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Four models have been developed and assessed for fitting the measured noise parameters up to 26GHz and Sparameters up to 4OGHz, for a commercial HEMT chip.The first treats the intrinsic noise sources as uncorrelated, thermal sources. The second is an extension of this, allowing a better fit to be achieved by including the distributed nature of the gate and drain electrodes using a semidistributed, sliced model. The third model neglects the distributed effect but takes into account the partial correlation of the gate and drain noise sources. This causes a larger improvement in the quality of fit, allowing the model t o fit the measured data within reasonable measurement limits. Fourthly, the addition of the distributed effect to the correlated model allows a further marginal improvement, but the conditioning of the problem and the accuracy of the data appear to be insufficient to allow accurate extraction of the additional parameters needed.