An investigation of RF short-channel effects in state-of-the-art GaN and InGaAs HEMTs, in relation to effective gate length and aspect ratio, is performed through our full band Cellular Monte Carlo simulator. In particular, the short-circuit current gain cut-off frequency, fT, is extracted using two different methods for several gate lengths. The first method relates fT to the electron transit time in the gate region, and from the electron velocity profile allows a direct estimation of fT, the effective gate length Leff, and the investigation of the nananoscale carrier dynamics in the channel. The second extraction methods derives f T through small-signal analysis. Our results indicates that the increasing difference between effective gate length and metallurgical gate length, as the device is scaled, plays a major role in limiting the RF performance. Moreover, maintaining a minimum aspect ratio of 5 for InGaAs HEMTs, and 10 for GsN devices, helps mitigating the short-channel effects.