Research On The Physical Mechanism And Analytical Model Of Tunneling FET

The TFET?Tunneling Field-Effect-Transistor?works on the band-to-band tunneling principle,and its subthreshold swing can break the the theoretical limit of the MOSFET subthreshold swing,making it a potential device of great research value and application potential.Traditional TFET faces four major problems,which are follows:1.The on-state current is small.2.There is ambipolar current.3.The gate-drain capacitance is large.4.A highly doped abrupt junction is required.For the problem of small on-state current and ambipolar current,researchers proposed many novel device structures,including short-gate TFET^[18],multi-dielectric TFET^[19],gate-covering-source TFET^[20],dual-workfunction TFET^[22]and dual-workfunction heterojunction TFET^[22],etc.The novel device structures can improve on-state current and reduce ambipolar current of TFET,improving the performance of TFET.For novel TFET structures,the analytical model can describe the relationship between structural parameters and performance directly,and can provide theoretical guidance for further structural optimization and design.However,these novel device structures are often more complex than traditional TFET structures,with non-uniform boundary conditions,making it harder for analytical modeling.Till now there is lack of analytical models corresponding to these structures.In order to describe the performance of the device better and provide references for designing devices,this thesis establishes the two-dimensional analytic potential model of the common gate-aligned channel double-gate TFET with uniform gate boundary conditions.This analytic model is based on two-dimensional Poisson equation.A two-dimensional potential model of a double-gate TFET with non-uniform gate boundary conditions is established for the novel device structures with complex boundaries derived from the traditional structure,such as multi-gate electrode materials and multi-gate oxide thickness.Based on the continuity of potential and electric displacement,a two-dimensional potential model of the entire gate electrode coverage region is established.In this thesis,the results of analytical model calculation and TCAD numerical simulation are compared from the aspects of device channel surface potential,channel center potential and channel surface electric field.The comparison results show that the potential and electric field of the analytical model and the numerical simulation are basically consistent at the surface and the center of channel,which verifies the correctness of the analytical model.Under the guidance of the analytical model,the optimization design and application to reduce ambipolar current are studied in this thesis.The thickness of the gate dielectric layer of gate-covering-drain TFET is analyzed and optimized.It is found that the gate dielectric layer of gate-covering-drain TFET has an optimal thickness,under which the ambipolar current of the TFET is the minimum.Based on this conclusion,we propose a novel double-gate TFET?SSG-TFET?with a step-shaped-gate structure.By applying the analytical model for the double TFET with non-uniform gate boundary conditions to SSG-TFET,the analytical of SSG-TFET is obtained.In this thesis,the optimal thickness of SSG-TFET is studied from the perspective of numerical simulation and analytical model respectively.The simulation results show that the optimal thickness of SSG-TFET decreases with the increase of channel thickness,and increases with the increase of the gate-covering-drain length,which is consistent with the analytical model prediction.The analytical model of the device is a bridge between the circuit model and the device design and is of important significance.The analytical model established in this thesis can match the numerical simulation results and predict the direction of device optimization,which has practical significance.