The Upper Limit of the Cutoff Frequency in Ultrashort Gate-Length InGaAs/InAlAs HEMTs: A New Definition of Effective Gate Length

Ultrashort gate-length pseudomorphic high-electron mobility transistors have been modeled using a full-band cellular Monte Carlo simulator. The RF response and the cutoff frequency f_T have been obtained for physical gate lengths ranging from 10 to 50 nm. These results, in turn, have been used in a transit-time analysis to determine the effective gate length in each case. By interpolation, one can make an estimate of the absolute upper limit for f_T, which we find to be 2.9 THz in the device studied. Importantly, the effective gate lengths are considerably shorter than the depletion lengths. Thus, in general, any estimate of f_T based on the latter quantity is likely too small by a quite significant amount.