5.4 Tunneling CNT-FETs

To reduce the parasitic capacitance the spacing between the gate-source and the gate-drain contacts can be increased. The extension region can be of n or p-type leading to n/i/n or p/i/p devices. Unlike conventional semiconductors in which doping is introduced by ion implantation, doping of CNTs requires controlling the electrostatics of the CNT environment (see Section 2.8.3) by additional gates [270], molecules [271], or metal ions [272].

In n/i/n or p/i/p devices the gate controls the thermionic emission current [270]. Aggressively scaled devices of this type suffer from charge pile-up in the channel [273,274], which deteriorates the off-current substantially and ultimately limits the achievable $ \ensuremath{I_\mathrm{on}}/\ensuremath{I_\mathrm{off}}$ ratio [273]. To overcome this obstacle a gate-controlled tunneling FET (T-CNT-FET) has been proposed [275].

In T-CNT-FET devices either a p/i/n or n/i/p doping profile can be used. The gate voltage modulates the band to band tunneling current. T-CNT-FETs benefit from a steep inverse sub-threshold slope and a better controlled off-current. In this section the effect of symmetric and asymmetric doping on the device characteristics is discussed.

Figure 5.17: The electron density spectrum along the device with a) symmetric and b) asymmetric doping profile.


M. Pourfath: Numerical Study of Quantum Transport in Carbon Nanotube-Based Transistors