Ultra-submicron GaAs MESFETs have been fabricated with gate lengths ranging from 25 nm to 80 nm, using an electron-beam lithography process, in order to examine the operating characteristics as a function of gate length. The MESFETs were fabricated on wafers doped at 2×1017 cm-3 and 1.5×1018 cm-3 with active layer thicknesses of 250 nm and 60 nm, respectively. Measurements of the transconductance, and the inferred transit velocity, as a function of the effective gate length show a minimum in these quantities near 55 nm. The rise in transconductance below this gate length is attributed to the onset of velocity overshoot in the channel region, and both the inferred transit velocity and the variations between the lightly doped samples and the heavily doped samples support this interpretation. For gate lengths below about 40 nm, however, the transconductance again drops. We attribute this drop to the existence of a minimum acceleration length needed for the carriers to reach the high values of the overshoot velocity. We have investigated this behavior with a transient transport model based upon a parameterized velocity autocorrelation function incorporating both energy and momentum relaxation rates. The results of this model yield qualitative agreement with both the measurements and the interpretation given above.
- Velocity overshoot
- generalized Langevin equation
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics
- Electrical and Electronic Engineering
- Materials Chemistry