GaN Electronics: RF Reliability and Degradation Mechanisms

AlGaN/GaN high electron mobility transistors (HEMT) are a key enabling technology for future high efficiency military and civilian microwave systems. The aim of this proposal is to provide transformative insight into the underlying physical processes that cause degradation in GaN RF power amplifiers (PA). This is of strategic importance for the UK given its strong RF electronics base, due to the fact that GaN RF power electronics delivers a disruptive step change in systems capability through power densities as high as 40W/mm and frequencies exceeding 300GHz. The UK has internationally leading academic research groups in this field, including Bristol and Cardiff. The key issue addressed in this proposal is that device degradation under RF stress is distinctly different than under DC stress, often resulting in a large increase in source resistance, something that never occurs under DC stress and is not explicable by conventional models. This observation implies that a device in RF operation applies voltage/current stresses, which are inaccessible under static conditions, making it imperative to understand the interaction between the RF operating mode and the degradation mechanism.
Bristol has provided seminal contributions to the international effort to understand DC GaN transistor degradation, where an understanding is slowly emerging that includes oxygen related reactions and diffusion processes, and dislocation linked breakdown in GaN transistors. This includes electroluminescence imaging for detection of leakage pathways, dynamic transconductance and transient analysis to detect trapping states, and the simulation of the effect of pulsed operation on bulk and surface traps. Over the last 15 years, Cardiff has established a world leading capability in RF PA design and measurement. In particular waveform engineering systems enable RF current/voltage waveforms to not only be measured directly but also to be manipulated almost at will. This manipulation of the waveform has allowed Cardiff to make seminal contributions to the understanding of high efficiency RF PA operation. In this project, the unique capability to 'tune' RF operation into extremely well defined states to enable 'controlled' RF stressing will be used to gain the step change understanding of RF device degradation. Reverse engineering of failed devices, electrical and electro-optical measurement before/after and during the RF stress, combined with physical device simulation, will be used to determine the RF specific degradation mechanisms. This capability to predict, engineer and measure the RF waveforms is key to achieving an understanding of the RF stresses that devices undergo during PA operation, and then to determine and specify the safe-operating-area for HEMTs.
This project utilises a partnership with state-of-the-art foundries in Germany and the USA, allowing the project to use production quality devices, essential for the relevance of the work. The project will be guided in terms of its relevance through guidance and interaction with Selex for systems level issues and IQE for the materials. The key synergy of Bristol and Cardiff will address a vitally important issue for the uptake of this disruptive technology, the identification of the RF degradation mechanisms. This will enable the impact of different modes of RF operation to be predicted, and a novel robust RF reliability test methodology to be developed, thus delivering large UK benefit and international impact.

The UK has a strong capability in systems, which rely on RF PAs, and has roadmaps for employing RF and microwave GaN electronics in defence and space applications. Companies such as Astrium use microwave PAs in satcomm up/downlink and space based radar sub-systems, and high efficiency, compactness and small weight is key to their business. Similarly, military systems companies such as Selex, MBDA and BAE SYSTEMS require matched high power microwave modules for phased array radar, RF seekers and electronic counter measures (eg counter IED). In both environments, the more than 5x increased power of GaN devices offers a dramatic disruptive step-change in capability allowing new system architectures which deliver enormous system benefits in terms of parameters including reduced weight, cooling load, increased range, and waveform flexibility. Device lifetime under RF conditions and the ability to confidently predict degradation under all conditions is clearly of particular concern and absolutely essential for these applications. At the other end of the supply chain, IQE is a UK company that is the global leader in the supply of epitaxy. Their goal is the development of a successful competitive GaN product.
GaN technology is being aggressively implemented by competitors worldwide and for the UK to maintain competitiveness it has to have the confidence to rapidly deploy this technology. These companies require a detailed understanding of the threats represented by RF reliability, degradation and failure mechanisms, and this project would provide the essential underpinning intelligence that allows a successful UK uptake of this rapidly emerging disruptive new technology. This research will therefore be performed with the support of key UK and international industrial partners.