Pseudomorphic HEMT small signal equivalent circuit model scaling

In MMIC design the active devices may be represented either by measured S-parameter blocks or by an equivalent circuit model. The passive elements and transmission line interconnections may be similarly treated but in this paper only pseudomorphic InGaAs/AlGaAs HEMTs (pmHEMTs) are considered. In the first approach the S-parameters of a representative pmHEMT are measured and then defined as an S-parameter block in a circuit simulator. Bias dependent S-parameters can be used to examine the predicted response of the circuit as a function of the applied bias. Similarly S-parameters may be measured from a range devices over a wafer or lot or from lot to lot to deduce the performance spread or predicted yield of the circuit. In the second approach a small signal equivalent circuit model is fitted to the measured S-parameters. A bias dependent model can be developed from S-parameters measured at multiple bias points. In an extension to this approach physical modelling of pmHEMTs either by a full time dependent 2-D numerical simulation or by an approximate analytic technique allows the equivalent circuit to be generated from a description of the physical cross section of the HEMT gate length, recess depth and doping etc. The MMIC and HEMT may be designed together so that the active device can he optimised for a particular circuit function or level of yield. This methodology is expected to become more important at mm-wave frequencies where performance margins are too tight to allow a fixed FET process to he used. In this paper we examine the commonly accepted small signal HEMT equivalent circuit model for validity when applied to pmHEMTs with gate lengths from 100 to 300 nm and gate widths of 50 to 200 μm. We have assessed the quality of fit to the measured S-parameters, judged whether the derived circuit is physically realistic and scalable and examined the agreement of f_T and f_MAX between the measurement and the fitted circuit. The equivalent circuit that has been developed is intended for application in MMICs at 60 and 94 GHz and should provide a suitable database against which to judge a physical HEMT model