A. M. Kurakin

Mechanism of mobility increase of the two-dimensional electron gas in AlGaN∕GaN heterostructures under small dose gamma irradiation

The effect of a small dose of gamma irradiation on transport characteristics of the two-dimensional electron gas (2DEG) in AlGaN∕GaN heterostructures was investigated. It is shown that the carrier concentration remains practically unchanged after an irradiation dose of 106rad, while the 2DEG mobility exhibits a considerable increase. The results are explained within a model that takes into account the relaxation of elastic strains and structural-impurity ordering occurring in the barrier layer under irradiation.

Mechanisms of current formation in resonant tunneling AlN∕GaN heterostructures

This paper presents an analysis of transport and noise properties of double-barrier resonant tunneling diodes formed on the basis of AlN∕GaN heterostructures. Two stable states are registered in the I-V characteristics of the diodes. The temperature dependences of current and noise behavior are analyzed to understand the contribution of different mechanisms responsible for current formation in the structures. The evolution of the spectral density of the noise current with temperature reveals several recombination-generation components. The mechanisms responsible for the formation of current in AlN∕GaN∕AlN diodes are discussed taking into account the Poole-Frenkel effect.

Quantum confinement effect on the effective mass in two-dimensional electron gas of AlGaN/GaN heterostructures

We report the results of direct measurements and a theoretical investigation of the in-plane effective mass in the two-dimensional electron gas of nominally undoped AlGaN/GaN heterostructures with a different degree of quantum confinement. It is shown that in most cases the conduction band nonparabolicity effect is overestimated and the electron wave-function penetration into the barrier layer should be taken into account. The contribution of the wave-function hybridization is determined to play the dominant role. The band edge effective mass value is deduced to be (0.2±0.01)m0.

Non-invasive assessment of leaf water status using a dual-mode microwave resonator

The water status in plant leaves is a good indicator for the water status in the whole plant revealing stress if the water supply is reduced. The analysis of dynamic aspects of water availability in plant tissues provides useful information for the understanding of the mechanistic basis of drought stress tolerance, which may lead to improved plant breeding and management practices. The determination of the water content in plant tissues during plant development has been a challenge and is currently feasible based on destructive analysis only. We present here the application of a non-invasive quantitative method to determine the volumetric water content of leaves and the ionic conductivity of the leaf juice from non-invasive microwave measurements at two different frequencies by one sensor device. A semi-open microwave cavity loaded with a ceramic dielectric resonator and a metallic lumped-element capacitor- and inductor structure was employed for non-invasive microwave measurements at 150 MHz and 2.4 Gigahertz on potato, maize, canola and wheat leaves. Three leaves detached from each plant were chosen, representing three developmental stages being representative for tissue of various age. Clear correlations between the leaf- induced resonance frequency shifts and changes of the inverse resonator quality factor at 2.4 GHz to the gravimetrically determined drying status of the leaves were found. Moreover, the ionic conductivity of Maize leaves, as determined from the ratio of the inverse quality factor and frequency shift at 150 MHz by use of cavity perturbation theory, was found to be in good agreement with direct measurements on plant juice. In conjunction with a compact battery- powered circuit board- microwave electronic module and a user-friendly software interface, this method enables rapid in-vivo water amount assessment of plants by a handheld device for potential use in the field.

Origin of noise in AlGaN∕GaN heterostructures in the range of 10–100MHz

We report on the noise origin in AlGaN∕GaN heterostructures for the frequency range of 10–100MHz. High electron mobility transistor heterostructures were designed for high-power and high-frequency application and grown on SiC substrates. The structures were patterned with Ohmic transmission line model (5, 10, 15, 20, and 25micrometer working distances) contacts and were analyzed using I-V characteristics and noise figure measurements. Different possible mechanisms of noise origin were considered and investigated in detail. The results of our analysis show that the thermal noise and hot carrier noise play a minor role in the structure in the investigated frequency range. At the same time, a dominant generation-recombination (G-R) noise is revealed. Moreover, two different components of G-R noise are found demonstrating different temperature dependences and as a result different physical origins of the noise are established. A detailed analysis of potential profiles of the structure calculated self-consiste...

Dynamic redistribution of the electric field of the channel in AlGaN∕GaN high electron mobility transistor with nanometer-scale gate length

Transport peculiarities and the physical origin of noise properties in AlGaN∕GaN-based high electron mobility transistors (HEMTs) with a large ratio of channel length to gate length were investigated. Dependence of deviations of low-frequency noise spectra from the 1∕f law on applied gate voltages was studied in an extended range of frequencies. The behavior is explained in terms of a model based on the dynamic redistribution of the electric field along the two-dimensional channel of the HEMT. The results show that the main contribution to the noise originates from the region under the gate and adjacent to the gate channel regions.

AlGaN/GaN heterostructures for hot electron and quantum effects

This paper reports on the transport and material characterization of AlGaN/GaN heterostructures. We compared the overheating temperatures in the transmission line model patterned devices during normal operation and analyze their dependence on the buffer thickness in order to optimize the thermal budget of the structures. It is demonstrated that noise spectra can be used to monitor the transport mechanisms and determine the activation energy of the traps. Small-dose gamma irradiation is used as an effective treatment for improving the structural properties. Based on cathodoluminescence and X-ray diffraction spectra shape, we explain the improvement as a result of the relaxation of elastic strains and structural-impurity ordering in the AlGaN barrier layer under irradiation. An irreversible improvement in mobility of the electrons in the channel, controlled by a proper dose of gamma radiation treatment, at considerably reduced self-heating is promising to achieve novel quantum transport regimes on the nanoscale.

Low‐Temperature Transport in AlGaN/GaN 2D Electron Systems

In this work results of magnetotransport investigation in a two‐dimensional electron gas (2DEG) formed at the interface of an undoped GaN/AlGaN heterostructure are reported. The measurements were performed in wide temperature (from 300 mK to 10 K), and magnetic field (up to 10 T) range. The effective mass of investigated 2DEG were estimated from Shubnikov‐de‐Haas oscillation measurements and found to be as high as 0.265 me. The experimental data are analyzed within the framework of weak localization and interference corrections to conductivity together with alternative mechanisms specific for polar semiconductors taking into account a strong electron‐optical phonon interaction.

Dual-mode microwave cavity for fast identification of liquids in bottles

An open metal cavity loaded with a ceramic dielectric resonator and a lumped inductor - capacitor structure allows for simultaneous excitation of the TE01δ dielectric resonator mode at 2 GHz and a lumped element mode at 100 MHz. The measured quality factors are about 7,000 and 400, respectively. Both modes exhibit strongly overlapping evanescent fields above the aperture outside the cavity. These fields can be used for dielectric and conductivity measurements on liquids stored in non-metallic bottles. The simultaneous use of both modes enables liquid category identification within one second. This method has been successfully applied for bottle scanning in checkpoints.

Noise spectroscopy of AlGaN/GaN HEMT structures with long channels

In this paper we discuss the effect of dissipated power on the characteristic time of noise spectra transformation investigated in wide-bandgap AlGaN/GaN transistor structures with long channels. It is found that the characteristic time as a function of temperature demonstrates an exponential dependence with definite activation energy. Obtained results are explained based on the developed model of non-equilibrium fluctuations of the sample resistance. These fluctuations cause a local overheating due to local change of resistance. In the long channels these fluctuations may create overheating regions with two different self-heating temperatures, leading to different activation dependences, observed in the experiment.