth rate of 4.7 mu m/h. Both of the major and minor carrier concentrations in the drift layers of a p-n junction structure were optimized at the reasonable growth rate in terms of the short growth time. The C impurity concentration was well controlled at a concentration on the order of 10(15) cm(-3). (C) 2016 Elsevier B.V. All rights reserved.