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The paper investigates the capabilities of the COBRA FET model  to predict the harmonic and intermodulation distortion (IMD) characteristics of μ-wave and mm-wave PHEMT devces. The ability of the model to account for high order effects is proven by analysing the second and third order derivatives of the drain 1/V model and the gate Q/V model. A substantial and conclusive selection of results of single-tone large-signal tests (with harmonics reaching into the mm-wave region) and two-tone large-signal test (including third and fifth intermodulation products) are presented for the case of a 0.2μm PHEMT process designed for mm-wave applications. The simulations have been carried out with the HP-EEsof SeriesIV simulator, where the model has been recently implemented (in compiled form). These results show a very good overall agreement between simulated and measured harmonic and IM distortion characteristics, from μ-wave up to mm-wave frequencies and across the entire bias region.