Design and fabrication of GaN-based field effect power transistors up to W-band

Au cours des dernières décennies, des progrès remarquables ont été réalisés sur les transistors à haute mobilité électronique à base de GaN (HEMTs GaN) destinés aux applications d’amplification et de commutation de puissance à haute fréquence. Actuellement, les HEMTs GaN les plus matures sont basés sur des hétérostructures AlGaN/GaN. Plus récemment, les hétérostructures à barrières ultrafines (<10 nm) (In)(Ga)AlN/GaN riches en Al ont également présentées beaucoup d’intérêt pour les applications en gamme d’ondes millimétriques. En effet, contrairement aux structures AlGaN/GaN, les barrières ultrafines riches en Al peuvent fournir une densité d’électrons (2DEG) deux fois plus élevée tout en offrant un rapport d’aspect important (longueur de grille / distance grille-canal) y compris avec des grilles très courtes inferieures à 100 nm. Par conséquent, les HEMTs GaN à barrière ultrafine riche en Al permettent de fonctionner à une fréquence plus élevée de manière robuste. Dans ce contexte, plusieurs groupes de recherche ont démontré une combinaison unique de puissance plus élevée et une bande passante plus large jusqu’à 100 GHz par l’utilisation de transistors GaN par rapport aux autres technologies (GaAs ou silicium). Cependant, la plupart des applications nécessitent des amplificateurs de puissance à très haut rendement associé à une fiabilité éprouvée et une linéarité accrue. L’état de l’art des HEMTs GaN est limité aujourd’hui à environ 50% de rendement PAE (Power Added Efficiency) et peu de travaux reportés sur la fiabilité des composants GaN utilisant des grilles courtes inferieures à 150 nm. Néanmoins, l’une des limitations majeures des composants RF modernes est la dissipation thermique. En effet, la puissance dissipée s’améliore de 80% lorsque le rendement PAE passe de 50% à 80%. L’objectif de ce travail est de fournir une technologie de pointe dans ce domaine avec le développement et l’optimisation de transistors GaN à grille sub-150 nm pour les applications en gamme d’ondes millimétriques. En particulier, nous avons effectué une optimisation des couches tampons (buffer) tout en optimisant une barrière AlN ultrafine inférieure à 5 nm afin d’augmenter le gain de puissance, d’améliorer le confinement des électrons sous fort champ électrique et de simultanément réduire les effets de pièges. De plus, le développement d’un banc de mesures de puissance à 94 GHz a permis de démontrer une densité de puissance à l’état de l’art en bande W avec les composants fabriqués. Ces travaux constituent une base de travail prometteuse pour garantir des performances élevées (notamment le rendement PAE) et fiables des HEMTs GaN pour l’amplification de puissance en gamme d’ondes millimétriques liée aux futures applications de télécommunication 5G, spatiales ou militaires.In the last decades, remarkable progresses have been achieved with GaN high electron mobility transistors (HEMTs) for use in high-frequency power amplification and switching applications. Currently, the most matured GaN HEMTs are based on AlGaN/GaN heterostructures. More recently, Al-rich ultrathin sub-10 nm (In)(Ga)AlN/GaN heterostructures have also received much attention for millimter-wave applications. This is because in contrast to AlGaN/GaN, they can provide more than two times higher 2 Dimensional-Electron-Gas (2DEG) sheet carrier density while offering a high aspect ratio (gate length / gate to channel distance) down to sub-100 nm gate lengths. As a result, Al-rich ultrathin barrier GaN HEMTs are able to operate at much higher speed without the use of gate recess, thus potentially enabling high device reliability. In this frame, a number of research groups have demonstrated a unique combination of higher power and wider bandwidth using advanced GaN transistors all the way to 100 GHz as compared to other technologies (GaAs or Silicon). However, most of the applications require very high efficiency power amplifiers with high linearity and proven reliability under harsh conditions. Current state-of-the-art GaN HEMTs are limited to about 50% power-added-efficiency (PAE) in the Ka band and much lower at higher frequency. Moreover, very few reports are available on the device reliability for sub-150 nm gate lengths. On the other hand, one of the major limitations of modern RF devices is the thermal dissipation. The dissipated power improves by 80% when the PAE increases from 50% to 80%.The aim of this work is to provide leading edge technologies in this field through the development and the improvement of sub-150 nm GaN transistors for high frequency applications. In particular, we have performed an extensive buffer engineering while carefully optimizing an ultrathin sub-5nm AlN barrier layer in order to maximize the power gain, improve the electron confinement under high electric and simultaneously reduce the trapping effects. Furthermore, the development of a power bench at 94 GHz enabled the demonstration of a record W-band output power density with the fabricated devices. This is believed to constitute a decisive asset in securing high performances and reliable GaN devices for next-generation millimeter-wave amplifiers related to future 5G telecommunications, space or military applications

