Improving Strained-Si on Si1 xGex Deep Submicron MOSFETs Performance by Means of a Stepped Doping Profile

We have made use of a stepped doping profile to improve the performance of strained-Si ultra-short MOSFETs. Electron mobility curves are calculated by a Monte Carlo simulator including electron quantization and Coulomb scattering, in addition to phonon and surface roughness scattering. In the first part of the paper, the effect of Coulomb scattering due to both interface charges and bulk impurities is carefully analyzed. We show that the strain enhances the Coulomb-limited mobility due to the interface-trapped charges as a consequence of a better screening of these charges by mobile carriers. However, we also show that this improvement in the Coulomb-limited mobility does not occur if the Coulomb scattering is due to bulk doping impurities, since they share the same physical space with the carriers, and therefore the screening is the same for the same inversion charge concentration. Nevertheless, we have shown that the use of a stepped doping profile bypasses this inconvenience. The introduction of a low doped layer below the oxide reduces the scattering produced by the bulk ionized impurities, enhancing Coulomb-limited mobility in deep-submicron devices. On the other hand, we have seen (by using MINIMOS-NT) that the use of the low doped silicon layer significantly improves the drain current while degrade the turn-off behavior of very short-channel devices only moderately. This design provides the possibility of taking full advantage of the great reduction in phonon scattering produced by the strain in the Si layer in these MOSFETs.