Geometrical scaling effects on carrier transport in ultrathin-body MOSFETs

Abstract

Multiple-gate transistors featuring an ultrathin semiconductor body are widely recognized as promising candidates for future generation CMOS technology nodes. In this thesis, we have discussed the effects of reducing the channel length and the body thickness on carrier transport in ultrathin-body MOSFETs. We discussed the general features of the carrier distribution and the potential profile in long-channel Double-Gate ultrathin-body MOSFETs, including the effect of quantum confinement in the ultrathin body. In particular, we have investigated the validity of the Effective Mass Approximation by a systematic comparison of the relevant quantities with a full-band quantization approach based on the Linear Combination of Bulk Bands, for both silicon and germanium n-MOSFETs for various crystal orientations. Furthermore, we proposed a method to quantify shifts in energy band alignment due to e.g. structural quantum confinement in ultrathin-body MOSFETs, using the temperature dependence of the subthreshold current. The results were compared with the shifts in threshold voltage, which is commonly used to quantify the effect of quantum confinement. Besides reduction of the body thickness, we have also investigated the effect of decreasing the channel length to values comparable to the carrier mean-free-path, so that carrier transport becomes quasi-ballistic. We have systematically analyzed carrier back-scattering using a sophisticated Multi-Subband Monte-Carlo simulator, both in a realistic 32-nm ultrathin-body MOSFET and in a template device structure with a fixed potential and template scattering parameters. Finally, we have proposed a new model for the backscatter coefficient in nanoscale MOSFETs, assuming that carriers encounter on average just a single scattering event, so that carrier transport is nearly ballistic. Systematic comparison with results obtained from Multi-Subband Monte-Carlo simulations shows very good overall agreement, demonstrating that the proposed model provides a useful framework to help explain and predict quasi-ballistic transport phenomena in nanoscale MOSFETs.