New Evidence for Velocity Overshoot in a 200 nm Pseudomorphic HEMT

It is believed that significant velocity overshoot effects are responsible for the high performance of Pseudomorphic HEMTs (PsHEMTs) with InGaAs channels grown on GaAs substrates. The expected overshoot is associated with the low effective mass in the channel and the large ¿ - L separation and is clearly demonstrated in numerous Monte Carlo simulations. Average electron velocities well in excess of 3.0×10⁷ cm/s have been predicted. However, average electron velocities extracted form transconductance measurements of such devices are much lower, typically in the range 1.5 - 2.0×10⁷ cm/s. Although there are some explanations in the literature for why such discrepancies exist in the extracted effective velocity they are based on Monte Carlo simulation itself without direct links to real fabricated and measured devices. In this paper we analyse for the first time real device measurements by using Monte Carlo and drift diffusion simulations. We obtain clear evidence that the average velocity in the channel of 200 nm PsHEMT fabricated in the Nanoelectronics Research Centre of Glasgow University exceeds 3.0×10⁷ cm/s.