EFFECTS OF THE SCREENING AND MIRROR CHARGES ON ELECTRON-MOBILITY IN THE SUBTHRESHOLD REGION OF SI MOSFETS

Since the impurity concentration increases as the device size is reduc ed, it is crucial to understand the electron-impurity interaction to a ccurately predict transport mechanisms of carriers. In this paper, the surface mobility in the subthreshold region is Si MOSFETs where carri er density is very low, is investigated. With an increase in the impur ity concentration, the surface mobility can significantly deviate from the universal curves at lower carrier densities because of Coulomb sc attering by the impurities. Impurity scattering, however, can be suppr essed by screening. Screening mechanisms at low carrier densities are investigated. At a low carrier density, the mirror charges induced bot h at the interface of the gate electrode and at the edge of the deplet ion layer are shown to affect surface mobility. The surface mobility d ecreases with the surface carrier density, but in the threshold region where electron density is very small, this decrease is found to be su ppressed by the screening of the mirror charges. When the impurity sca ttering is included in the calculation, the threshold voltage shifts u pwards. The upward shift of the threshold voltage can be as large as s everal tens of mV, in addition to the previously reported quantization effects of the carriers in the inversion layer, for the devices used in deep submicron MOSFETs. The calculated results are in good agreemen t with measurements. Copyright (C) 1996 Elsevier Science Ltd