Guoyong Duan

Lateral Current Confinement Determines Silicon Avalanche Transistor Operation in Short-Pulsing Mode

The transient in a Si bipolar junction transistor was investigated in high-current short-pulsing ( 2 ns) mode both experimentally and numerically. A comparison of measured and simulated waveforms clearly showed that only a small fraction of the perimeter of the emitter-base interface (in the lateral direction) takes part in the switching transient when a capacitor of relatively small value (80 pF) is discharged across the transistor to obtain a current pulse of a few nanoseconds in duration. A good agreement was found between measurements and simulations in the 2-D numerical model when the effective operating perimeter was used as a parameter in the model. The results allowed reliable analyses of the thermal regime to be performed. Possible reasons for the significant current confinement in short-pulsing mode and relatively homogeneous transistor switching with longer current pulses are discussed, and a mechanism of fast lateral turn-on spread is assumed. One conclusion of practical importance is that a short-pulsing relatively high-current mode could not be realized without current confinement in the lateral direction.

Three-dimensional peculiarities in an avalanche transistor provide a broadened range of amplitudes and durations of the generated pulses

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.

Turn-on spread determines the size of the switching region in an avalanche transistor

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.

Pulsed time-of-flight radar for fiber-optic strain sensing

This article describes a fiber-optic interrogation device based on the pulsed time-of-flight technique. The apparatus is capable of measuring time delays between wideband reflectors, such as connectors, along a fiber path with a precision of about 280 fs (rms value) and a spatial resolution of about 3 ns (0.30 m) in a measurement time of 25 ms. Potential application areas include measuring integral strain and its derivatives such as cracks, deflections, and displacements, particularly in large civil engineering and composite structures. The operation and basic blocks of the measurement system are presented in detail together with measurement results obtained in laboratory and field conditions. It is shown that by using a fiber loop sensor with a reference fiber, it is possible to achieve a strain precision below 1 microstrain and a measurement frequency of 4 Hz. System performance proved adequate for the study of both static and dynamic phenomena in a bridge deck.

Switching Mechanisms Triggered by a Collector Voltage Ramp in Avalanche Transistors With Short-Connected Base and Emitter

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 short-connected emitter and base, thanks to the 2-D effects.

3-D Properties of the Switching Transient in a High-Speed Avalanche Transistor Require Optimal Chip Design

We have recently shown that only a small part of a Si bipolar junction transistor (BJT) conducts the current in a short-pulsing mode (≤ 2 ns), and a complicated temporal variation takes place in the size of operating emitter-base perimeter. Namely, the switched-on region in the corner of an emitter finger first shrinks down to just a few micrometers and only then spreads to ~ 100 μm by the end of the transient. Additionally important is the demonstrated ability of a tiny filament (≤ 10 μm) to quench the switching in the entire perimeter (1.6 mm). This creates the impression that an initial triggering inhomogeneity of the smallest size will always win the switching competition. It has been shown experimentally, however, that the sharpest corners (in size) “lose out” to the ~ 100 μm corners, a fact that has not been explained so far. It is shown here using quasi-3-D modeling that an optimal curvature for the corner of an emitter finger exists that provides minimal switching delay, resulting in the shortest current pulses of the highest amplitude. This finding is especially important when designing unique subnanosecond avalanche BJTs, the 3-D transient properties of which are of major importance.

Chalcogenide glass surface passivation of a GaAs bipolar transistor for unique avalanche terahertz emitters and picosecond switches

The ultra-narrow “collapsing” field domains discovered recently in avalanching GaAs bipolar junction transistor provide a physical basis for designing unique THz emitters and superfast switches. Reliability in devices operating near their volume breakdown voltage requires decisive suppression of premature surface breakdown. We demonstrate here complete, durable surface breakdown suppression through simple deposition of a massive chalcogenide glass layer on the mesa surface by means of a negative charge formed at the interface.

Peculiarities of Surface Breakdown in GaAs Bipolar Junction Structures

An avalanching GaAs bipolar junction transistor can operate as an effective terahertz source or as a superfast voltage/current switch, with each unique function offering prospects for various applications. As the transistor is operating near its breakdown voltage, the most probable destruction mechanism is device shortening at the mesa surface caused by surface breakdown. This manifests itself in measured I -V curves as a “soft” increase in surface current within the voltage range lying well below the volume breakdown. Surprisingly, the mechanism of surface breakdown has not properly been investigated or interpreted, despite the long history of the problem. We show here by comparing experimental results with those of 2-D numerical simulations that the “soft” increase in the surface current is, in fact, a premature breakdown that is suppressed by impact-generated electrons trapped at the surface. These negatively charged surface traps cause expansion of the space charge region and reduce the peak electric field near the surface, thus drastically increasing the voltage range over which avalanching can exist at the surface without fatal current growth. This mechanism explains various peculiar features of surface breakdown and should be taken into account when analyzing device reliability, surface breakdown transients, or passivation methods.

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

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.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 pr...

Modified High-Power Nanosecond Marx Generator Prevents Destructive Current Filamentation

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.