Impurity-limited mobility and variability in gate-all-around silicon nanowires

We discuss the scattering of electrons and holes by charged dopant impurities in ⟨001⟩,⟨110⟩, and ⟨111⟩ gate-all-around silicon nanowires (Si NWs) with diameters in the 2-8 nm range. We show that the mobility of minority carriers follows simple trends resulting from band-structure effects. In the inversion mode, the ⟨110⟩ and ⟨001⟩ [respectively, ⟨111⟩ and ⟨110⟩] Si NWs are the best n-type [resp. p-type] channels. The choice of a high-κ gate oxide is critical to achieve large mobilities in ultimate Si NWs. The mobility of majority carriers is found to increase with decreasing NW diameter and is more weakly dependent on the orientation. We also discuss the variability of single impurity resistances as a function of the structural parameters and nature of the carriers.