Limitations of Characterizing mm-Wave MODFETs with Microwave Probes

The accuracy of microwave probing is not adequate for the characterization of MODFETs used in the most demanding mm-wave circuit applications. Accuracy of 5 % for most device model circuit elements is desireable. This requirement demands accuracies of 1 fF, 1 pH, 0.08 Ω and 0.1 pS. Examples are given demonstrating why this accuracy is desired and the calibration errors limiting this accuracy. The pads normally used to allow contact of the probes to the devices introduce parasitic capacitances that are significant (e.g., Cgd = 1.8 fF) for small mm-wave devices. The standard calibration technique does not account for the pads. It is shown that the pad capacitances can not be simply subtracted because of 3 body capacitance effects. The pads also move the reference planes back from the intrinsic device; this effect appears as inductances that have a significant effect on the model for larger MODFETs. For example, an error of 10 pH in the gate inductance of a 350 μm wide MODFET reduces the modelled gain of a narrow-band 42 GHz amp by 3 dB. Measurements of the standard Load-Open-Short-Thru calibration reflection standards using a Load-Reflect-Thru calibration suggest that the open capacitance accuracy is worse than 3 fF. Modelling showed that an error 0.1 pS in the transmission reflection planes produces an Rgs error of 10 % for active MODFETs and 50 % for passive MODFET. Experiments suggest the error is worse than 0.1 pS.