A novel concept for field-effect transistors - the tunneling carbon nanotube FET

We present experimental and simulation results on a novel concept for a nanowire-field-effect transistor (NW-FET) based upon band-to-band tunneling. In contrast to a conventional FET, the NW-FET enables in principle extremely small subthreshold swings and is insensitive to changes of temperature.

1 Introduction

In recent years, improvements of CMOS technology have led to an enormous down-scaling ofmetal-oxide-semiconductor field-effect transistors and devices with channel lengths less than 10nmhave been demonstrated. However, besides all fabrication-related progress the power consumption of highly integrated circuits becomes more and more the central issue calling for high-performance, low power devices. In this respect the limitation of conventional FETs to a minimum subthreshold swing of becomes a major obstacle to further reduce the operational voltage while leaving the on/off-ratio of the devices constant. The reason for this is shown in Fig.1 where the conduction band of an FET in the off-state is depicted for two different gate voltages. Only carriers in the exponential tail of the source Fermi distribution with energies higher than contribute to the current. For large enough bias the drain current where is the density of states, the carrier velocity and is the source Fermidistribution. Since the product and because the integral can easily be solved leading to theminimum at room temperature (in the ideal case that the conduction band changes one-to-one with a change of gate potential). The minimalS implies a minimal value of the operational voltage in order to provide a certainon/off-ratio. Therefore it is highly desirable to explore new device architectures that overcome the 60mV/dec-limit of conventional FETs. Some concepts to realize this have been discussed in the literature. Gopalakrishnan and coworkers were able to demonstrate a subthreshold swing smaller 60mV/dec in a special FET structure [1]. However, their device relies on impact ionization and has a very strong drain voltage dependence. A better strategy can be deduced from the expression for the drain current given above:altering the product in an appropriate way will allow to change the subthreshold swing. In particular, since S is determined by carrier injection from the highenergy tail of the source Fermi distribution, cutting off this high energy tail of is a way to realize subthreshold swings below the thermal limit. Figure 1. Conduction band of a conventional MOSFET for two different gate voltages in the device's off-state. Thermal emission over limitsS to60mV/dec at room temperature.