Reverse modeling of E/D logic submicrometer MODFETs and prediction of maximum extrinsic MODFET current gain cutoff frequency

A method for estimating the source resistance, fringe capacitance, gate length, and effective saturation velocity from the microwave Y -parameters of MODFETs with known vertical structure is discussed. The scheme is applied to a variety of MODFETs fabricated on molecular-beam-epitaxial (MBE) material using a submicrometer enhancement/depletion- (E/D-) mode IC process. More than 100 MODFETs were measured and analyzed. Both the values and variances of the extracted parameters are very physical. In particular, it is found that the extracted saturation velocity (1) is independent of the gate length in the regime studied (0.25-0.91 μm); (2) is rather independent of process and threshold voltage variations; (3) is marginally higher when the Al mole fraction is increased from 20% to 28%; (4) is not significantly higher in pseudomorphic InGaAs than in GaAs; and (5) is quite a bit higher than is often assumed or extracted, with a value close to the stationary peak velocity in undoped GaAs. There is little sign of overshoot above this limit. Using the extracted peak velocity and a simple analytical MODFET model, the extrinsic current gain cutoff frequency (f_Tx) is predicted well in the gate-length regime studied