A. E. Belyaev

Separation of hot-electron and self-heating effects in two-dimensional AlGaN/GaN-based conducting channels

We address experimental and theoretical study of a two-dimensional electron gas transport at low and moderate electric fields. The devices under study are group-III nitride-based (AlGaN/GaN) gateless heterostructures grown on sapphire. The transmission line model patterns of different channel lengths, L, and of the same channel width are used. A strong dependence of the device I–V characteristics on the channel length has been found. We have developed a simple theoretical model to adequately describe the observed peculiarities in the I–V characteristics measured in steady-state and pulsed (10−6 s) regimes. The effect of the Joule heating of a heterostructure is clearly distinguished. The thermal impedance and the channel temperature rise caused by the Joule self-heating have been extracted for the devices of different L at different values of dissipated power. The current reduction due to both self-heating and hot-electron effects is determined quantitatively as a function of the electric field.

Internal strains and crystal structure of the layers in AlGaN/GaN heterostructures grown on a sapphire substrate

In this paper, we investigate the structural properties of AlGaN/GaN heterostructures grown by metal organic chemical vapor deposition on sapphire substrates with different thicknesses using high-resolution x-ray diffraction and Raman scattering methods. We discuss the microscopic nature of spatial-inhomogeneous deformations and dislocation density in the structures. Microdeformations within mosaic blocks and the sizes of regions of coherent diffraction are determined. We reveal a gradient depth distribution of deformations in the mosaic structure of nitride layers, as well as at the interface regions of the sapphire substrate on the microscale level using confocal micro-Raman spectroscopy. We determine that an increase in substrate thickness leads to a reduction in dislocation density in the layers and an increase in the elastic deformations. The features of the block structure of nitrides layers are shown to have a significant influence on their elastic properties.

Hot-electron transport in AlGaN/GaN two-dimensional conducting channels

We report on experimental studies of high-field electron transport in AlGaN∕GaN two-dimensional electron gas. The velocity–electric field characteristics are extracted from 10to30ns pulsed current–voltage measurements for 4.2 and 300K. An electron drift velocity as high as 1.7×107cm∕s was obtained in the fields 150kV∕cm. Estimates of thermal budget of the system show that overheating of the electrons exceeds 1700K at highest electric fields achieved in the experiment.

Mechanism of dislocation-governed charge transport in schottky diodes based on gallium nitride

A mechanism of charge transport in Au-TiBx-n-GaN Schottky diodes with a space charge region considerably exceeding the de Broglie wavelength in GaN is studied. Analysis of temperature dependences of current-voltage (I–V) characteristics of forward-biased Schottky barriers showed that, in the temperature range 80–380 K, the charge transport is performed by tunneling along dislocations intersecting the space charge region. Estimation of dislocation density ρ by the I–V characteristics, in accordance with a model of tunneling along the dislocation line, gives the value ρ ≈ 1.7 × 107 cm−2, which is close in magnitude to the dislocation density measured by X-ray diffractometry.

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.

AlGaN/GaN High Electron Mobility Transistor Structures: Self-Heating Effect and Performance Degradation

This paper reports on the results of the experimental and numerical investigation into the self-heating effect in AlGaN/GaN heterostructures grown on sapphire and SiC substrates. It shows that temperature increase has an opposite dependence on the buffer thickness for sapphire and SiC substrates. Noise spectroscopy is also used to monitor the self-heating effect. Moreover, it is shown that the room-temperature spectra can be used to determine the activation energy of the traps. An irreversible improvement in mobility and quantum scattering time is registered after the irradiation of AlGaN/GaN heterostructures at a total dose of 1 times106 rad of 60Co gamma rays.

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.

Excess low-frequency noise in AlGaN'GaN-based high-electron-mobility transistors

The low-frequency 1/f noise characteristics of AlGaN/GaN high-electron-mobility transistors with gate length scaled down to 150 nm grown on sapphire by metalorganic chemical vapor deposition have been studied. Certain features of the 1/f noise have been revealed in these short-gate transistors. The low-frequency noise spectra show drastically different behavior depending on the gate voltage VG in the range of low (VGt⩽VG⩽0) and high (VG<VGt) biases. The noise spectra-gate bias dependences allow one to distinguish a spatial redistribution of effective noise sources in the transistor channel. The Hooge parameter has been deduced separately for the ungated region, αHp≃10−3, and for the gated region, αHa≃2×10−4, of the transistor channel. These values are as low as those previously observed in nitride heterostructures grown on silicon carbide substrates.

Fine structure of photoresponse spectra in a double-barrier resonant tunnelling diode

The photoresponsivity spectra of double-barrier resonant tunnelling diodes have been measured in a wide range of light wavelength as well as applied voltage. The complex behaviour of measured spectra is analysed, taking into account different channels for electron injection into the quantum well (QW). The results obtained yield evidence for modulated electron-hole (e-h) recombination in the QW provided by direct excitation of e-h pairs in the QW.

Mechanism of contact resistance formation in ohmic contacts with high dislocation density

A new mechanism of contact resistance formation in ohmic contacts with high dislocation density is proposed. Its specific feature is the appearance of a characteristic region where the contact resistance increases with temperature. According to the mechanism revealed, the current flowing through the metal shunts associated with dislocations is determined by electron diffusion. It is shown that current flows through the semiconductor near-surface regions where electrons accumulate. A feature of the mechanism is the realization of ohmic contact irrespective of the relation between the contact and bulk resistances. The theory is proved for contacts formed to III-V semiconductor materials as well as silicon-based materials. A reasonable agreement between theory and experimental results is obtained.