  We analyze the terahertz detection characteristics of resonant tunneling
diodes (RTDs) using a set of simple equations that covers three detection
modes; (i) direct detection, (ii) amplified detection, and (iii) self-homodyne
(coherent) detection. (i) and (ii) are based on the square-law detection, and
(iii) is on the homodyne detection with the RTD used as an injection-locked
local oscillator. The calculated results exhibit small- and large-signal areas
depending on irradiation power. In the small-signal area, the detection current
is proportional to irradiated power for (i) and (ii), and to square root of
irradiated power for (iii). The detection current has a peak at the bias
voltage at the boundary between (ii) and (iii). Effect of frequency fluctuation
of irradiated wave is analyzed for (iii), and it is shown that the detection
current is proportional to irradiated power if the fluctuation becomes wider
than injection-locking range. The analytical results in this paper reasonably
explain the reported experiments.
