NEGF simulations of a junctionless Si gate-all-around nanowire transistor with discrete dopants

We have carried out 3D Non-Equilibrium Green Function simulations of a junctionless gate-all-around n-type silicon nanowire transistor of 4.2 × 4.2 nm2 cross-section. We model the dopants in a fully atomistic way. The dopant distributions are randomly generated following an average doping concentration of 1020 cm−3. Elastic and inelastic phonon scattering is considered in our simulation. Considering the dopants in a discrete way is the first step in the simulation of random dopant variability in junctionless transistors in a fully quantum mechanical way. Our results show that, for devices with an “unlucky” dopants configuration, where there is a starvation of donors under the gate, the threshold voltage can increase by a few hundred mV relative to devices with a more homogeneous distribution of dopants. For the first time we have used a quantum transport model with dissipation to evaluate the change in threshold voltage and subthreshold slope due to the discrete random donors in the channel of a junctionless nanowire nMOS transistor. These calculations require a robust convergence scheme between the quantum transport equation and the Poisson equation in order to achieve convergence in the dopant-induced resonance regime.

► Quantum transport simulations of random dopant variability in junctionless FET’s.
► Nanowires with 20 nm channel length and 4.2 × 4.2 nm2 cross-section are studied.
► Starvation of donors under the gate increases threshold voltage by 300 mV.
► Elastic and inelastic phonon scattering mechanisms are considered.
► Quasi-bound states of the impurities produce regions of local cooling or heating.