Optimization of Source/Drain Doping Level of Carbon Nanotube Field-Effect Transistors to Suppress OFF-State Leakage Current while Keeping Ideal ON-State Current

Abstract

The effects of doping concentration variation in source/drain junctions on the characteristics of carbon nanotube field-effect transistors (CNFETs) have been studied in order to realize high performance CNFETs, where suppressed OFF-state leakage current and ideal maximum ON-state current were obtained. The characteristics of CNFETs with doped source/drain regions have been studied by solving the Poisson and carrier transport equations self-consistently. The transmission coefficient through the bandgap (Eg) has been calculated using the Wentzel–Kramers–Brillouin (WKB) approximation in order to take into account the band-to-band tunneling (BTBT) leakage current. The doping is characterized by the Fermi level (Ed) in a doped region which is measured from the conduction band edge. In this study, it is demonstrated that, when the power supply voltage (Vdd) is greater than the bandgap of carbon nanotubes (CNTs), optimized doping level Ed of Vdd-Eg shows the lowest OFF-state current (IOFF). On the other hand, when Vdd is smaller than Eg, IOFF monotonically decreases as Ed decreases, although the aggressively lowered doping concentration results in lower ON-state current. We also demonstrated that CNFETs with low source/drain doping concentration exhibit current saturation at higher gate voltages. The current saturation results from the fact that the injection velocity to the channel is limited by the velocity in the source region which is determined by the source doping level. We showed that, in order to avoid the current saturation, doping level should be higher than 0.3 eV, regardless of carbon nanotube diameter.