Novel Sub-Threshold Methodologies for GaN Electronic Devices: A Study of Device Reliability and Degradation Mechanisms

GaN power electronics, in particular, AlGaN/GaN high electron mobility transistors (HEMT) are currently being developed and starting to be applied for power conversion, radar, satellite and communication applications. Switched mode power systems based on this will deliver improved efficiency, hence forming a key enabling technology for the low carbon economy. Although performance of these devices is fully sufficient to enable disruptive changes for many system applications, reliability is presently still in question, not only in the UK and Europe, but also in the USA and Japan. This proposal aims at developing a new electrical methodology to study and understand reliability of GaN based HEMTs, in particular to identify the nature of electronic traps generated during the operation of GaN HEMTs, and which affect their lifetime. The programme is supported by key UK, European and US industries (International Rectifier UK, Fraunhofer Institute IAF Germany, UMS Germany, TriQuint USA), and builds on leading expertise in the field of GaN HEMT reliability developed at the Center for Device Thermography and Reliability (CDTR) in Bristol, established in various research programmes in Bristol funded by EPSRC and the US Office of Naval Research (ONR). The focus of this work will lie in overcoming the challenge that the highly accurate standard Capacitance-Voltage (CV) or Conductance technique for probing electronic traps in semiconductor devices cannot be performed on transistor structures relevant to real applications. This is because these techniques require large transistor structures to have enough capacitance to be measurable. Realistic devices have short gate length with consequently too low a capacitance to be accurately measured at the typical measurement frequency of 1kHz-1MHz, also any damage introduced into a device during device operation is typically in too small an area to be easily detectable using traditional techniques. In contrast, methods which can be applied to small III-V FET devices such as current-DLTS or transconductance dispersion respectively use a non-equilibrium pulse technique which is prone to misinterpretation, or have only given qualitative information to date. A key insight which underpins this proposal is that electronic traps in or near the channel primarily generate dispersion in a device below the pinch off voltage in the sub-threshold regime of operation which will be exploited in this programme. We will develop a dynamic transconductance method for GaN HEMT reliability analysis, suitable for small HEMT devices and insensitive to gate leakage currents. The development of this new electrical methodology which delivers the advantages of the quasi-equilibrium capacitance techniques but in small devices, will allow accurate measurements of degradation induced trap properties to be made for the first time. Noise measurements will complement this novel trap analysis, in additional we will benefit from the pulsed electrical-optical trapping analysis technique we developed in the ONR funded DRIFT programme. The work will advance the understanding of GaN HEMT device degradation during operation, i.e., device reliability, and will keep the UK at the forefront of internationally leading semiconductor device reliability research. The methodologies to be developed will also have direct applicability to the burgeoning worldwide effort in III-V CMOS technology for scaled low-power logic.

This proposal is focused on the development and application of new electrical methodologies for semiconductor device reliability research, to gain novel insight into the reliability of electronic devices and their degradation & failure mechanisms, in particular of AlGaN/GaN high electron mobility transistors (HEMTs). The benefits are such that it is now clear that GaN technology will replace GaAs in most microwave applications, and increasingly likely that it will displace Si in many power electronic systems over the next few years. This is a genuinely disruptive technology which will have profound negative implications for UK industry and academia if the UK does not maintain an active role in its deployment. GaN technology has already demonstrated impressive microwave performance for high power amplifiers (HPAs) in terms of power level, power density, power-added efficiency (PAE), bandwidth and robustness, clearly exceeding those permitted by the existing Si LDMOS, GaAs PHEMT or HBT technologies. Similarly for the power switching market, GaN switches deliver up to a 10x improvement in key figures of merit such as specific on-resistance compared to Si, giving higher efficiency, smaller size or higher operating frequency for application in a wide range of power conversion products. Companies around the world are gearing up to exploit the opportunity to support the low carbon economy which flows from the adoption of this exciting new technology. However, device reliability is still a major issue of concern. GaN HEMT reliability therefore needs to be addressed to achieve full uptake of this new technology, and gain the associated UK economic benefit, which therefore is the key aim of this proposal. The CDTR in Bristol has been the only UK academic group involved in the major European projects aimed at establishing an independent supply chain for GaN microwave technology (KORRIGAN, MANGA and GREAT2 funded by European Defence Agency and European Space Agency). This has arisen due to the demonstrated benefit to industry over the last decade of the techniques developed at the CDTR for thermal/strain measurement and reliability assessment. While achieving reliable GaN HEMT technology is naturally an industry focused activity, this success shows that there exist significant opportunities for UK academia to contribute to obtaining reliable devices, in terms of understanding why devices fail (i.e., device degradation mechanisms and physics of device failure) and the development of new analysis techniques and reliability testing concepts. This approach is embodied in this proposal and will not only directly benefit the partners of this project (IR, IAF, UMS, TriQuint) with whom regular meetings will be established to achieve direct industrial impact, but also other UK industries the CDTR in Bristol has links with (Selex UK, RFMD UK). Further benefits to the UK and to UK systems industry from this project would be through these links to world state-of-the-art device manufacturers. This gives UK academia an invaluable insight into the direction and capability of technology development in this rapidly evolving disruptive technology. Furthermore this project will train young researchers in device reliability. We note that IR UK is currently hiring GaN Process Scientists. The expertise gained by young researchers in this programme is an essential key knowledge base needed in UK power, defence and aeronautics industry. Furthermore, Bristol and UK industries will benefit as new analysis technique developments have great potential to provide the basis for the UK to be partners not only in future European but also US programmes. A previous successful example of the economic impact of Bristol research in the GaN HEMT research field is the CDTR / Bristol developed measurement technique for the quantification of temperature in devices, Raman thermography currently being commercialised via Quantum Focus Instruments (QFI), San Diego, USA.