  We show that Si MOSFETs, AlGaN/GaN HEMTs, AlGaAs/InGaAs HEMTs, and p-diamond
FETs with feature sizes ranging from 20 nm to 130 nm could operate at room
temperature as THz spectrometers in the frequency range from 120 GHz to 9.3 THz
with different subranges corresponding to the transistors with different
features sizes and tunable by the gate bias. The spectrometer uses a
symmetrical FET with interchangeable source and drain with the rectified THz
voltage between the source and drain being proportional to the sine of the
phase shift between the voltages induced by the THz signal between
gate-to-drain and gate-to-source. This phase difference could be created by
using different antennas for the source-to-gate and drain-to gate contacts or
by using a delay line introducing a phase shift or even by manipulating the
impinging angle of the two antennas. The spectrometers are simulated using the
multi-segment unified charge control model implemented in SPICE and ADS and
accounting for the electron inertia effect and the distributed channel
resistances, capacitances and Drude inductances.
