Band-Structure Effects in Ultrascaled Silicon Nanowires

In this paper, we investigate band-structure effects on the transport properties of ultrascaled silicon nanowire FETs operating under quantum-ballistic conditions. More specifically, we expand the dispersion relationship epsiv(kappa) in a power series up to the third order in kappa² and generate the corresponding higher order operator to be used within the single-electron Hamiltonian for the solution of the Schrodinger equation. We work out a hierarchy of nonparabolic models accounting for the following: 1) the shift of the subband edges and the change in the transport effective masses; 2) the higher order Hamiltonian operator; and 3) the splitting of the fourfold unprimed subbands in nanometer-size FETs. We then compute the device turn-on characteristics, the threshold shift versus diameter, and the subthreshold slope (SS) versus gate length. By compensating for the different threshold voltages, i.e., by reducing the turn- on characteristics to the same leakage current at zero gate bias, it turns out that the current discrepancies between the most general model and the bulk-parabolic model are contained within 20%. Finally, it turns out that the nonparabolic band structure gives an improved SS at the lowest gate lengths due to a reduced source-drain tunneling, reaching up to 30% enhancement.