Quantitative understanding of mobility degradation due to remote charge scattering

Si MOSFETs face the scaling limit of the gate SiO/sub 2/ thickness t/sub ox/. It is reported that the drive current does not increase, even though one may make gate oxide thinner than 1.3 nm (Timp et al, 1998). Besides the large direct tunneling currents, there is concern over the mobility degradation inherent to MOSFETs with an ultrathin gate oxide (Yu et al, 2000). However, the mechanism of the mobility degradation has not been fully elucidated to date. Mobility degradation due to remote charge scattering (RCS) is considered to be one cause of the drive current reduction. RCS results from the depletion charge in the poly-Si gate. Since the depletion charge is separated from the channel carriers by the gate oxide, the RCS-limited mobility, /spl mu//sub RCS/, is negligible when the gate oxide is thick. Recently, the use of an ultra-thin SiO/sub 2/ with t/sub ox/ less than 1.0 nm has been reported for sub 50 nm MOSFETs (Chau et al, 2000). In such MOSFETs, /spl mu//sub RCS/ may well reduce the total effective mobility, /spl mu//sub eff/, since the number of scatters is as large as the inversion charge. We have extended previous theories (Krishnan et al, 1998; Yang et al, 2000; Ando et al, 1982) by precise calculation of the potential, and found that the calculated mobility is consistent with recent experimental results (Takagi et al, 2001).