Modeling MESFETs for intermodulation analysis of mixers and amplifiers

The problem of modeling GaAs MESFETs for calculations of intermodulation and spurious responses is examined. It is shown that an adequate model must express not only the absolute I/V characteristics of the device, but also the derivatives of those characteristics. It is demonstrated that these derivatives are dominant in determining intermodulation levels, and that the common approaches to modeling MESFETs do not model those derivatives very well. Finally, a new model for the MESFET gate I/V characteristic (the dominant nonlinearity in most FETs) that is accurate through at least the third derivative is proposed.>