We present experimental evidence that current gain cutoff frequency (ft) values equal to or greater than those achieved with high electron mobility transistors (HEMTs) and pseudomorphic HEMTs can also be achieved by ion‐implanted GaAs and InGaAs metal‐semiconductor field‐effect transistors. These measuredftresults clearly suggest that the average electron velocity under the gate is determined primarily by the high‐field electron velocity rather than the low‐field electron mobility. Hence, we conclude that the transport properties of the two‐dimensional electron gas in HEMTs and pseudomorphic HEMTs do not make a significant contribution to the high‐frequency and high‐speed performance of these devices.