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The thermal mechanism of the transition from t h e superconducting to normal state as a result of the superconducting film heating by a strong electromagnetic wave has been theoretically investigated. The thermal balance equation for a thin film on dielectric substrate, stabilized at a temperature To is solved. The absorption of radiation in the film is calculated using a two-fluid model of a superconductor. The hysteresis character of t h e SN transition is shown. The MW and IR power limiter is proposed. The limiter is a superconducting film mirror, which limits the reflected power. The dependences of the threshold value of incident power against To for some frequencies are calculated.

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 Rsh 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 Rsh. The corresponding values for samples with Rsh > 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.