In this work, our earlier method for measuring resistance R_sh of semiconductor films with a near-field scanning microwave microscope [A. N. Reznik and S. A. Korolyov, J. Appl. Phys. 119, 094504 (2016)] is studied in a 0.1 kΩ/sq < R_sh < 15 kΩ/sq range. The method is based on a microscope model in the form of a monopole or dipole antenna interacting with an arbitrary layered structure. The model fitting parameters are determined from the data yielded by calibration measurements on a system of etalon samples. The performance of the method was analyzed experimentally, using strip-probe and coaxial-probe microscopes in the frequency range of 1-3 GHz. For test structures, we used doped GaN films on the Al₂O₃ substrate and also transistor structures based on the AlGaN/GaN heterojunction and AlGaAs/GaAs/InGaAs/GaAs/AlGaAs quantum well with a conducting channel. The obtained microwave microscope data were compared with the results of measurements by the van der Pauw method. At the first stage of the experiment, the calibration etalons were bulk homogeneous samples with different permittivity/conductivity values. In this case, satisfactory agreement between the microscope and the van der Pauw data was obtained with a strip probe on all tested samples in the entire range of R_sh. With a coaxial probe, such accordance was observed only in high-ohmic samples with R_sh > 1 kΩ/sq. The use of GaN film structures as a calibration system helped to increase the accuracy of the coaxial-probe-aided measurement of R_sh to a level of ∼10%.

We have developed an analytical model of a near-field microwave microscope based on a coaxial resonator with a sharpened tip probe. The probe interacts with a layered sample that features an arbitrary depth distribution of permittivity. The microscopic tip end with the accumulated charge is regarded as a monopole antenna radiating an electric field in near zone. The impedance of such an antenna is determined within a quasi-static approximation. The proposed model is used for calculating the sample-sensitive parameters of the microscope, specifically, resonance frequency f₀ and quality factor Q₀, as a function of probe-sample distance h. The theory has been verified experimentally in studies of semiconductor structures, both bulk and thin films. For measurements, we built a ∼2.1 GHz microscope with an effective tip radius of about 100 μm. The theoretical and experimental dependences f₀(h) and Q₀(h) were found to be in a good agreement. The developed theory underlies the method for determining sheet resistance R_sh of a semiconductor film on a dielectric substrate proposed in this article. Studies were performed on doped n-GaN films on an Al₂O₃ substrate. The effective radius and height of the probe determined from calibration measurements of etalon samples were used as the model fitting parameters. For etalon samples, we employed homogeneous sapphire and doped silicon plates. We also performed four-probe dc measurements of R_sh. The corresponding values for samples with R_sh > 1 kΩ were found to be 50% to 100% higher than the microwave results, which are attributed to the presence of microdefects in semiconductor films.

A receiver based on a low-barrier diode has been developed for use in a 94 GHz frequency-modulated continuous-wave radar. The receiver has a very simple design with the radio frequency (RF) front-end consisting of only a mixing diode mounted on a planar slot antenna. The low-barrier diode operates at a zero bias, thus the RF front-end does not require any power supply. A transimpedance amplifier transforms the mixer intermediate frequency current to voltage, which is the output signal of the receiver module. The receiver works at a low local oscillator (LO) power keeping conventional performance: a noise-equivalent power of 1.4×10 $^{-19}$ W/Hz at a LO power of 0.1 mW is achieved.

A new approach to millimeter-wave imaging was suggested and experimentally studied. This approach can be considered as the evolution of the well-established focal-plane array (FPA) millimeter-wave imaging. The significant difference is the use of a direct-conversion array receiver, instead of the direct-detection array receiver, along with the frequency-modulated continuous-wave (FMCW) radar technique. The sensitivity of the direct-conversion receiver is several orders higher than the sensitivity of the direct-detection one, which allows us to increase the maximum imaging range by more than one order of magnitude. The additional advantage of the direct-conversion technique is the opportunity to obtain information about the range to an object. The realization of the direct-conversion FPA imaging system was made possible due to original sensitive simple-designed receiving elements based on low-barrier Mott diodes. The suggested imaging method’s main characteristics, which include the achievable angular and range resolution and the achievable maximum imaging range, were studied. A maximum range of up to 100 m was experimentally determined. A 94 GHz 8 × 8 imaging system was developed for demonstration purposes and studied in detail. The suggested technique is assumed to be useful for creating a long-range millimeter-wave camera, in particular, for robotic systems that operate in poor environmental conditions.