Numerical Study of Lightly Doped Drain and Source Carbon Nanotube Field Effect Transistors

In this paper, we investigate the transport properties of carbon nanotube field-effect transistors (CNTFETs), with a nonequilibrium Green's function (NEGF) method. Tunneling leakage currents with respect to gate voltages are known effects for MOSFET-like CNTFETs (MOSCNTs). To minimize this phenomenon, we have proposed a structure with a simple modification of the MOSCNT by using lightly doped regions between the intrinsic channel and the highly doped source and drain regions, which we call the "lightly doped drain and source CNTFET (LDDS-CNTFET)." Simulations have shown that LDDS-CNTFET characteristics are related to the lightly doped region concentration. In comparison with an MOSCNT and a linearly doped CNTFET (LD-CNTFET), an LDDS-CNTFET with appropriately doped lightly doped drain and source regions has demonstrated a larger ON current (I-on), a larger ON-OFF ratio (I-on/I-off), a superior ambipolar characteristic, a shorter delay time, and also a smaller power-delay product. Furthermore, our results show that the channel length for an LDDS-CNTFET is shorter than that for an LD-CNTFET having the same OFF-state characteristics. Finally, the effect of the unavoidable Schottky barriers at the interface of the heavily doped source/drain regions and their metal electrodes has been taken into account. Simulations have demonstrated that these Schottky barriers have almost the same deteriorating effects on the characteristics for both LD-CNTFETs and LDDS-CNTFETs. Hence, all discussions regarding the superiority of the proposed structure are also valid in presence of the Schottky barriers.