High Power AlN/GaN HEMTs with record power-added-efficiency >70% at 40 GHz

International audienceWe report on breakthrough power-added-efficiency (PAE) Q-band performances using a vertically scaled AlN/GaN HEMT technology. The comparison between a 3 nm and 4 nm barrier thickness shows both superior performance and robustness for the thinner barrier layer attributed to the reduced mechanical strain into the heterostructure. Large signal characteristics at 40 GHz revealed an outstanding PAE of 73% at VDS = 30V associated to an output power density > 5 W/mm in pulsed mode. Also, the load-pull measurements mapping across the 4-inch wafer demonstrates a high uniformity and reproducibility of the results. Consequently, significantly improved PAE can be expected for next generation of high power MMICs operating in the millimeter-wave range

Status and progress of millimeter-wave GaN transistors for next generation high-power radar systems

With the development of wireless communication such as 5G or SATCOM, the need and requirements for millimeter-wave compact solid-state high power amplification has significantly increased. In this presentation, we will discuss some potential device design enabling to properly operate in the millimeter-wave range under high drain bias &gt; 20 V with high performance

Thermal and statistical analysis of various AlN/GaN HEMT geometries for millimeter Wave applications

International audienceDownscaling HEMT devices is nowadays substantial to allow their operation in the millimeter wave frequency domain. In this work, the electrical parameters of three different AlN/GaN structures featuring various GaN channel thicknesses were compared. After a DC electrical stabilization procedure, 96 HEMT devices under test exhibit a minor dispersion in DIBL and lag rates, which reflects an undeniable technological mastering and maturity. Evaluation of the sensitivity of devices with different geometries at temperatures of up to 200°C revealed that the gate-drain distance impacts Ron variation and not I dss variation with temperature. We also showed that DIBL at moderate electrical field and the drain lags exhibit athermal behavior; unlike gate lag delays which can be thermally activated and exhibit a linear temperature dependence regardless of the size of the gate length and gate-to-drain distance

Role of AlGaN back-barrier in enhancing the robustness of ultra-thin AlN/GaN HEMT for mmWave applications

International audienceIn this work, the robustness of three different AlN/GaN structures targeting high frequency applications with ultra-thin 3 nm AlN barrier and devices with different gate-to-drain spacing are evaluated. Devices were step-stress tested in off-state, semi-on-state, and on-state conditions. Different failure mechanisms were identified. Devices without AlGaN back-barrier show a fast increase of gate leakage current, leading to a significant degradation of electrical parameters such as the threshold voltage Vth, extrinsic transconductance GM, drain saturation current IDSsat and on-resistance Ron. This study reveals the role of the AlGaN back-barrier in the improvement of the device robustness presenting a good electrical parameters stability. The presence of an AlGaN back-barrier on low carbon- doped buffer leads to better gate leakage current stability, low trapping-related and self-heating mechanisms. The AlGaN back-barrier shows an extension of the operational Safe Operating Area for AlN/GaN technology

High Efficiency AlN/GaN HEMTs for Q-Band Applications with an Improved Thermal Dissipation

International audienc