Electron mobility in extremely thin single-gate silicon-on-insulator inversion layers

Inversion-layer mobility has been investigated in extremely thin silicon-on-insulator metal–oxide–semiconductor field-effect transistors with a silicon film thickness as low as 5 nm. The Poisson and Schrœdinger equations have been self-consistently solved to take into account inversion layer quantization. To evaluate the electron mobility, the Boltzmann transport equation has been solved by the Monte Carlo method, simultaneously taking into account phonon, surface-roughness, and Coulomb scattering. We show that the reduction of the silicon layer has several effects on the electron mobility: (i) a greater confinement of the electrons in the thin silicon film, which implies an increase in the phonon-scattering rate and therefore a mobility decrease; (ii) a reduction in the conduction effective mass and the intervalley-scattering rate due to the redistribution of carriers in the two subband ladders as a consequence of size quantization resulting in a mobility increase; and (iii) an increase in Coulomb scattering because of a greater number of interface traps in the buried Si–SiO2 and to a closer approach of these charged centers to the mobile carriers. The dependence of these effects on the silicon-layer thickness and on the inversion-charge concentration causes the mobility to be a nontrivial function of these variables. A detailed explanation of the mobility behavior is provided. Mobility for samples with silicon thickness below 10 nm is shown to increase in an electric field range that depends on the charged center concentration, while for silicon layers over 10 nm mobility always decreases as the silicon-layer thickness is reduced.