Terahertz Response of Carbon Nanotube Transistors

We present an approach for time-dependent quantum transport based on a self-consistent nonequilibrium Green function formalism. The technique is applied to a ballistic carbon nanotube transistor in the presence of a time-harmonic signal at the gate. In the on state, the dynamic conductance exhibits plasmonic resonant peaks at terahertz frequencies. These vanish in the off state, and the dynamic conductance displays smooth oscillations, a signature of single-particle quantum effects. We show that the nanotube kinetic inductance plays an essential role in the high-frequency behavior.

Figure 1

(a) Cross section of the cylindrical NTFET geometry. (b) dc transfer characteristics for $L=30\mathrm{nm}$. (c) Band diagram in the on and off states, as marked by arrows in panel (b); the dashed-dotted line at $-1\mathrm{eV}$ is Fermi level.

Figure 2

ac response in the off state for a $L=30\mathrm{nm}$ NTFET: (a) real/imaginary part of $g(\omega )$. (b) Color plot of the density of states along the channel, showing the resonant photoexcitation of carriers through energy and spatially oscillating quantum states. The valence band edge is marked with the solid black line, and $E_F=-1\mathrm{eV}$.

Figure 3

Panel (a) shows the frequency-dependent conductance for a NTFET with $L=30\mathrm{nm}$. The top inset compares the self-consistent and non-self-consistent results. The bottom inset shows the dielectric function versus frequency. Panel (b) shows the periodic potential near the plasmon frequencies for $L=90\mathrm{nm}$. Panel (c) shows the response for three different channel lengths.

Figure 4

Real part of the dynamic capacitance $C(\omega )$ for $L=30\mathrm{nm}$ in the on state. Inset: RLC model for the dynamic capacitance.