M. F. Romero

Optical properties of magnesium doped AlxGa1−xN (0.61 ≤ x ≤ 0.73)

We investigate the optical properties of AlxGa1–xN:Mg with aluminum content of 0.61 ≤ x ≤ 0.733 in comparison to undoped and silicon doped reference samples. The ordinary dielectric functions, excitation, and emission spectra are reported at different temperatures. A comprehensive analysis yields quantitative data on the valence band structure of the ternary alloy, i.e., splitting and order of valence bands with different symmetries. Finally, the near band gap emission in AlGaN:Mg is found to be most probably dominated by donor to free-hole recombination.

Luminescence from two-dimensional electron gases in InAlN/GaN heterostructures with different In content

The luminescence properties of InxAl1−xN/GaN heterostructures are investigated systematically as a function of the In content (x = 0.067 − 0.208). The recombination between electrons confined in the two-dimensional electron gas and free holes in the GaN template is identified and analyzed. We find a systematic shift of the recombination with increasing In content from about 80 meV to only few meV below the GaN exciton emission. These results are compared with model calculations and can be attributed to the changing band profile and originating from the polarization gradient between InAlN and GaN.

Trapping phenomena in AlGaN and InAlN barrier HEMTs with different geometries

Trapping effects were evaluated by means of pulsed measurements under different quiescent biases for GaN/AlGaN/GaN and GaN/InAlN/GaN. It was found that devices with an AlGaN barrier underwent an increase in the on-resistance, and a drain current and transconductance reduction without measurable threshold voltage change, suggesting the location of the traps in the gate-drain access region. In contrast, devices with an InAlN barrier showed a transconductance and a decrease in drain associated with a significant positive shift of threshold voltage, indicating that the traps were likely located under the gate region; as well as an on-resistance degradation probably associated with the presence of surface traps in the gate-drain access region. Furthermore, measurements of drain current transients at different ambient temperatures revealed that the activation energy of electron traps was 0.43 eV and 0.38 eV for AlGaN and InAlN barrier devices, respectively. Experimental and simulation results demonstrated the influence of device geometry on the observed trapping effects, since devices with larger gate lengths and gate-to-drain distance values exhibited less noticeable charge trapping effects.

Impact of device geometry at different ambient temperatures on the self-heating of GaN-based HEMTs

The influence of the device geometry on the self-heating for GaN-based HEMTs was assessed at different ambient temperatures, from 25 °C to 175 °C. The results showed that the gate width can significantly affect the heat dissipation. In addition to this, the distribution of the generated heat in the channel has been demonstrated to be dependent on the distance between the gate and drain contacts. Besides the device geometry, the ambient temperature was also found to be relevant for the thermal resistance, mainly due to the temperature-dependent thermal conductivity of the layers and the substrate. The channel temperature and the thermal resistance extracted from the measurements were in good agreement with the simulations.

Physics-based analytical model for input, output and reverse capacitance of a GaN HEMT with the field-plate structure

This paper presents an analytical model for input, output, and reverse capacitance of a normally on AlGaN/GaN high-electron mobility transistor (HEMT) with a gate field-plate structure, when the device is in the subthreshold regime. Together with the existing model for the output I-V characteristics, the proposed capacitance model provides the complete set of analytical equations that relate the physical design parameters to the electrical characteristics of the device. The model was verified by the experimental characterization of a HEMT. In comparison to the physics-based models implemented in Finite Element Analysis tools, the obtained capacitance model has substantially lower level of complexity and, therefore, it is more suitable for implementation into iterative design optimization algorithms. In order to verify the proposed model for such usage, the prototype of a high-frequency buck converter was built, using previously modeled GaN HEMT as the main switch. The hybrid analytical-behavioral power loss model of a high-frequency buck converter was implemented into Simplorer simulation tool, using the proposed physics-based model as the device description for the capacitive part. The efficiency measurements showed good agreement with the simulation results, even at 20 MHz of switching frequency in the low range of the output power.

Fabrication and characterization at high temperature of AlGaN/GaN enhancement-mode HEMTs

Enhancement-mode (E-mode) high electron mobility transistors (HEMTs) based on a standard AlGaN/GaN heterostructure have been fabricated using two different methods: 19F implantation and fluorine-based plasma treatment. The need of a thermal annealing after both treatments has been proven in order to restore the ID and gm levels. DC characterization at high temperature has demonstrated that ID and gm decrease reversibly due to the reduction of the electron mobility and the drift velocity. Pulsed measurements (state period and variable pulse width) have been performed to study the self-heating effects.

Thermal stability study of AlGaN/GaN MOS-HEMTs using Gd 2 O 3 as gate dielectric

Thermal stability of AlGaN/GaN MOS-HEMTs and -diodes using Gd₂O₃ are investigated by means of different thermal cycles and storage tests up to 500°C for one week. IV DC and pulsed characteristics of the devices before and after the processes are evaluated and compared with conventional HEMTs. Results show that the devices with Gd₂O₃ dielectric layer have lower leakage current and a more stable behavior during thermal treatment processes compared with conventional devices. In fact, an excellent on/off ratio of about 10⁸ and a stable V_t is observed after storage at high temperature. The beneficial effects of Gd₂O₃ on trapping effects of MOS-HEMTs are also discussed.

Analysis of InAl(Ga)N/GaN wet-etching by structural, morphological and electrical methods

Wet etching of InAl(Ga)N/GaN structures has been studied in detail by means of Rutherford backscattering spectroscopy, x-ray diffraction, atomic force microscopy and capacitance–voltage profiling (C–V). The samples used for the study were grown on three different substrates (sapphire, silicon carbide and silicon(111)). Nearly lattice-matched compositions were measured for all the samples. We obtained different etching rate depending on the homogeneity and root-mean-square roughness of the surface as well as the underlying substrate, attributing the difference possibly to the presence of threading dislocation in the sample. The study interest is correlated to the possibility to control at a very precise level the thickness etching of the material, making it possible to fabricate normally-off recessed gate high-electron-mobility-transistors.

Systematic Optical Characterization of Two-Dimensional Electron Gases in InAlN/GaN-Based Heterostructures with Different In Content

Model calculations have been performed to study systematically the formation of a two-dimensional electron gas (2DEG). The results are used for analyzing the photoluminescence properties of corresponding InAlN/GaN heterostructures (HS) for various In concentrations (x = 6.7–20.8%). We found a luminescence peak, clearly dependent on the In content, that is attributed to the recombination between electrons in the 2DEG at the second level (En=2) and photoexcited holes in the GaN buffer. The results can be understood with the changing band profile attributed to the different polarization gradient between InAlN and GaN.

Effects of Gd 2 O 3 Gate Dielectric on Proton-Irradiated AlGaN/GaN HEMTs

AlGaN/GaN high electron mobility transistors (HEMTs) and MOS-HEMTs using Gd2O3 as gate dielectric were irradiated with 2-MeV protons up to fluence of