Jury

Thesis Director
Mr. Farid MEDJDOUB CNRS scientist (IEMN, France)

Reviewers
Mrs. Nadine COLLAERT (Professor, Vrije Universiteit Brussel)
Mr. Jean-Pierre RASKIN (Professor, UC Louvain)

Examiners
Mr. Didier THERON (DR CNRS, IEMN)
Mr. Cesar RODA NEVE (Program manager, SOITEC)
Mr. Jean-Marc TANGUY (Engineer, DGA)

Invited
Mr. Didier FLORIOT (Thales technical director)

 Summary:

In recent years, significant progress has been made with GaN high electron mobility transistors (HEMTs) to advance the next generation of 5G networks, radar systems and satellite communications. However, to further improve power amplification and high-frequency operation (>30 GHz), innovative architectures have been developed. These designs feature redesigned structures, including sub-150nm gate lengths, thinner barrier layers or optimised epitaxial layers. Several research groups have achieved impressive results, with high added power efficiency (PAE > 50 %) and high output power (POUT > 3 W/mm), in frequency ranges from Ka-band (30 GHz) to W-band (94 GHz). Despite this progress, the reliability of short devices remains a major challenge due to the high electric field, self-heating and electron trapping effects. This research integrates device fabrication, structural and electrical characterisation and TCAD simulations to provide state-of-the-art information in this area. One of the most promising technologies, the degraded AlGaN channel HEMT, has been studied using advanced simulations to better understand its unique properties. In addition, an optimised buffer architecture using an AlGaN back barrier and an ultra-thin AlN barrier has made it possible to achieve peak power performance at 40 GHz in fabricated devices. Finally, a new unbuffered architecture featuring an ultra-thin AlGaN barrier was also investigated, with promising results that could rival existing technologies. Short-term reliability tests were carried out to identify the main shortcomings and guide future developments.

Abstract:

In recent years, significant progress has been achieved with GaN high electron mobility transistors (HEMTs) in advancing the next generation of 5G networks, radar systems, and satellite communications. However, to further enhance power amplification and high-frequency operation (>30 GHz), innovative architectures have been developed. These designs feature reengineered structures, including sub-150 nm gate lengths, thinner barrier layers, or optimized epitaxial layers. Several research groups have demonstrated impressive results, achieving high power-added efficiency (PAE > 50%) with substantial high output power (POUT > 3 W/mm), across frequency ranges from the Ka-band (30 GHz) to the W-band (94 GHz). Despite these advancements, the reliability of short devices remains a significant challenge due to high electric field, self-heating, and electron trapping effects. This research integrates device fabrication, structural and electrical characterizations, and TCAD simulations to provide cutting-edge insights in this field. One of the most promising technologies, the graded AlGaN channel HEMT has been explored through advanced simulations to better understand its unique properties. Furthermore, an optimized buffer architecture using an AlGaN back barrier and an ultra-thin AlN barrier has enabled state-of-the-art power performance at 40 GHz in fabricated devices. Finally, a novel buffer-free architecture featuring an ultra-thin AlGaN barrier has also been investigated, showing promising results that may rival existing technologies. Short-term reliability tests have been conducted to identify key shortcomings and guide future development.