Model for the cutoff frequency in RF MOSFETs

We present a simple analytical model for the drain current dependent cutoff frequency f(T) in submicron high frequency MOSFET's. The model combines the SPICE MOS3 drain current model and a modified McMacken-Chamberlain model for the current dependent transit time, which allows the description of the I-V characteristics and f(T), os. current with the same set of SPICE parameters. The model is verified by comparing simulated results to measured data for 0.6 mu m RF MOSFET's fabricated at VTT Electronics. The model explains well the observed behaviour. Furthermore, the comparison indicates that when the channel length is reduced to a half-micron range and below, most of the charging delay is caused by the transit time.