Semiclassical and Quantum Transport in CNTFETs Using Monte Carlo Simulation

The effects of quasi-ballistic and quantum transport on the operation and the performance of carbon nanotube FETs are examined by means of both Boltzmann and Wigner Monte Carlo simulations including phonon scattering. The semiclassical simulation of transistors of gate length in the range 10-100 nm shows the strong ballisticity of the transport in the gated part of the channel, while the emission of high-energy phonons occurs only beyond the drain end of the channel. For a gate length of 25 nm, the fraction of ballistic electrons reaches 89%, instead of typically 31% in the silicon FET of the same gate length with the undoped channel. For a gate length higher than 20 nm, the main quantum-transport effect is the reflection of carriers due to the sharp potential drop at the drain end of the channel. In spite of strong microscopic differences observed when comparing Wigner and Boltzmann functions, this effect just makes the quantum current slightly smaller than the semiclassical one on the full range of the gate voltage. For a smaller gate length, the source-drain tunneling becomes possible in the subthreshold regime, which enhances the subthreshold slope and the off-current in the quantum simulation.