A new remote Coulomb scattering model for ultrathin oxide MOSFETs F.G´ amiz, A.Godoy, and J.B.Rold´ an Departamento de Electr´ onica y Tecnolog´ ıa de Computadores. Universidad de Granada. Avd.Fuentenueva s/n, 18071 Granada (Spain) Phone: 34-958246145 Fax: 34-958243230 E-mail: fgamiz@ugr.es Abstract— 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 has been developed. It is shown that if the oxide is thin enough the remote Coulomb scattering due to the depletion charge in the polygate 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. The model is implemented in a Monte Carlo simulator, where the effects of the ionized impurities 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 2nm, but that its effects for tox > 5nm are negligible. Good agreement with experimental results was obtained. I. I NTRODUCTION In order to scale CMOS (Complementary Metal-Oxide- Semiconductor) devices to smaller dimensions while main- taining 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 0.1μm, gate oxides below 2nm 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 concen- tration, 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. II. SIMULATION RESULTS AND DISCUSSIONS. A. Coulomb Scattering Rate We employed the new remote-coulomb scattering model to study the effect of the poly-depletion charge and oxide thick- ness on electron mobility. The depletion charge distribution in the poly gate was calculated by solving the Poisson equation. Thus, the actual distribution of the charge in the poly is taken into account, for each value of the gate voltage, and each value of N D-poly . The particular value of the poly-depletion layer thickness depends on the poly doping concentration and on the voltage applied to the gate. We observed that for the same value of inversion charge concentration (i.e. the same value of charge in the poly depletion layer) the lower N D-poly , the wider the thickness of the depletion layer, and the lower the effect of RCS, i.e., the higher the electron mobility. Unless otherwise stated, we considered a silicon inversion layer with the following parameters: substrate doping concen- tration N A =5 × 10 17 cm -3 , poly impurity concentration, N D-poly =1 × 10 20 cm -3 , and an interface trap concentration of N it =5 × 10 10 cm -2 at the oxide/silicon and poly/oxide interfaces. Different values of the oxide thickness ranging from t ox =1nm to t ox = 10nm are assumed. Figure 1 shows the Coulomb scattering rate for the electrons in the ground subband taking into account the poly depletion charge (solid line) and ignoring the effects of the poly depletion charge (dashed line). The contributions of the substrate doping charge (N A =5 × 10 17 cm -3 ) and of the interface trapped charge (N it =5 × 10 10 cm -2 ) are considered in evaluating the Coulomb scattering rate. In both cases, the effects of the oxide thickness and of the different dielectric constant between the oxide and the polysilicon are considered. 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