Velocity Overshoot and Its Degradation in Sub-50-nm Gate-length GaAs/AlGaAs High-electron-mobility Transistors

Ultra-short-channel AlGaAs/GaAs high-electron-mobility Transistors (HEMTs) have been fabricated with gate lengths ranging from 25 nm to 95 urn in order to examine the characteristics of sub-50-nm HEMT scaling. The measured transconductance showed gradual rise with decreasing gate length down to 70 mu. The sharp rise in transconductance at gate lengths below 70 nm gate length is attributed to the onset of velocity overshoot. For gate lengths below 40 nul, however, the transconductance drops suddenly. We investigated this behavior by using a transient transport model, which indicated the existence of a minimum acceleration length being needed for carriers to reach high values of the overshoot velocity.