Ultimate Drift Velocity Research Articles

Analytical Study of Drift Velocity in Low Dimensional Devices

Understanding of quantum limit in low dimensional devices helps to develop the new device types same as Carbon Nanotube Field Effect Transistor (CNTFET) and Naonowire. For each dimensionality the limitations on carrier drift velocity due to the high-field streaming of otherwise randomly velocity vector in equilibrium is reported. The results are based on the asymmetrical distribution function that converts randomness in zero-field to streamlined one in a very high electric field. The ultimate drift velocity for all dimensions is found to be appropriate thermal velocity for a nondegenerately doped sample of silicon, increasing with the temperature, but independent of carrier concentration. However, the ultimate drift velocity is the Fermi velocity for degenerately doped silicon increasing with carrier concentration but independent of the temperature.

The high-field drift velocity in degenerately-doped silicon nanowires

The charge transport in nanowires suitable for high-speed applications depends on charge carrier's mobility and saturation velocity in the conducting channel. It is shown that the high mobility does not always lead to higher carrier drift velocity. The ultimate drift velocity (the intrinsic velocity) due to the high-electric-field streaming is based on the asymmetrical distribution function that converts randomness in zero-field to a streamlined one in a very high electric field. The saturation velocity limited to the intrinsic velocity is an appropriate thermal velocity for a non-degenerately doped nanowire, increasing with the temperature, but independent of carrier concentration. However, this saturation velocity is the appropriate Fermi velocity for a degenerately doped silicon nanowire, increasing with carrier concentration but independent of temperature. The results obtained are applied to the modelling of a silicon nanowire transistor.

Current–voltage characteristics of a silicon nanowire transistor

The nanowires and nanotubes are being considered as the best candidates for high-speed applications. It is shown that the high mobility does not always lead to higher carrier velocity. The ultimate drift velocity due to the high-electric-field streaming are based on the asymmetrical distribution function that converts randomness in zero-field to streamlined one in a very high electric field. The limited drift velocity is found to be appropriate thermal velocity for a nondegenerately doped sample of silicon, increasing with the temperature, but independent of carrier concentration. However, the limited drift velocity is the Fermi velocity for a degenerately doped silicon nanowire, increasing with carrier concentration but independent of temperature. The results obtained are applied to the modeling of the current–voltage characteristics of a nanowire transistor.

The Ultimate Ballistic Drift Velocity in Carbon Nanotubes

The carriers in a carbon nanotube (CNT), like in any quasi‐1‐dimensional (Q1D) nanostructure, have analog energy spectrum only in the quasifree direction; while the other two Cartesian directions are quantum‐confined leading to a digital (quantized) energy spectrum. We report the salient features of the mobility and saturation velocity controlling the charge transport in a semiconducting single‐walled CNT (SWCNT) channel. The ultimate drift velocity in SWCNT due to the high‐electric‐field streaming is based on the asymmetrical distribution function that converts randomness in zero‐field to a stream‐lined one in a very high electric field. Specifically, we show that a higher mobility in an SWCNT does not necessarily lead to a higher saturation velocity that is limited by the mean intrinsic velocity depending upon the band parameters. The intrinsic velocity is found to be appropriate thermal velocity in the nondegenerate regime, increasing with the temperature, but independent of carrier concentration. However, this intrinsic velocity is the Fermi velocity that is independent of temperature, but depends strongly on carrier concentration. The velocity that saturates in a high electric field can be lower than the intrinsic velocity due to onset of a quantum emission. In an SWCNT, the mobility may also become ballistic if the length of the channel is comparable or less than the mean free path.