High frequency substrate losses for RF MOSFETs are analyzed using numerical device simulation. The results show that losses in devices made on low resistivity substrate occur through the substrate while losses in devices made on high resistivity substrate in the high frequency region occur along the surface through the device (source-drain). An equivalent circuit model is developed which accurately describes the off-state losses. Based on the model significant improvements in terms of output resistance are demonstrated, using an improved device on high resistivity substrate. (C) 2003 Elsevier Ltd. All rights reserved.