Although Si and Ge are the staples of semiconductor industry, many different materials are being considered for field effect transistor (FET) applications to boost performance. Examples include Si, Ge, III-Vs, carbon nanotubes (CNTs), and graphene. As the channel length drop below 10 nm, direct source-to-drain tunneling through the channel limits the OFF current for both standard FETs and TFETs. Therefore, a natural question arises:
“How well do different materials block the direct tunneling current in both n-type and p-type NW FETs or CNTFETs with actual 5 nm channels?”
We investigated a wide range of materials including Si, Ge, InAs, InSb, InP, GaAs, GaN nanowires and CNTs with varying diameters to compare their ability in blocking the tunnel current at the 5-nm length scale. We found that, overall, Si gives the lowest tunnel current for electron and hole tunneling.
Tunneling currents are determined by the attenuation factor κ in the bandgap. Although κ is the key parameter, most device people do not have an intuitive feel for quantitative values of κ. For example, what values of κ would be considered ‘large’ versus ‘small’ decay constants? Device people are, however, well-calibrated to values of the effective mass, and they do have an intuitive feel for what constitutes a ‘heavy’ mass versus a ‘light’ mass. Therefore, to compare the relative size of tunnel currents among different materials without performing expensive numerical simulation, we mapped the values of κ into a tunneling a effective mass m*. The larger the tunneling mass, the smaller the tunneling current and vice versa. Si has the heaviest tunnel mass and the lowest current.