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It has been shown recently that only a small part of the emitter-base interface in a Si bipolar junction transistor participates in short-pulsing avalanche switching. This lateral current shrinkage attributed to the “winner takes all” effect reduces the transistor switching size from 1600 to ∼100 μm, still remaining much larger than the transistor structure thickness. We show using quasi-3-D transient modelling that the size of the operating perimeter, which is critically important for switching efficiency and device reliability, is determined by competition between lateral turn-on shrinkage and spread. The latter has never been demonstrated in avalanche transistors before.

Although Marx-bank connection of avalanche transistors is widely used in applications requiring high-voltage nanosecond and subnanosecond pulses, the physical mechanisms responsible for the voltage-ramp-initiated switching of a single transistor in the Marx chain remain unclear. It is shown here by detailed comparison of experiments with physical modeling that picosecond switching determined by double avalanche injection in the collector-base diode gives way to formation and shrinkage of the collector field domain typical of avalanche transistors under the second breakdown. The latter regime, characterized by a lower residual voltage, becomes possible despite a shortconnected emitter and base, thanks to the 2-D effects.Index Terms-Avalanche transistor, device phenomenon, experiment and physical modeling, high-speed avalanche switching, Marx generators.

It has been shown that the same avalanche transistor can generate both short (8A/2 ns) and longer high-current pulses (90 A/7 ns), but the operating perimeter length self-organized by the transistor is much smaller in the first case (∼0.1 mm) than in the second (1.6 mm). Since the two-dimensional approach failed to explain this experimental fact, we present here an interpretation using quasi-three-dimensional modelling. Spatial triggering inhomogeneity should not exceed ∼5% for the transistor to survive when generating long pulses, while the same moderate inhomogeneity ensures short-pulsing operation because powerful current filamentation quenches the switching in the rest of the perimeter.

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.

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A traditional Marx circuit (TMC) based on avalanche transistors with a shortened emitter and base was investigated numerically using a 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 explains 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.