Material Selection for Minimizing Direct Tunneling in Nanowire Transistors

When the physical gate length is reduced to 5 nm, direct channel tunneling dominates the leakage current for both field-effect transistors (FETs) and tunnel FETs. Therefore, a survey of materials in a nanowire geometry is performed to determine their ability to suppress the direct tunnel current through a 5 nm barrier. The materials investigated are InAs, InSb, InP, GaAs, GaN, Si, Ge, and carbon nanotubes. The tunneling effective mass gives the best indication of the relative size of the tunnel currents when comparing two different materials of any type. The indirect-gap materials, Si and Ge, give the largest tunneling masses in the conduction band, and they give the smallest conduction band tunnel currents within the range of diameters considered. Si gives the lowest overall tunnel current for both the conduction and valence bands, and therefore, it is the optimum choice for suppressing tunnel current at the 5 nm scale. A semianalytic approach to calculating tunnel current is demonstrated, which requires considerably less computation than a full-band numerical calculation.