5.1.1 Ambipolar Conduction

A single-gate CNT-FET with symmetric gate-source and gate-drain spacer lengths is shown in Fig. 5.1. It is assumed that the SCHOTTKY barrier heights for electrons and holes are equal. The output and transfer characteristics of this device for different biases are shown in Fig. 5.2 and Fig. 5.3, respectively. To obtain a better insight into the device operation the band-edge profiles along the device are also shown.

Figure 5.1: Sketch of the single-gate (SG) structure. $T_\textrm {Ins}$=10 nm and $L_\textrm {GS}$= $L_\textrm {GD}$=4 nm.

Fig. 5.2 and Fig. 5.3 show that electron current through the source-sided barrier is both tunneling and thermionic emission. However, there is a parasitic current due to the thermionic emission of holes across the drain sided barrier. If the drain voltage becomes higher than the gate voltage, the thickness of the drain-sided SCHOTTKY barrier for holes is reduced. As a result, the parasitic band-to-band tunneling current of holes increases and ambipolar conduction occurs.

Figure 5.2: Right figures show the output characteristics and left ones the corresponding band-edge profile. a) $V_\textrm {D}$=0.2 V, b) $V_\textrm {D}$=0.4 V, and c) $V_\textrm {D}$=0.6 V.

The current contributions of electrons and holes are represented by blue and red curves, respectively. Note that at transition points electrons and holes have the same contribution to the total current, whereas in other regions either the electron or hole contribution will dominate. The results indicate that the ambipolar conduction has a detrimental effect on the device operation in both the on- and off-state.

Figure 5.3: Right figures show the transfer characteristics and left ones the corresponding band-edge profile. a) $V_\textrm {G}$=0.3 V, b) $V_\textrm {G}$=0.1 V, and c) $V_\textrm {G}$=0.0 V.

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