We report on the photoresponse dependence on the terahertz radiation intensity in ALGaN/GaN HEMTs. We show that the ALGaN/GaN HEMT can be used as a THz detector in CW and in pulsed regime up to radiation intensity of several kW/cm(2). The dynamic range in the pulsed regime of detection can be more than 2 decades. We observed that the photoresponse of the HEMT could have a compound composition if two independent parts of the transistor are involved in the detection process; this result indicates that a more simple one channel device may be preferable on the detection purpose.

In this study we report on the micro-, And nanostructural, morphological and electrical properties of Ti(20 nm)/Cr(120 nm)/Al(50 nm) contacts to n-GaN by electron microscopy, X-ray diffraction and I-V measurements. Tested contacts were annealed at the temperature of 400, 700 and 900 degrees C for 10 min in vacuum. It was found that Cr appeared in the top part of the GaN epitaxial layer at 700 degrees C. High resolution electron microscopy (HREM) revealed Ti2AlN MAX phase as well as Al2O3 phase at the interface of n-GaN/Ti/Cr/Al contact at 900 degrees C. X-ray diffraction examinations showed that new Ti2AlN MAX phase formed and Cr3GaN as well as CrN phases developed at 900 degrees C. I-V characterizations exhibited that while the as-deposited and annealed Ti/Cr/Al contacts are rectifying up to 700 degrees C the diffusion of Cr into the epi GaN layer led to ohmic behaviour at 700 degrees C. (c) 2013 Elsevier Ltd. All rights reserved.

Optically induced electronic transitions in nitride based polar heterostructures have been investigated by absorption and emission spectroscopy. Surface photovoltage (SPV), photocurrent (PC), and photo luminescence spectroscopy have been applied to high quality InAlN/AlN/GaN structures to study the optical properties of two dimensional electron gas. Energy levels within the two dimensional electron gas (2DEG) well at the interface between the GaN and AlN have been directly observed by SPV and PC. Moreover, a strong enhancement of the photoluminescence intensity due to holes recombining with electrons at the Fermi Energy, known as fermi energy singularity, has been observed. These analyses have been carried out on InAlN/AlN/GaN heterojunctions with the InAlN barrier layer having different In content, a parameter which affects the energy levels within the 2DEG well as well as the optical signal intensity. The measured energy values are in a very good agreement with the ones obtained by Schrödinger–Poisson simulations.

AlGaN/GaN based FETs have great potential as sensitive and fast operating detectors because of their material advantages such as high breakdown voltage, high electron mobility, and high saturation velocity. These advantages could be exploited for resonant and non-resonant terahertz detection. We have designed, fabricated, and characterized AlGaN/GaN based FETs as single pixel terahertz detectors. This work focuses on non-resonant detection and imaging using GaN field plate FETs. To evaluate their performances as terahertz detectors, we have measured the responsivity as a function of gate voltage, the azimuthal angle between the terahertz electric field, the source-to-drain direction, and the temperature. A simple analytical model of the response is developed. It is based on plasma density perturbation in the transistor channel by the incoming terahertz radiation. The model shows how the non-resonant detection signal is related to static (dc) transistor characteristics and it fully describes the experimental results on the non-resonant sub-terahertz detection by the AlGaN/GaN based FETs. The imaging performances are evaluated by scanning objects in transmission mode and an example of application of terahertz imaging as new non-destructive technique for the quality control of materials is given. Results indicate that these FETs can be considered as promising devices for terahertz detection and imaging applications.

In order to develop high voltage high current and high temperature HEMT transistors, we have studied an isolated gate based on a thin Al2O3. TCAD simulations have been used to explain abnormal C(V) results depending on the density and localization of positive charges under the gate. We have found and explained the dependence between degradation of Al2O3 interface and electrical characteristics.

AlInN/GaN HEMTs have shown outstanding power performances for high frequency applications, due in particular to their high current densities and their thinner barrier layers than in AlGaN/GaN HEMTs that minimize short channel effects. In this paper, we present the first published power results of two K-band hybrid amplifier demonstrators at 20GHz and 26.5GHz using 0.25µm gate length devices. At these frequencies, respectively, cw RF output power of 4.5 Watts with 20% PAE and 1.65 W with 15.5 % of PAE were obtained. These state-of-the-art results confirm the potential of AlInN/GaN technology for high frequency applications.

We present experimental results on the Terahertz radiation from high electron mobility transistors at room temperature, which clearly show the tunability of the emission frequency by the gate voltage.

We evaluate the optical performance of AlGaN/GaN MISFETs as a non-resonant sub-terahertz, room temperature detector. The single-pixel responsivity and the noise equivalent power are determined. The efficiency of the detection is demonstrated by the room temperature imaging of different solutions of acetone in cyclohexane.

