Insights into the design and optimization of tunnel-FET devices and circuits

Abstract

Improving the on-current has been the focus of enhancing the performance of tunnel field-effect transistors (TFETs). In this paper, we show that the increase in I_ON is not sufficient to improve the circuit performance with TFETs. As TFETs show a drain-barrier voltage in their output characteristics below which the drain current drastically reduces, the rise/fall time significantly increases. This reduces the dynamic noise margin and limits the performance achievable from TFETs. We show that, in TFETs, the delay of the circuit is determined by the rise/fall time rather than by the propagation delay. The saturation voltage is much higher compared with that of complementary metal-oxide-semiconductor (CMOS) devices, leading to a lower gain and a lower static noise margin in digital circuits, as well as impeding the performance of latch/regenerative circuits. We present a design space comprising of I_ON, a drain saturation voltage, and a drain threshold voltage for minimizing the propagation delay of circuits using TFETs. Finally, for the same off-current and speed of operation, TFET devices tend to suffer from a higher gate capacitance compared with CMOS devices. If this behavior is not taken into account during the circuit design, these devices (although designed for low-power applications) can dissipate more power at the same speed of operation than CMOS counterparts.