The influence of parasitic capacitance on high frequency performances of SiGe n-HFET is reported. These capacitances arising from the fringe and electrostatic contributions of the gate are extracted from HF measurements and estimated with a 2D hydrodynamic modelling and with a 2D electrostatic modelling of the device. The main electrostatic contribution arises from the strong direct coupling between the mushroom-shaped gate and the 2D electron gas in the quantum well increased by the thin oxide layer above the undoped Si cap layer. The height of the gate foot, the shape of the top of the gate are investigated, showing that if the mushroom shape increase the capacitance by 22% compared to a simple-shaped gate, the increase of its size has less impact than the oxide thickness and the aspect ratio. The extrinsic capacitances can degrade by 27% the intrinsic properties of the transistors reported here. (C) 2007 Elsevier Ltd. All rights reserved.