In the investigated InAlN/GaN layers, it is shown that the surface morphology and the crystallinity of the alloy critically depend on the In composition. Atomic force microscopy analysis points out that step flow growth is not easily attained in this system. When the InAlN or AlN interlayer thickness is increased, the growth mode becomes three‐dimensional. However, the formed islands are hundred of nanometers apart, and were not observed in the transmission electron microscope.

In order to assess possible mechanisms of gate reverse-bias leakage current in AlInN/GaN high electron mobility transistors (HEMTs) grown by metalorganic chemical-vapor deposition on SiC substrates, temperature-dependent current-voltage measurements combined with Fourier transform current deep level transient spectroscopy (FT-CDLTS) are performed in the temperature range of 200–400 K. In this range of temperature reverse-bias leakage current flow is found to be dominated by Poole–Frenkel emission. Based on CDLTS measurements, a model of leakage current transport via a trap state located at the AlInN/metal interface with an activation energy of 0.37 eV is suggested. The trap nature is shown to be an extended trap, most probably associated with dislocations in the AlInN barrier layer.

We report on room temperature terahertz generation by a submicron size AlGaN/GaN-based high electron mobility transistors. The emission peak is found to be tunable by the gate voltage between 0.75 and 2.1 THz. Radiation frequencies correspond to the lowest fundamental plasma mode in the gated region of the transistor channel. Emission appears at a certain drain bias in a thresholdlike manner. Observed emission is interpreted as a result of Dyakonov–Shur plasma wave instability in the gated two-dimensional electron gas.

An AlGaN/GaN based field effect transistor (FET) has been designed, fabricated, and used as a resistive mixer for heterodyne detection in the 140-220 GHz frequency range. A double VNA heterodyne measurement setup has been used in an on-wafer configuration to accurately quantify the incident radiation absorbed by the device. The appropriate selection of optimum biasing conditions for minimum conversion losses is investigated. 47.3 dB conversion losses are obtained at 150 GHz and the device linearity is confirmed.

InAlN/GaN layers grown by metalorganic vapor phase epitaxy have been investigated by transmission electron microscopy (TEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). It is shown that the surface morphology and the crystallinity of the InAlN layers critically depend on the In composition. AFM analysis points out that step flow growth may not be easily attained in this system: when the InAlN or AlN interlayer thickness is increased, the growth mode becomes three dimensional. However, the formed islands are hundred of nanometers apart, and were not observed in the transmission electron microscope. From the TEM investigation, it is pointed out that v-shaped defects are present at the surface of some layers and they may be as deep as the whole ternary alloy, they are connected to threading dislocations originated from the underlying template layers. As expected the measured residual strain is minimum when the ternary layer composition is close to 16-17% In.

InAlN/GaN is indeed an alternative to the common AlGaN/GaN heterostructure in electronics and sensing. It enables operation at extremely high temperature once problems with contact metallization and passivation have been solved. It is the only heterostructure known presently, which allows overgrowth of high quality diamond films to combine two of the most stable semiconductors. Thus, applications reach from high power microwaves systems and high temperature electronics to sensing in harsh environment.

Two experiments demonstrate that high electron mobility transistors (HEMTs) are well suited for terahertz (THz) imaging. We have shown that HEMTs can be effectively used as plasma wave detectors in a THz imaging system at frequencies higher than 1 THz at room temperature and this device are sensitive to the polarization direction of THz radiation. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Deep centers in AlGaN/GaN high electron mobility transistors (HEMTs) on SiC substrate have been characterized by capacitance deep level transient spectroscopy (DLTS) and conductance deep level transient spectroscopy (CDLTS). These measurements reveal the presence of three kinds of electron defects E1, E2 and E3 with activation energies of 0.52, 0.29 and 0.09eV, respectively and a hole-like trap HL1 with activation energy 0.905eV. The conductance DLTS using a gate pulse, shows an additional trap HL2, located at the surface. The localization and the identification of these traps are presented. Finally, these experimental results demonstrate the complementarities of these two techniques.

In this work we present experimental results on room temperature terahertz (THz) detection/emission from nanotransistors as well as double grating gates structures. It shows that the emission spectra from GaN/AlGaN HEMT transistor can be successfully interpreted in the frame of the Dyakonov-Shur model which predicts that by heating/accelerating the electrons up to velocities comparable with plasma wave velocity can lead to instability and generation of plasma waves in the transistor channel. Studies of the current dependence of THz detection by InGaAs based HEMT is also presented. It is shown that in realistic transistor structures the room temperature resonant THz detection can be observed only upon applying a current high enough to produce fast/hot electrons. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.