Scaling of gate length in ultra-short channel heterostructure field effect transistors

Ultra-short channel AlGaAs/GaAs modulation doped field effect transistors (MODFETs) have been fabricated with gate lengths ranging from 25 to 115 nm, using an electron beam lithography process, in order to examine the fundamental limitation of transistor scaling. For gate lengths in the tens of nanometer range, where the gradual channel approximation is no longer valid, it is observed that the transconductance is improved by electron velocity overshoot. Velocity overshoot starts to occur in our devices with sub-70 nm gate lengths, which is smaller than that expected from other published experimental results. This is due to the suppression of the velocity overshoot by a parasitic gate-fringing effect in a reduced gate aspect ratio. However, we find that the enhancement in transconductance by the velocity overshoot is limited with further scaling and the transconductance starts to drop at a gate length below 40 nm. Short-channel electron tunneling is suggested experimentally to explain the degradation of transistor performance in the shortest channel devices.