Bias dependent and scalable small-signal modeling of MESFETs

Results of small-signal modeling of 0.7 um gate length ion-implanted MESFETs are presented here. Modeling included scalability with respect to number of gate fingers and gate bias dependence of equivalent circuit parameters (E.C.Ps). GaAs MESFETs with unit gate width of 150 mum and varying number of gate fingers (from 4 to 8) keeping all other structural parameters constant were fabricated for this experiment. To find small-signal E.C.Ps we used method proposed by Dambrine et.al. On-wafer measurement of S-parameters for all devices was done from 100 MHz to 26.5 GHz under different bias-conditions. Using this data, all the E.C.Ps were then extracted for each device, at various gate-biases and V_ds = 5 V. Scaling of all parameters was done with respect to number of gate fingers and finally bias-dependence of intrinsic parameters was studied. Finally we reached to a model that can give the E.C.Ps of any device we fabricated, given the gate-bias and number of gate-fingers. This equivalent circuit can be used to generate S-parameters of devices with good accuracy in the whole frequency range of measurement.