Contents I. Introduction
In order to scale CMOS (Complementary Metal-Oxide-Semiconductor) devices to smaller dimensions while maintaining good control of the short-channel effects, the gate oxide thickness should be reduced in close proportion to the channel length [1]. Thus, for devices with gate lengths below , gate oxides below could be needed. However, oxide scaling results in several effects that impose serial limitations on MOS devices [1], including an important degree of remote Coulomb scattering due to the poly-gate charge, which strongly degrades electron mobility, [2], [3], [4], [5], [6]. In this work we show the importance of the RCS effect on electron mobility. In particular, we find that, depending on the oxide layer thickness and the poly impurity concentration, this scattering mechanism could become as important as the main scattering mechanisms that control the transport properties of carriers in the MOSFET channel. An improved theory for remote-charge-scattering limited mobility in silicon inversion layers has been developed [7]. A Monte Carlo method is used to solve the Boltzmann transport equation (BTE) taking into account the effect of RCS mechanism. A detailed description of the Monte Carlo simulator can be found elsewhere [8]. The contribution of other scattering mechanisms (phonon scattering, surface roughness scattering and Coulomb scattering due to ionized bulk doping impurities and interface charges) is simultaneously taken into account. Prior to this, the one-dimensional Schroedinger and Poisson equations are selfconsistently solved in the whole structure, and the charge in the polysilicon carefully evaluated; thus we take into account the actual distribution of the charge in the polysilicon gate, instead of using the depletion approximation to evaluate the RCS rate. Using this remote-Coulomb scattering model (detailed in Ref. [7])in a Monte Carlo simulator, we made an extensive study of the effect of the polysilicon depletion charge on electron mobility. Section II discusses the effect of the poly depletion doping concentration and the role of the oxide thickness. The effect of the substrate doping is also analyzed. A comparison with experimental results is provided. Finally the main conclusions of our work are drawn in Section III.
