A simple miniature source generating pulse trains with a central frequency of $100 GHz and a duration of 50-100 ps has been demonstrated recently. The source is based on nanometer-scale collapsing field domains (CFDs) generated in the collector of an avalanching bipolar GaAs transistor. The central frequency is determined by the domain transient time across the collector, and thus, a routine increase in the oscillation frequency from 0.1 to 0.3-0.5 THz would require a reduction in the collector thickness by a factor of 3-5. This is not acceptable, however, since it would reduce the maximum blocking voltage affecting the achievable peak current across the avalanche switch. We suggest here a solution to this challenging problem by reducing the CFD travel distance while keeping the collector thickness unchanged. Here, the discovered and interpreted phenomenon of CFD collapse when entering a dense carrier plasma zone made it possible by means of bandgap engineering. A CFD emitter generating $200 GHz wavetrains of $100 ps in duration is demonstrated. This finding opens an avenue for the increase in the oscillation frequency without any reduction in the emitted power, by using a smart structure design.
Progress in terahertz spectroscopy and imaging is mostly associated with femtosecond laser-driven systems, while solid-state sources, mainly sub-millimetre integrated circuits, are still in an early development phase. As simple and cost-efficient an emitter as a Gunn oscillator could cause a breakthrough in the field, provided its frequency limitations could be overcome. Proposed here is an application of the recently discovered collapsing field domains effect that permits sub-THz oscillations in sub-micron semiconductor layers thanks to nanometer-scale powerfully ionizing domains arising due to negative differential mobility in extreme fields. This shifts the frequency limit by an order of magnitude relative to the conventional Gunn effect. Our first miniature picosecond pulsed sources cover the 100–200 GHz band and promise milliwatts up to ∼500 GHz. Thanks to the method of interferometrically enhanced time-domain imaging proposed here and the low single-shot jitter of ∼1 ps, our simple imaging system provides sufficient time-domain imaging contrast for fresh-tissue terahertz histology.
Electric-field-induced reversible avalanche breakdown in a GaAs microcrystal due to cross band gap impact ionization Appl.Multiple "collapsing" field domains are a physical reason for superfast switching and sub-terahertz (sub-THz) emission experimentally observed in powerfully avalanching GaAs structures. This phenomenon, however, has been studied so far without considering carrier energy relaxation and that essentially has restricted the possibility of correct interpretation of experimental results. Here, we apply a hydrodynamic approach accounting for non-local hot-carrier effects. The results confirm the collapsing domain concept, but show that the domains cannot reduce well below 100 nm in width, since a moving collapsing domain leaves behind it a tail of hot carriers, which causes broadening in the rear wall of the domain. This puts principal restrictions on the emission band achievable with our unique avalanche mm-wave source to about 1 THz. Another finding suggested here is a physical mechanism for the single collapsing domain's quasi-steady-state motion determined by powerful impact ionization. The results are of significance for physical interpretation of properties of our pulsed sub-THz source, which has recently demonstrated its application potential in mmwave imaging in both amplitude and time-domain pulse modes with picosecond time-of-flight precision. V C 2015 AIP Publishing LLC. [http://dx.
Проведены исследования чувствительности параметров классических n+/n- GaAs и AlGaN/GaN структур с двумерным электронным газом (НЕМТ) и полевых транзисторов на их основе к gamma-нейтронному воздействию. Определены уровни их радиационной стойкости. Развит метод экспериментального исследования структур на основе дифференциального анализа вольт-фарадных характеристик, позволяющий определять слои структуры, в которых накапливаются радиационные дефекты. Впервые предложена методика, позволяющая учесть изменение площади обкладок измеряемой емкости барьерного контакта, связанной с возникновением кластеров радиационных дефектов, формирующих диэлектрические включения в слое двумерного электронного газа.
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