Assessing the Role of a Semiconductor's Anisotropic Permittivity in Hafnium Disulfide Monolayer Field-Effect Transistors

2-D semiconductors show great promise to serve as channel materials in next-generation field-effect transistors (FETs). The permittivity of many 2-D semiconductors is anisotropic, though many recent simulation works studying 2-D FETs have treated these materials as though they have isotropic permittivities. Because there have been no works that investigate the role of each element of a semiconductor's anisotropic permittivity on a device's performance, the impact that this isotropic approximation has on a simulation's accuracy is unknown. Furthermore, the impact of a semiconductor's anisotropic permittivity on a device's performance cannot be explained using existing theory. In this simulation study, we investigate, for the first time, the impact of a semiconductor's anisotropic permittivity on the performance of FETs. Our main findings are that the isotropic approximation becomes inaccurate as the channel lengths of FETs are scaled down and that short-channel effects become less significant when the semiconductor's in-plane permittivity decreases or its out-of-plane permittivity increases. We also find that the capacitance of the semiconductor in the out-of-plane direction (i.e., the capacitance associated with the out-of-plane permittivity) more significantly influences a device's gate capacitance when equivalent oxide thickness (EOT) decreases. Therefore, EOT alone cannot be used to assess total gate control in aggressively scaled 2-D devices.