Priyank Rastogi

Quantum Confinement Effects in Extremely Thin Body Germanium n-MOSFETs

We explore the impact of varying channel thickness (from 8 to 1.5 nm) on extremely thin germanium n-MOSFETs, by explicitly incorporating the quantum confinement effects in the band structure calculations using the first principle density functional theory. In Ge (001) thin films in the sub-10-nm regime, the X valley becomes the lowest conduction band valley and is mostly responsible for the charge transport as in silicon. Considering device parameters as per the international technology roadmap for semiconductors (ITRS) projected device specifications for the year 2024, we use the confinement-modulated effective mass to calculate the drain current employing the fully ballistic nonequilibrium Greens function transport model. The best suited thickness for digital applications is found to be 1.5 nm with subthreshold slope of 83.8 mV/decade, ION/IOFF of 1.8 × 104, and an ION exceeding ITRS targets.

Impact of Channel Thickness Variation on Bandstructure and Source-to-Drain Tunneling in Ultra-Thin Body III-V MOSFETs

In nanoscale MOSFETs with sub-10 nm channels, the source-to-drain tunneling is expected to be a critical bottleneck, especially in III-V devices on account of their extremely low effective masses. Also, to maintain electrostatic integrity at extremely small gate lengths, the channels need to be made ultrathin. In such devices, the bandstructure of the channel material becomes thickness dependent due to quantum confinement effects, and deviates remarkably from that of the bulk material. In this paper, we use first principle density functional theory calculations to evaluate the variation of the effective mass and bandgap with channel thickness. Then, we perform semi-classical ballistic and full quantum non-equilibrium Greens function transport simulations to study the impact on source-to-drain tunneling in III-V nMOSFETs. We demonstrate that the severity of the expected degradation due to source-to-drain leakage is reduced significantly, when the beneficial impacts of change in bandstructure, and multi-valley transport are taken into account.

Effective Doping of Monolayer Phosphorene by Surface Adsorption of Atoms for Electronic and Spintronic Applications

ABSTRACT We study the effect of surface adsorption of 27 different adatoms on the electronic and magnetic properties of monolayer black phosphorus using density functional theory. Choosing a few representative elements from each group, ranging from alkali metals (group I) to halogens (group VII), we calculate the band structure, density of states, magnetic moment and effective mass for the energetically most stable location of the adatom on monolayer phosphorene. We predict that group I metals (Li, Na, K), and group III adatoms (Al, Ga, In) are effective in enhancing the n-type mobile carrier density, with group III adatoms resulting in lower effective mass of the electrons, and thus higher mobilities. Furthermore, we find that the adatoms of transition metals Ti and Fe produce a finite magnetic moment (1.87 and 2.31 μB) in monolayer phosphorene, with different band gap and electronic effective masses (and thus mobilities), which approximately differ by a factor of 10 for spin-up and spin-down electrons, opening up the possibility for exploring spintronic applications.

Band-to-band tunneling in Γ valley for Ge source lateral tunnel field effect transistor: Thickness scaling

The direct and indirect valleys in Germanium (Ge) are separated by a very small offset, which opens up the prospect of direct tunneling in the Γ valley of an extended Ge source tunnel field effect transistor (TFET). We explore the impact of thickness scaling of extended Ge source lateral TFET on the band to band tunneling (BTBT) current. The Ge source is extended inside the gate by 2 nm to confine the tunneling in Ge only. We observe that as the thickness is scaled, the band alignment at the Si/Ge heterojunction changes significantly, which results in an increase in Ge to Si BTBT current. Based on density functional calculations, we first obtain the band structure parameters (bandgap, effective masses, etc.) for the Ge and Si slabs of varying thickness, and these are then used to obtain the thickness dependent Kanes BTBT tunneling parameters. We find that electrostatics improves as the thickness is reduced in the ultra-thin Ge film ( ≤ 10 nm). The ON current degrades as we scale down in thickness; howeve...