Quantum mechanical analysis of transport phenomena through 2d transition metal dichalcogenide material based tunnel field effect transistor

Abstract

In this thesis, a self-consistent simulator using Non Equilibrium Green’s Function (NEGF) method has been designed for single layer two dimensional Transition Metal Dichalchogenide (TMD) based vertical heterojunction tunnel FET. Single layer n-doped 〖MoS〗_2 and p-doped 〖WTe〗_2 have been used as the two layers coupled by weak van der walls force. In the work, coupled Hamiltonian with both top layer conduction band and bottom layer valence band has been formed to use in NEGF and self-consistently solve with poisson’s equation. Coherent ballistic transport mechanism has been applied to find device current which does not take into account the scattering mechanism when current flows through the device. It is a fairly simplified assumption for ultra-short devices as in this case. High ON current and low subthreshold swing is obtained from the device making it suitable for low power applications. The incorporation of TMD material as the channel makes it possible to obtain defect free surfaces which in turn make it suitable for high speed devices. Findings obtained from the simulation are compared with previously simulated works and satisfactorily similar was the result. For 15 nm channel overlap length and 15 nm gate extension length on both sides, theoretical subthreshold swing of 10mV/decade has been measured in the work. For 20 nm channel overlap length, ON current of 18μA/μm has been obtained as well. It has been found that device parameters like chemical doping, top and bottom gate voltages, dielectric layer thickness have significant impact on device performance. The effect of dimensions of top gate extension and overlap length of two layers on ON and OFF state current transport has been studied which will help the designer to optimize between low subthreshold swing and miniaturization of the device. A thorough understanding of the impact that different device parameters have on the device performance makes the study a valuable framework for evaluating the device performance.