A semidistributed model for the noise and scattering parameters of a HEMT

The authors present details of a new model for millimetric HEMT devices taking into account the distributed effects of the gate and drain lines and the non-uniformity of the structure at the gate feed region. The effects of distribution are considered by dividing the active region into equal slices whose dimensions are small when compared to the minimum guided wavelength. Each slice is modelled as an intrinsic HEMT with thermal noise sources. The slices are connected together through lossy reactances. The first slice differs from the remaining slices to allow for non-uniformity. The model was implemented and optimised using a commercial CAD package. It is shown to provide a good fitting of the manufacturer´s measured S-parameters and noise parameters from 2 to 40 GHz for a super low noise millimetric HEMT chip. A significant improvement in the fitting of the noise parameters is shown. The significance of the distributed effects is shown by comparing the predictions of the optimised model with those of an unsliced model. The effect of the non-uniform structure is shown by a comparison of the parameters of the first slice and those of the remaining slices in the optimised model