5.4.1 Symmetric and Asymmetric Doping

The operation of the device can be well understood by considering the spectrum of the electron density along the device (Fig. 5.17). At high negative gate voltages, due to strong band bending near the source contact, band to band tunneling contributes significantly to the total current. By increasing the gate voltage to positive values the band bending near the source contact decreases, and as a result band to band tunneling decreases.

On the other hand, the increase of the gate voltage results in strong band to band tunneling near the drain contact. As a result the total current increases in the off-regime (Fig. 5.18-a) which has a detrimental effect on the device performance. Fig. 5.19-a shows that the parasitic current increases if the drain voltage becomes much higher than the gate voltage. For the device with symmetric doping we assumed that the donor and acceptor concentrations at the source and drain contacts are $N_\mathrm{D_S}=N_\mathrm{A_D}= 2 \times 10^{9}~\mathrm{m^{-1}}$. By decreasing the doping of the drain side, the band bending decreases for the same gate voltage (Fig. 5.17-b) and the band to band tunneling current near the drain contact decreases considerably (see Fig. 5.18-b and Fig. 5.19-b). For the device with asymmetric doping profile, $N_\mathrm{D_S}= 2\times 10^{9}~\mathrm{m^{-1}}$ and $N_\mathrm{A_D}= 5\times 10^{8}~\mathrm{m^{-1}}$.

Figure 5.18: The transfer characteristics for a) symmetric and b) asymmetric doping.

Figure 5.19: The output characteristics for a) symmetric and b) asymmetric doping.

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