Why tunneling FETs don't work, and how to fix it

Abstract

To date, TFET results have been unsatisfying. The best reported subthreshold swings have been measured at a current density of around a nA/um and get significantly worse as the current increases. In order to achieve a better performance, there are fundamental design issues that need to be engineered. We can understand these issues by analyzing the three types of devices shown in Fig 1. The voltage required to operate a TFET can be given by: V_DD = V_SS × Log(I_on /I_off)+ V_OV. V_SS is the subthreshold swing and V_OV is the overdrive voltage needed to achieve the desired on-current after threshold. V_OV will be determined by the device geometry as shown in Fig 2 [1]. Introducing quantum confinement in the direction of tunneling increases the conductance by 1-2 orders of magnitude at low voltage. V_SS is given by the following model [2]: SS = 1/ η_el × (1/S_Barrier + η_conf/S_DOS)^-1 (1) η_el is the electrostatic gate efficiency. η_conf is the quantum confinement efficiency and comes from energy level shifts that occur when the quantum well shape changes with bias. S_Barrier represents the steepness in mV/decade that comes from changing the thickness of the tunneling barrier. S_DOS is the steepness of the joint density of states (DOS) and represents the rate at which the joint DOS fall off as the band edges are misaligned.