Résumé
The nonresonant detection of terahertz detectors based on asymmetric dual-grating-gate (A-DGG) high electron mobility transistors has been reported at both terahertz and sub-terahertz frequencies in several papers; but its behavior could not be fully understood in the range of the gate voltages close to and below the threshold voltage Vth. Two different contributions can attenuate the photoresponse u in the sub-threshold voltage region. The model developed in Ref. 3 showed that the gate leakage current suppresses the detector terahertz photoresponse in the sub-threshold region, leading to a nonresonant maximum in photoresponse versus gate dependence. While, in the model of Ref. 4, the detector loading effects were considered as responsible for the signal drop in the sub-threshold range. These effects are related to the voltage divider formed by input impedance of the measurement apparatus and the channel resistance of the detector which is exponentially increasing below threshold [4]. In the framework of the ANR-JST WITH project, we performed detailed studies of both the voltage u and the current i photoresponse of A-DGG devices. The photoresponse i was determined using the transfer characteristics measured with and without applied terahertz radiation. The measurements were carried out as a function of gate voltage and temperature (4 - 300 K), for incoming radiation frequencies of 292 and 655 GHz. As a result, we show that both effects are important and their relative contributions depend on gate voltage and temperature. We determine the separate contributions of the gate leakage current and the loading effect to the terahertz rectification signal in the sub-threshold region.