High electron mobility transistors (HEMTs) have become important for high frequency and low noise applications. There are devices now operating with a cutoff frequency, f(T), of several 100 GHz. Through simulation, we have been investigating how these frequencies may be pushed even higher, and have found that it may be possible to achieve an f(T) of over 3 THz. For this, we have used a full-band, cellular Monte Carlo transport program, coupled to a full Poisson solver, to study a variety of InAs-rich, InGaAs pseudomorphic HEMTs and their response at high frequency, concentrating on devices with a structure (from the substrate) InP/InAlAs/InGaAs/InAlAs/InGaAs, with the quantum well composed of In(0.75)Ga(0.25)As. We have studied gate lengths over the range 10-70 nm and various source-drain spacings. The performance of scaled devices has been evaluated to determine the ultimate frequency limit. Here, the importance of the effective gate length has been evaluated from the properties internal to the device.