NEGF Method for Design and Simulation Analysis of Nanoscale MOS Devices

Abstract

The growing demand for miniaturized transistors with increased performance and low power consumption has scaled down the device to the nanoscale regime. The design and modeling of such small devices needs a reliable computing technique for simulation. Because the flow of electrons in such short channels is due to differences in electrochemical potential between the source and drain, it is called ballistic transport. The study of novel microscaled and nanoscaled electronic devices using the classical approach is inaccurate and the results are not reliable. The complexity of the current formulation increases as the size of the device is reduced. The general concept for the current in the field-effect transistor (FET) device is considered due to the drift and diffusion of carriers in the device. But in modern concepts, the flow of electrons is explained as a combination of two different processes, elastic transfer and heat generation. The elastic transfer is force driven and the heat generation is entropy driven. Considering the previously mentioned process, the transport of electrons is studied using two theories, semi-classical transport theory and quantum transport theory. The semi-classical transport theory explains the flow of electrons by a combination of Newtonian mechanics for force-driven and entropy-driven thermodynamics called the Boltzmann transport equation (BTE). In mesoscopic physics, the quantum transport theory is used in which the force-driven transport of electrons is well explained by quantum mechanics. The quantum transport theory is an advanced one that properly describes the current flow in the electronic device and is reliable. In the quantum transport theory, force-driven flow is expressed using the Schrödinger wave equation and entropy-driven thermodynamics, and the formal integration of these two processes is obtained by the non-equilibrium Green's function (NEGF) method. The NEGF utilizes the process of the many-body perturbation theory (MBPT) to explain the dispersed entropy-generating practices, which are dependent on the second quantization language. In both the equilibrium and non-equilibrium conditions, NEGF has evolved to examine the many-particle quantization systems. The application of NEGF formalism is used in the field of quantum optics, quantum correction of BTE, transport in bulk systems corresponding to the high field, quantum electron and holes transport in different materials and devices, resonant tunnel diodes of III–V groups, electron waveguides, quantum cascade lasers, Si nanopillars, carbon nanotubes, graphene nanoribbons, metal wires, organic molecules, spintronic devices, and thermal and thermoelectric devices. A meticulous framework is provided by the NEGF method to deal with the whole interaction, either elastic (non-dissipative) or inelastic (dissipative), with the help of MBPT, which occurs in the channel. This chapter will provide a comprehensive understanding of the accurate carrier transport model, which can be employed with advanced nanoscale devices where the classical theory fails. It is also capable of setting a strong background for readers and research groups who are actively working in the quantum and ballistic transport regime. The chapter is also beneficial to graduate/undergraduate students and industry people who are pursuing the same domain.