  We report on the numerical and theoretical results of sub-THz and THz
detection by a current-driven InGaAs/GaAs plasmonic Field-Effect Transistor
(TeraFET). New equations are developed to account for the channel length
dependence of the drain voltage and saturation current. Numerical simulation
results demonstrate that the effect of drain bias current on the
source-to-drain response voltage (dU) varies with the device channel length. In
a long-channel TeraFET where plasmonic oscillations cannot reach the drain, dU
is always positive and rises rapidly with increasing drain current. For a short
device in which plasmonic oscillations reach the drain, the current-induced
nonuniform electric field leads to a negative response, agreeing with previous
observations. At negative dU, the amplitude of the small-signal voltage at the
drain side becomes larger than that at the source side. Thus, the device
effectively serves as a THz amplifier in this condition. Under the resonant
mode, the negative response can be further amplified near the resonant peaks. A
new expression of dU is proposed to account for this resonant effect. Based on
those expressions, a current-driven TeraFET spectrometer is proposed. The ease
of implementation and simplified calibration procedures make it competitive or
superior compared with other TeraFET-based spectrometers.
