GaN is a potential sensor material for harsh environments due to its piezoelectric and mechanical properties. In this paper an 8mm diameter sensor structure is proposed based on a GaN / AlGaN / sapphire HEMT wafer. The discs will be glass-bonded to an alumina package, creating a 'drumskin' type sensor that is sensitive to pressure changes. The electromechanical behaviour of the sensor is studied in an attempt to optimise the design of a pressure sensor (HEMT position and sapphire thickness) for operation in the range of 10 - 50 bar (5 MPa) and above 300°C. ©2010 IEEE.