  The mean free path (MFP) in silicon nanowire field effect transistors limited
by surface roughness scattering (SRS) is calculated with the non-perturbative
approach utilizing the non-equilibrium Green's function method. The entrance
scattering effect associated with finiteness of the channel length is
identified and a method to eliminate it in the calculation of the MFP is
developed. The behavior of the MFP with respect to channel length (L), channel
width (W), and the root-mean-square (RMS) of the surface roughness is
investigated extensively. Our major findings are that the single parameter,
RMS/W, can be used as a good measure for the strength of the SRS effects and
that the overall characteristics of the MFP are determined by the parameter. In
particular, the MFP exponentially decreases with the increase of RMS/W and the
MFP versus the gate electric field shows a distinctively different behavior
depending on whether the strength of the SRS effects measured by RMS/W is
smaller or greater than 0.06.
