Selection of appropriate channel material is the key to design high performance tunnel field effect transistor (TFET), which promises to outperform the conventional metal oxide semiconductor field effect transistor (MOSFET) in ultra-low energy switching applications. Recently discovered atomically thin GeSe, a group IV mono-chalcogenide, can be a potential candidate owing to its direct electronic band gap and low carrier effective mass. In this work we employ ballistic quantum transport model to assess the intrinsic performance limit of monolayer GeSe-TFET. We first study the electronic band structure by regular and hybrid density functional theory and develop two band k · p hamiltonian for the material. We find that the complex band wraps itself within the conduction band and valence band edges and thus signifies efficient band to band tunneling mechanism. We then use the k · p hamiltonian to calculate self-consistent solution of the transport equations within the non-equilibrium Green’s function formalism and the Poisson’s equation based electrostatic potential. Keeping the OFF-current fixed at 10 pA/μm we investigate different static and dynamic performance metrics (ON current, energy and delay) under three different constant-field scaling rules: 40, 30 and 20 nm/V. Our study shows that monolayer GeSe-TFET is scalable till 8 nm while preserving ON/OFF current ratio higher than 104.

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Dissipative transport in 20 nm long phosphorene metal oxide semiconductor field effect transistor (MOSFET) is studied in armchair (AC) and zigzag (ZZ) directions. In a multiscale approach, the unit cell of phosphorene is first relaxed and bandstructure is calculated using hybrid density functional theory (DFT). The transport equations are then solved quantum mechanically under the nonequilibrium Green's function (NEGF) formalism using DFT-calibrated two band k•p hamiltonian. The treatment of electron phonon scattering is done under the self consistent Born approximation (SCBA) in conjunction with deformation potential theory. It is found that, optical phonon modes are largely responsible for degradation of ON-current apart from p channel AC MOSFET where acoustic phonon modes play a stronger role. It is further observed that electron-phonon scattering is more pronounced in ZZ direction whereas the diffusive ON-current of p-MOSFET in a given direction is higher than n-MOSFET. Further study on the complex bandstructure of phosphorene reveals band wrapping within bandgap region in AC direction and multiple crossings in ZZ direction. This signifies strong phonon assisted tunneling (PAT) in ZZ direction in comparison to AC direction. For completeness, drain current in AC tunnel field effect transistor (TFET) is calculated and electron-phonon scattering is observed only in the near vicinity of the OFF current.

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Ballistic transport in monolayer Germanane metal oxide field effect transistors (MOSFETs) are investigated for high performance(HP) applications. Characteristics of both n-and ptype transistors having channel lengths of 7 nm, 5 nm and 3 nm are studied and compared against the International Technology Roadmap for Semiconductor (ITRS) target of 2028. For this purpose we conceive single band effective mass Hamiltonian and double-band k.p Hamiltonian for n-and p-type channels respectively. The quantum transport model is based on the Non-Equillibrium Greens Function (NEGF) formalism self consistently coupled with Poisson equation. The ON current (I ON ) in 3 nm channel n-and p-MOSFETs, for a fixed OFF current I OFF =100 nA/µm, are found to be ∼ 890 µA/µm and 700 µA/µm respectively, which is indeed remarkable. We also observe that with the increase of channel lengths, the p-MOSFET starts to outperform the n-MOSFET in terms of I ON requirements as the direct source-to-drain tunneling gets suppressed. Other performance metrics like total gate capacitance, intrinsic switching delay and switching energy have also been calculated and found to be comparable to the ITRS 2028 HP technology requirements.