But D. Process of charge transfer and registration of terahertz radiation in structures based on silicon

The thesis presents the state of the art of terahertz detectors, as well as the possible practical applications for terahertz systems. The development of concepts of terahertz field-effect transistors detector, which can operate under non-cryogenic conditions and which can be fabricated as multi-pixel imaging sensors have progressed over the last few years. Complementary metal–oxide–semiconductor technology manufacturing techniques provide cost-efficient solutions for most parts of the electromagnetic spectrum and can provide large arrays of detectors. This thesis is devoted to study of terahertz detectors based on field-effect transistors fabricated using low-cost silicon complementary metal–oxide–semiconductor technology and they are compared to InGaAs/InP ones. The main research effort was devoted to the problem of detectors linearity at high radiation intensities. The photoresponse of field effect transistors to terahertz radiation in a wide range of intensities from 0.5 mW/cm^2 up to 500 kW/cm^2 and for frequencies from 0.13 THz to 3.3 THz was studied. This work shows that the photoresponse of all studied detectors increases linearly with increasing radiation intensity up to a few kW/cm2 range and are followed by the nonlinear and saturation parts for higher radiation intensities. This effect has led to the new model of broadband field-effect transistor detectors. The model is based on the phenomenological knowledge of the transistor static transfer characteristic and explains the photoresponse nonlinearity as related to non-linearity and saturation of the transistor channel current. Although the model uses the metal–oxide–semiconductor field-effect transistor approach, it can be easily be generalized to describe also terahertz detection by III-V high-electron-mobility transistors and used to account of loading impedances and parasitic circuit elements. The developed model explains consistently experimental data both in linear and nonlinear regions of terahertz detection. Also the thesis presents studies of broadband terahertz detection by different GaN/AlGaN, and Si field-effect transistors in the temperature range of 5–380 K. This work has analyzed the results using a theoretical model and it was shown that the shape of the photoresponse on the gate voltage dependence is defined by the transfer characteristics at all temperatures. The photoresponse increased by about one order of magnitude with decreasing of the transistor temperature. For lower temperatures, the photoresponse value was saturated probably due to the change of the dominant electron transport mechanism. Our results clearly show that terahertz detectors based on InGaAs or Si field effect transistors can be improved by lowering temperature. They also show that below nitrogen temperature the further improvement is hindered by the physics of the detection process itself. For the case of temperatures above 300 K, we proposed to take into account the temperature dependence of transistor parameters which made it possible to fit the experimental results of the terahertz field-effect transistors detection. The possibility of using field-effect transistor detectors in communication systems at a frequency of 0.2 THz with a bandwidth of over 10 GHz are experimentally demonstrated in the thesis. Also, the thesis shows a practical idea on how the terahertz detectors can be utilized to produce terahertz images and characterize integrity and homogeneity of polymer materials. Thus, the thesis presents techniques that were used in our laboratory for single pixel raster-scanned imaging. Along the way, the result that confirmed the assumption about the coefficient of frequency dependence of responsivity for broadband detection by terahertz field-effect transistor detector in linear region at frequency range from 1 THz to 3.3 THz was obtained in the thesis. This result shows an estimate of the lower threshold of responsivity for broadband detection. A responsivity can be improved by the use of a narrow-band antenna for the especial narrow frequency band. Experimental data were successfully interpreted in the frame of the extended model of THz FET detection. Both experiments data and theoretical model show that dynamic range of field effect transistors based terahertz detectors extends over many orders of magnitude of intensity of incoming terahertz radiation.