High frequency devices aimed for power applications cannot be characterized by on-wafer measurements due to power dissipation constraints. Therefore characterization using test fixture is necessary. Latest techniques applied to onwafer devices use a four-port characterization of the extrinsic network surrounding the device. Nevertheless, this cannot be applied straightforwardly to the test fixture case due to the fact that the ground reference of the device is not the same as the measurement, and this effect increases with frequency. A fiveport definition would lead to an accurate characterization, but with the increase of the complexity of the problem. This paper presents an alternative technique based on four-port deembedding technique with a correction of the local ground effect by using two cold-FET measurements and a simplification of the four-port matrix model. © 2006 EuMA.