Correlation of IP3 in a phemt to the third derivative of Ids wrt Vgs, or Gm3 is addressed. A non-linear large-signal Phemt model was developed based on the fitting RF Gm characteristics as well as IV/CV. Systematic extraction approach is described. In extraction, RF Gm fitting minimizes the errors between modelled and measured Gm3. Scale-ability in terms of DC, S-parameters and IP3 is demonstrated. For large-size devices, other factors, such as RF-Gds characteristics, are also important. The over-all model includes also physics-based selfheating and scaleable measurement-based noise equations that makes it the most comprehensive and most practical LN_FET model in industry.