Electron viscosity dispersion and terahertz and ultra-short pulse detection

Abstract

The channel lengths of deep submicron transistors and photodetectors reached dimensions smaller than the mean free path. These devices could operate at frequencies comparable to or higher than the electron momentum collision frequency. Their ballistic and quasi-ballistic operation regimes are dramatically different from a more conventional collision-dominated transport. In these regimes, the measured apparent electron mobility becomes proportional to the channel length (so-called "ballistic mobility"). At high mobility values, the electron transport becomes viscous, and the electron density oscillations in ultra short channels are damped by the electronic viscosity rather than by the collisions with impurities or lattice vibrations. In this paper, we show that viscosity dispersion is an important effect in determining the effective viscosity at high frequencies and/or for very short excitation pulses. The characteristic frequency determining the viscosity dispersion is the electron-electron collision frequency, which depends on the electron concentration and temperature. Using a Drude-like equation for the viscosity, we evaluate the viscosity dispersion effect for deep submicron devices implemented in Si, AlGaN/ GaN, p-diamond, and AlGaAs/InGaAs. The applications of the viscosity dispersion effect include terahertz and IR photodetectors. This effect is especially important for the detection of femtosecond and terahertz pulses.