Impact of Super-Steep-Retrograde Channel Doping Profiles on the Performance of Scaled Devices

Super-steep retrograded (SSR) channels were compared to uniformly doped (UD) channels as devices are scaled down from 250 nm to the 50 nm technology node, according to the scheme targeted by the National Technology Roadmap for Semiconductors (1997). The comparison was done at the same gate length Lgate and the same off-state leakage current Io , where it was found that SSR profiles always have higher threshold voltages, poorer subthreshold swings, higher linear currents, and lower saturation currents than UD profiles. Using a simulation strategy that takes into account the impact of short-channel effects on drive current, it was found that the improved shortchannel effect of retrograde profiles is not enough to translate into a higher performance over the UD channels for all technologies. Hence, if the effective gate-dielectric thickness scales linearly with technology, retrograde doping will not be useful from a performance point of view. However, if the scaling of the gatedielectric is limited to about 2 nm, SSR profiles can give higher drive current than UD channels for the end of the roadmap devices. Thus, the suitability of SSR channels over UD channels depends on the gate-dielectric scaling strategy. Simulations using a self-consistent Schrödinger–Poisson solver were also used to show that the impact of quantum mechanical (QM) effects on the long-channel characteristics of SSR and UD MOSFET’s will be similar.