It is shown both experimentally and via MINIMOS simulation that the subthreshold swing S = ∂VG ∂log10ID in a MOSFET may decrease significantly as gate length LG is reduced before increasing catastrophically when LG becomes so short that punchthrough current flows. The effect is largest in devices with lightly doped substrates, deep source/drain junctions, and heavy threshold adjust implants operated at high drain bias. An explanation for the effect is provided in terms of sharing of the depletion region charge between gate and drain.
Solid State Electronics
Department of Electronics

Tarr, N.G, Walkey, D.J., Rowlandson, M.B., Hewitt, S.B., & MacElwee, T.W. (1995). Short-channel effects on MOSFET subthreshold swing. Solid State Electronics, 38(3), 697–701. doi:10.1016/0038-1101(94)00147-8