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

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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DD</sub> = V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SS</sub> × Log(I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</sub> /I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</sub> )+ V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OV</sub> . V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SS</sub> is the subthreshold swing and V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OV</sub> is the overdrive voltage needed to achieve the desired on-current after threshold. V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OV</sub> 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SS</sub> is given by the following model [2]: SS = 1/ η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">el</sub> × (1/S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Barrier</sub> + η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">conf</sub> /S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DOS</sub> ) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> (1) η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">el</sub> is the electrostatic gate efficiency. η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">conf</sub> is the quantum confinement efficiency and comes from energy level shifts that occur when the quantum well shape changes with bias. S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Barrier</sub> represents the steepness in mV/decade that comes from changing the thickness of the tunneling barrier. S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DOS</sub> 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.