On the Interpretation of Ballistic Injection Velocity in Deeply Scaled MOSFETs

The ballistic injection velocity is examined in state-of-the-art Si extremely thin SOI MOSFETs using ballistic quantum simulations and a virtual source (VS) compact model. The results indicate that the device performs at around 50%-60% of its ballistic limit and that the ballistic injection velocity at the top of the potential barrier (ToB), as obtained by numerical simulation, can be significantly lower than its counterpart extracted at the VS. This occurs because, at high drain bias, the ToB moves under the influence of gate bias toward the source contact, where additional mobile charge resides that are not directly induced by the gate contact but by the source contact. This effect becomes increasingly important as channel length shrinks and is affected by several factors, including the details of the source design. The accurate estimation of a physically meaning “injection velocity” under ballistic limit could be therefore very difficult in very short channel MOSFETs.