Electrical Transport in Nanoscale Semiconductors: A Quantum Transport Approach
As predicted by the International Roadmap for Semiconductors, III-V metal-oxide semiconductor field-effect transistors(MOSFETs) are the prime candidates for the 7-nm node and beyond owing to their high mobilities. However, several challenges need to be overcome before III-V materials can replace silicon in extremely scaled devices. As the size of semiconductor devices enters into the nanoscale regime, it is important to understand quantum mechanical effect in electron transport because quantum mechanical effects such as microscopic scattering, quantum mechanical tunneling, carrier confinement and interference effects play a crucial role. We have investigated quantum transport properties of III-V and silicon in nano-scale devices using the non-equilibrium Green’s function (NEGF) formalism coupled with a 20 orbital sp3d5s*-SO tight-binding model. A mode space NEGF approach combined with the full-band WKB model has been introduced to calculate band-to-band tunneling current under the valence band where an atomistic full-band NEGF approach provides a zero current. Self-energy functions for carrier scattering have been implemented within the mode space NEGF formalism to instigate the influence of microscopic scattering effects on quantum transport. This approach has been benchmarked by comparing the scattering rates obtained from Fermi’s golden rule which is widely used in traditional semi-classical calculations. The scattering model is used to study the role of phonon scattering and surface roughness scattering on the carrier transport characteristics of Si and III-V channel devices. In the presence of the electron-phonon interactions, the drain current decreases compared with its ballistic limit, and the current reduction ratio increase as the channel length increases. Effects of indium mole fraction and source/drain doping density (NSD) on the performance of nanoscale III-V MOSFETs are explored and their performance is compared with that of Si transistors. For III-V's, we have found an optimum indium mole fraction and NSD that maximize the Ion/Ioff ratio and Ion by balancing injection velocity and short channel effect.