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Résumé

Depuis le début des années 90, une nouvelle catégorie de filières dites à large bande interdite est venue compléter l'éventail déjà large des technologies utilisées pour les capteurs, pour l'optique, pour l'électronique de puissance et pour l'électronique des hautes fréquences. Les technologies carbure de silicium (SiC) et nitrure de gallium (GaN) possèdent des qualités intrinsèques remarquables, et représentent ainsi une véritable rupture technologique avec les technologies GaAs et Si/SiGe. Leur développement offre de nouvelles opportunités en terme de conception de circuits, voire d'architecture de systèmes (réseau de transpondeurs des applications radar, gestion de l'énergie des systèmes embarqués, ...) en panachant les différentes technologies disponibles pour des applications en optique, en opto-électronique et en électronique. Ce papier propose un état de la technologie GaN et de ses applications HF : les principaux acteurs mondiaux sont présentés, ainsi que les performances des technologies, quelques démonstrateurs de circuits HF et premiers résultats sur la fiabilité de ces filières.

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Abstract

Low frequency noise (LFN) is a reliable diagnostic tool to evaluate and locate the defects of a technology. In this study, LFN is used to assess effects of deuterium (H+ ions) in diffusion condition on the robustness of 0.25 *2*75 ¼m² gate area AlGaN/GaN high electron mobility transistors (HEMT) grown on Si substrate. H+ Ions are diffused from the above AlGaN/GaN layer through the AlGaN/GaN interface and GaN layer, notably under the gated channel where the defects are located. Two batches of devices are stressed under high temperature condition at 400°C during 5 minutes (step 1) and 500°C during 15 minutes (step 2). The first batch is composed with 8 deuterated transistors while the second batch is composed with 8 non deuterated transistors. Static measurements and low frequency noise spectral density measurements of the drain current (SID) are examined after each step of temperature. The first step does not reveal any degradation, while the second step highlights significative differences between the deuterated and non deuterated devices: LFN of deuterated devices remains constant, whereas LFN of non deuterated devices increases (GR superimposed with 1/f flicker noise). The deuteration of the devices can open the way to robust temperature devices, as AlGaN/GaN HEMT are dedicated to applications at high power and high temperature.

Abstract

GaN related devices have demonstrated excellent performances for high power, high temperature up to X-band applications. However, even if the reliability studies on AlGaN/GaN high electron mobility transistors (HEMT) have led to higher mean time to failure (MTTF), physical mechanisms induced by stresses are still not well known. This paper proposes an original solution to improve the robustness of the devices by passivating the traps that are supposed to be related to the degradation process. Based on the experience of previous works, we use Deuterium H+ to block the traps located at the AlGaN/GaN interface above the gated zone of the device, and the traps in the bulk of the conducting channel (2 dimensions electron gas : 2DEG). 2 batches of devices are processed with and without deuterium, and submitted to temperature stresses at 500°C. Low frequency noise (LFN) measurements are performed to track the evolution of the spectral current density of the drain current, which is known to be related to the structural evolution of the traps and of the crystal structure perfection. Devices with deuterium feature stable LFN spectra, while LFN spectra of the devices without deuterium evolve during the different stress steps. Thus, deuterium can offer an interesting alternative to enhance the robustness of AlGaN/GaN devices operating under stringent temperature conditions.

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Abstract

An electrical stress test has been conducted on 98 Zener conventional reference diodes structures and it has been observed that 45 % of these devices failed after 3000 hours. However this high failure ratio can be reduced to below 25 %, or less, providing that an appropriate low frequency noise (LFN) characterization is initially performed and that all the devices exhibiting the larger LFN are subtracted from the lot subjected to electrical stress test. Further results obtained after a 4500 hours electrical stress test conducted on a reduced number of diodes fully validate this finding.

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Abstract

Wide bandgap devices such as AlGaN/GaN High Electron Mobility Transistors (HEMT) grown on silicon carbide (SiC) substrate are investigated. Low frequency noise (LFN) measurements have been carried out to evaluate the structural perfection of dual gated HEMT devices featuring 0.25x2x75µm² gate area: generation-recombination (GR) processes are evidenced. Two sets of GR-bulges related respectively to AlGaN/GaN interface and quantum well are identified. Each GR-bulge is composed of two GR centers. The devices are then characterized in a temperature controlled oven, and these GR centers are extracted from LFN spectra versus temperature. Activation energies of the defects located at the AlGaN/GaN interface are measured at 0.38±0.05eV and 0.21±0.05eV using Arrhenius plots under saturated biasing conditions. Equivalent activation energies are extracted under ohmic biasing conditions. These results are compared with SIMS measurements, using the deuterium in diffusion condition as a probe to integrally explore the presence of defects throughout the AlGaN-GaN HEMT structure. Large concentrations of deuterium (more than 10E20 D concentration per cm3) are measured at the AlGaN/GaN interface and in the 2DEG layer, thus proving the presence of numerous vacations at the AlGaN/GaN interface as well as in the 2DEG. From the confrontation with previously published results, the defects might be assigned to the nitrogen vacancy and to MgGa-VN complexes.

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The evolution of spectrally pure microwave sources is described, together with the problem of modeling the spectral shape of these signals. The noise sources interaction with the intrinsic nonlinear behavior of the oscillator is pointed out, and various solutions are proposed. Original techniques are shown to investigate on the phase noise modeling directly at the transistor level. Then, the problem of noise in some new approaches for frequency generation, in which other noise sources than the transistor noise contributes to the oscillator noise, is described.

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