Study On Simulation In Tunnel Field-Effect Transistors

In recent years, rapid development of integrated circuit(IC) technology allows conventional CMOS faced its limits of characteristics, so that power consumption issue is increasingly prominent. The tunnel field-effect transistor(TFET) is a promising candidate that can get a subthreshold swing(SS) below 60 mV/dec thanks to the band-to-band tunneling(BTBT) mechanism, so that the device can reduce power consumption. The models on tunneling in the mainstream TCAD tools and the quantum transport boundary method(QTBM) based on non-equilibrium Green’s function method are introduced in this paper. These models are used to study the effects in tunneling current due to different tunneling directions and an additional gate.The followings are brief introductions on each chapter in this paper. Chapter 1 introduces the research background and the significance of TFETs. Chapter 2 describes a band-to-band tunneling model based on the WKB approximation. Chapter 3 introduces a quantum transport boundary method(QTBM) based on the non-equilibrium Green’s function method. Chapter 4 studies the relationship between tunneling current and tunneling directions based on a WKB-based model in Si1-xGex. Chapter 5 studies the effect of an additional gate introduced into an L-shaped GaSb-InAs heterojunction TFET with QTBM. Chapter 6 is the summary and prospect.These studies found that:(1) crystal direction have little effects on the indirect BTBT generation;(2) for silicon, the indirect BTBT generation is higher than the direct BTBT generation, and for germanium, it’s the direct BTBT generation that’s higher;(3) in the Si1-xGex TFET, as the drain source voltage increases, the tunneling direction gradually transforms to the vertical from the horizontal and to the germanium in the source from the silicon in the channel, with the tunneling generation rate and the area where inter-band tunneling phenoma take place increasing gradually, which increases the tunneling current;(4) for germanium, the direct BTBT current is higher than the indirect BTBT current, especially when the gate-source voltage is large;(5) in the L-shaped GaSb-InAs heterojunction TFET, an additional gate structure reduces a subthreshold swing of 88.78 mV/dec to 39.15 mV / dec, and the subthreshold swing is becoming more sensitive to the position of the additional gate than its length;(6) increasing the length of the gate(i.e., gate becomes L-shaped) and reducing the thickness of the channel device, an additional gate structure still is able to reduce the subthreshold swing of these devices, but a L-shaped gate does not seem conducive to the additional gate structure. Overall, the additional gate structure for reducing the subthreshold swing of TFETs is feasible.