In this paper, we present a new and improved analytical mobility model, which explains the effect of transconductance overshoot observed in ultra-short-channel length MOSFETs quite well. The model expresses the mobility as a function of the critical electric field, channel length, oxide thickness, and applied drain-to-source and gate-to-source voltages. The results obtained from our model have been compared with the experimentally reported values available in literature, and a good match between the two is seen.