In this work, a metal nitride interlayer semiconductor (MN-I-S) source/drain (S/D) model is suggested to investigate the effect of titanium nitride (TiN) and tantalum nitride (TaN) on the specific contact resistivity (rho(c)) of an MN-I-S S/D structure in a sub-14 nm n-type Si FinFET. The work function (WF) variation of TiN and TaN was considered based on a Rayleigh distribution. In this model, an undoped interlayer (undoped-IL) or heavily doped interlayer (n(+)-IL) were included to identify the effect of IL doping on rho(c). The structure with an n+-IL provides a very low variation in rho(c) as well as lower rho(c) values (i.e., similar to 4 x 10(-9) Omega.cm(2)). By using three-dimensional technology computer-aided design (TCAD) simulation, we also investigated the impact of rho(c) variation on device performance. The MN-I-S S/D with an n(+)-IL showed a higher on-state drive current with highly suppressed variation.