  We studied time resolution and response power dependence of three terahertz
detectors based on significantly different types of field effect transistors.
We analyzed the photoresponse of custom-made Si junctionless FETs, Si MOSFETs
and GaAs-based high electron mobility transistors detectors. Applying
monochromatic radiation of high power, pulsed, line-tunable molecular THz
laser, which operated at frequencies in the range from 0.6-3.3 THz, we
demonstrated that all these detectors have at least nanosecond response time.
We showed that detectors yield a linear response in a wide range of radiation
power. At high powers the response saturates varying with radiation power P as
$U = R_0 P/(1+P/P_s)$, where $R_0$ is the low power responsivity, $P_s$ is the
saturation power. We demonstrated that the linear part response decreases with
radiation frequency increase as $R_0 \propto f^{-3}$, whereas the power at
which signal saturates increases as $P_s \propto f^3$. We discussed the
observed dependences in the framework of the Dyakonov-Shur mechanism and
detector-antenna impedance matching. Our study showed that FET transistors can
be used as ultrafast room temperature detectors of THz radiation and that their
dynamic range extends over many orders of magnitude of power of incoming THz
radiation. Therefore, when embedded with current driven read out electronics
they are very well adopted for operation with high power pulsed sources.
