Effect of polysilicon depletion charge on electron mobility in ultrathin oxide MOSFETs

We studied the effect of the depletion charge in the polysilicon gate on electron mobility in ultrathin oxide MOSFETs. An improved theory for remote-charge-scattering-limited mobility in silicon inversion layers is developed. The model takes into account the effects of image charges, screening, inversion layer quantization, the contribution of different subbands, oxide thickness, the actual distribution of charged centres inside the structure, the actual distribution of carriers in the inversion layer, the correlation of charged centres and the charged centre sign. It is shown that if the oxide is thin enough the remote Coulomb scattering due to the depletion charge in the poly-gate becomes an effective scattering mechanism, whose effect is comparable to those of the main scattering mechanisms that control the movement of the carriers in the MOSFET channel. As a consequence, this scattering mechanism must be taken into account in order to satisfactorily explain the experimental results obtained in ultrathin oxide MOSFETs. The model is implemented in a Monte Carlo simulator, where the effects of the ionized impurity charge in the substrate, the interface trapped charge and the contribution of other scattering mechanisms are taken into account simultaneously. Our results show that RCS cannot be neglected for oxide thicknesses below 2 nm, but that its effects for tox > 5 nm are negligible. Good agreement with experimental results was obtained.