Modified high-power nanosecond Marx generator prevents destructive current filamentation

Abstract A traditional Marx circuit (TMC) based on avalanche transistors with a shortened emitter and a base was investigated numerically by using a two-dimensional (2-D) physics-based approach and experimentally, and compared with a special Marx circuit (SMC) suggested here, in which an intrinsic base triggering of all the stages protects the transistors, especially the second one, from thermal destruction due to current filamentation. This is because the entire emitter-base perimeter in the SMC participates in switching, whereas in a TMC the switching is initiated across the entire area of the emitter but then changes to current filamentation due to certain 3-D transient effects reported earlier. Very significant difference in local transient overheating in the transistors operating in TMC and SMC determines the difference in reliability of those two pulse generators. The results suggest a new circuit design for improving reliability and explain the difference in the operating mode of different transistors in the chain which makes the second transistor most prone to destructive thermal filamentation. This new understanding points additionally to ways of optimizing the design of the transistors to be used in a Marx circuit

Contactless terahertz sensing of ultrafast switching in Marx generator based on avalanche transistors

Abstract In this letter, we have studied the temporal evolution of switching for each stage of the Marx generator with picosecond temporal and millimeter spatial resolutions employing terahertz measurements. The Marx circuit utilizes collapsing-field-domain (CFD)-based avalanche switches, which are formed in a bipolar GaAs structure and result in the picosecond speed of powerful carrier generation and electrical switching. The application of the CFD-based avalanche switches emitting mm-wave pulsed radiation in the Marx generator provides a unique opportunity to accurately track the switching instants for each of the circuit stages with a picosecond time precision. The collapsing domains cause the sub-THz pulses radiated by each of the avalanche switches, and the same domains generate the electron-hole plasma thus causing simultaneously the electrical switching. In this work, we report the direct measurements of the switching instants for each of the four stages Marx generator and suggest an interpretation of non-trivial experimental results

Suppression of dynamic current leakage in avalanche S-diode switching circuits

Abstract This work investigates the dynamic current leakage of $S$-diode, which is a GaAs-based avalanche switch doped with deep Fe acceptor traps. The dynamic leakage has negative effect on superfast switching parameters of this unique device, and here we suggest an original way of reducing the leakage by means of circuit design. It is shown that an additional bias for avalanche S-diode in the current pulse generation circuit forms a negatively charged layer of iron traps near the electron-injecting junction. As a result, the concentration of nonequilibrium electrons goes down, which leads to a decrease in leakage current by ∼3–4 times, and a rise in S-diode switching voltage. The results were obtained in the experimental study and are approved by calculation

Time-domain terahertz imaging of layered dielectric structures with interferometry-enhanced sensitivity

Abstract This article presents a time-domain imaging technique for layered dielectric slabs using a solid-state wavelet generator with subterahertz carrier frequency. The technique utilizes the dual nature of a wavelet, i.e., both the applicability of time-of-flight measurements and the ability of wavelets to interfere in thin dielectric layers at a carrier frequency that is preserved in spite of the ultrawideband character of the signal. This results in a very high sensitivity of the time delay of the resultant pulse to variations in the effective thickness (thickness × refractive index) of the dielectric layer. It is shown using a plane-wave analysis of the pulse propagation that under certain conditions, this sensitivity enhancement can reach an order of magnitude. The experimental setup for the reflection-mode operation is described and its performance in the discrimination of healthy and malignant tissues and in the detection of corrosion under paint is demonstrated

Miniature high-power nanosecond laser diode transmitters utilizing simplest avalanche drivers

Abstract The-state-of-the-art in long-distance near-infrared optical radars is the use of laser-diode-based miniature pulsed transmitters producing optical pulses of 3–10 ns in duration and peak power typically below 40W. The duration of the transmitted optical pulses becomes a bottleneck in the task of improving the radar ranging precision, particularly due to the progress made in developing single photon avalanche detectors (SPADs). The speed of miniature high-current drivers is limited by the speed of the semiconductor switch, either a gallium nitride (GaN) field-effect transistor, the most popular alternative nowadays, or a silicon avalanche bipolar junction transistor (ABJT), which was traditional in the past. Recent progress in the physical understanding of peculiar 3-D transients promises further enhancement in speed and efficiency of properly modified ABJTs, but that is not the only factor limiting the transmitter speed. We show here that a low-inductance miniature transmitter assembly containing only a specially developed capacitor, a more advanced transistor chip than that used in commercial ABJTs and a laser diode has allowed peak power from 40 to 180 W to be reached in optical pulses of 1–2 ns in duration without after-pulsing relaxation oscillations. This finding is of interest for compact low-cost, long-distance decimetre-precision lidars, particularly for automotive applications

Avalanche delay and dynamic triggering in GaAs-based S-diodes doped with deep level impurity

Abstract The article is concerned with a detailed switching delay effect exhibited by avalanche S-diodes-superfast GaAs closing switches doped with deep Fe centers. The current and voltage time dependences are simulated in a simplified generator. The dynamic electric field and charge profiles in the structures are calculated. This article describes an impact that Fe capture cross sections of free charge carriers have on delayed switching. The simulation results show that delayed switching is associated with deep center recharging in a double injection mode due to three different processes. There are two different delay mechanisms to be herewith distinguished. A delay effect is experimentally viewed to control the dynamic switching voltage (and the avalanche breakdown voltage) using constant voltage adjustment capability enabled by a triggering circuit supply. The authors demonstrate the way it is possible to adjust the amplitude of current nanosecond pulses in the range of 20—45 A through a lidar transmitter circuit with a semiconductor laser and nonoptimized S-diode. The findings are consistent with the results of numerical simulation

Collapsing-field-domain-based 200 GHz solid-state source

Abstract 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

Interferometrically enhanced sub-terahertz picosecond imaging utilizing a miniature collapsing-field-domain source

Abstract 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