In this paper we investigate the performance of GaN high electron mobility transistors (HEMTs) on a Si substrate in comparison with those on a sapphire substrate. The GaN-on-Si devices show much better static DC performance than the GaN-on-sapphire devices thanks to the better thermal conductivity of Si. However, their RF performance is limited by parasitic loading effects induced by the conductive substrate. To analyze the influence of the parasitic loading effects on device power performance in detail, a large-signal model is constructed for the GaN-on-Si devices. From the large-signal simulations, it is found that the conductive substrate not only limits the output power but also severely degrades the operating power gain and power added efficiency (PAE). (C) 2008 Elsevier Ltd. All rights reserved.