Vladimir Egorkin

Effect of hot-carrier energy relaxation on main properties of collapsing field domains in avalanching GaAs

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 domains quasi-steady-state motion determined by powerful impact ionization. The results are of si...

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.

Investigation of Electro-Physical and Transient Parameters of Energy Accumulating Capacitors Applied in Nanosecond and Sub-nanosecond High-Current Avalanche Switches

Peak power of nanosecond and sub-nanosecond high-power pulsed optical transmitters was found to be drastically affected not only by the speed of the avalanche switch, but also by structure and geometry of the capacitive energy accumulator, and assembly construction. Together with trivial effect of parasitic inductance of the entire switching loop, it was found that some capacitors, even those designed for microwave use, are unable to release the charge in sub-nanosecond time scale when large signal operation (high current) is required. The problem was successfully solved using specially developed surface-mounted capacitors utilizing high-quality silicon nitride using plasma-deposition method. Finally miniature capacitors permitting direct assembling with avalanche transistor chips and laser diode with minimum possible (~1 nH) parasitic inductance withstand up to 600 V and allow nanosecond 20–30 A current pulse generation and optical pulses exceeding 30 W.

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...

Optimization of ohmic contacts to n -GaAs layers of heterobipolar nanoheterostructures

This paper investigates ohmic contacts to n-GaAs layers of the heterobipolar nanoheterostructures obtained through electron-beam evaporation of Ge, Au, Ni, and Au layer-by-layer. The effect of the firing time and temperature on the contact resistance is considered. Based on the analysis of the characteristics of the ohmic contacts, a firing installation of a special design and a firing technique are developed. The technique ensures the minimum contact resistance for the minimum size of a transition layer, satisfactory morphology, and even edges of the contacts.

Nanosecond Miniature Transmitters for Pulsed Optical Radars

The-state-of-the-art in long-distance near-infrared optical radars is utilization the laser-diode-based miniature pulsed transmitters producing optical pulses of 3–10 ns in duration and peak power typically below 40 W. The bandwidth of the receiving channel nowadays exceeds 300 MHz, and thus the duration of the optical pulses exceeding 3 ns is a bottleneck in the task of high practical importance, namely increase in the radar ranging precision. Nowadays the speed of the high-current drivers is limited by the speed of a semiconductor switch that is typically field-effect transistor or an avalanche switch. The last one provides faster switching, and development of new avalanche switches is very challenging and important task, but this is not the only factor limiting the transmitter speed. Here we show that not only the switch, but also parasitic inductance in the miniature assembly and type of the capacitor play very important role in solving the problem of long-distance decimeter-precision radar.