Projects per year
I am Royal Academy of Engineering Chair in Emerging Technologies, Fellow of the Institute of Electrical and Electronics Engineers (IEEE), Materials Research Society (MRS), Society of Photo-Optical Instrumentation Engineers (SPIE), IET (Institute of Engineering and Technology) and IoP (Institute of Physics), and Royal Society Merit Award Holder.
I am leading the Center for Device Thermography and Reliability (CDTR), a research centre focusing on improving the thermal management, electrical performance and reliability of novel devices, circuits and packaging. Since 2001 we have been developing and applying new techniques for temperature, thermal conductivity, electrical conductivity and traps analysis, especially for microwave and power electronic semiconductor devices, made of wide and ultra-wide bandgap materials, such as GaN, Ga2O3, SiC and diamond. We pioneered numerous experimental techniques which are now widely used in acadamia and industry, including Raman thermography (used for high spatial resoltution measurement of semiconductor device temperature), substrate backbiasing (for power electronic device development), and many more, and develop new microwave and power device concepts and their implentation. Our team of about 20 international researchers and PhD students works with industry and academia from across the globe to develop the next generation of technology for communications, microwave and power electronics to enable the low carbon economy.
I am leading numerous large research programmes, the EPSRC Programme Grant GaN-DaME and Platform Grant MANGI to develop and implement new GaN-on-Diamond device technology; my group is also part of the US Department of Energy (DOE) funded Energy Frontier Research Center (EFRC) ULTRA developing new ultra-wide bandgap semiconductor materials and devices for smart grid applications. We are furthermore in process setting up the first UK site for Ga2O3 material growth for >2kV power device technology.
I am co-funder of TherMap Solutions, a spin-out company from the University of Bristol, providing industry the tools for accurate thermal conductivity measurements of materials used in a wide range of applications, ranging from electronics, to aerospace, to nuclear applications and beyond. Good heat sinking is critical for many applications which the thermal measurement tools support to develop.
Please visit the CDTR website for more information on our research, the group and its team members, latest news, and open positions.
We are presently looking for PhD students to join our group in the following areas:
Ultra-high thermal conductivity semiconductor device packaging - metal diamond composites and nano-silver die attaches
Many semiconductor devices operate nowadays at power density much greater than traditional Si and GaAs devices. When these devices are packaged, traditional CuMo based device packages are employed. These have been used for decades, but there is innovation on the horizon (and this is urgently needed). This project will explore exciting new materials, metal-diamond composites as well as nano-silver based die attaches to increase the ability of a semiconductor package to extract heat from the semiconductor chip. Challenges exist in how to optimize heat transport across interfaces including the diamond-metal interface. Heat transport in diamond is phonon based but in a metal it is mainly electron based which causes natural challenges and is still poorly understood.
Gallium Oxide – Next generation power electronic semiconductor devices
Unless more energy efficient semiconductor devices are developed, there will be major energy shortages in the future. If it progresses at the current rate, Artificial intelligence (AI) will consume most of the energy humans generate in a few decades. There is an exiting material on the horizon, Gallium Oxide (Ga2O3), with a bandgap of 4.9eV , that will allow high breakdown voltage, energy efficient power electronics, electric cars and electric planes. We have demonstrated its excellent device performance with collaborators in Japan and the USA, but also the limits it faces, namely excessive device heating as it is a low thermal conductivity material as well as carrier trapping. This project will address to understand the physical origins of these device limits and develop mitigation strategies; this will include the integration of this new material with diamond to aid heat extraction.
Phonon & heat transport in diamond – The challenges to make it the best material
The high thermal conductivity of diamond has been widely exploited in the thermal management of semiconductor devices, enabling cooling of high temperature areas in high power electronic devices. To make production cost-effective, instead of using single crystalline diamond, heat is managed with the use of polycrystalline diamond. However, this material exhibits an extensive microstructure which impacts on phonon and, as a result, on heat transport. This process is still poorly understood and even more so if the diamond is integrated with electronic materials such as GaN for ultra-high-power microwave electronics (GaN-on-Diamond). The research project focuses on developing and applying phonon-based heat transport models to gain unprecedented insight into the thermal properties of GaN-on-Diamond ultra-high-power microwave electronics. The project benefits from our current EPSRC Programme Grant GaN-DaME project and will also contain experimental characterization of materials.
Beyond graphene materials and devices
Graphene has generated lots of excitement over many years; but what comes after graphene? In this project we explore Te-based 2D semiconductors which have demonstrated ultra-high optical sensitivity suitable for detector applications. The challenge is these materials oxidize quickly and need to be encapsulated, which can be achieved using graphene but also BN; we will explore advanced devices using these new materials including GaTe but also using BN in particular, next generation detectors (optical and neutrons) as well as transistors.
Current-collapse-free devices? It can happen!
GaN power switching devices offer outstanding on-resistance, breakdown voltage and high-speed switching performance - and tremendous progress in their development had already been achieved. However, a continuing issue has been their dynamic on-resistance (or current-collapse: CC), which is a trap-related increase in on-resistance, following high off-state bias operation. Over the last few years considerable progress has been reported with CC-free devices, but some results are still dependent on measurement conditions such as switching time, switching type (hard or soft), temperature etc. We offer a complete study of electrical characteristics of the device and its performance, working towards the goal of achieving a fundamental understanding of potential problems – and solutions, in this technology. Detailed measurements of dynamic on-resistance at different switching and temperature conditions, followed by a simulation to understand what the measurements are highlighting, finishing with a model that explains the physical mechanism – a very exciting process, from beginning to end!
RF switching and hot electron related degradation using electroluminescence
A full understanding of reliability is essential for RF transistors. Various aspects of RF reliability testing of GaN-based devices have already been addressed by different research projects, such as gate metal instabilities, inverse piezoelectric effects, passivation breakdown, and generation of trap states; comparisons between dc and RF reliability testing were also performed. Hot-electron degradation depends not only on the concentration of hot electrons involved in the transport, but also on the energy the electrons obtain due to the high accelerating electric field. The stress high hot-electron concentrations put on devices can cause defects. Electroluminescence (EL) (which originates from hot electron scattering with defects - and consequently Bremsstrahlung) is widely used for the assessment of the hot electron- related degradation rate in GaN-based devices. Recently, EL has been successfully used to understand how hot-electrons act on the device under class B and class J RF operations and compared to dc operation on the same load line. The focus of this work is to compare the hot-electron behaviour of the device under different classes of RF operation with EL (classes A, B, and F - working with Cardiff University) and using different electrical characterization techniques.
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Bicer, M., Valle, S., Brown, J. G., Kuball, M. H. H. & Coimbatore Balram, K., 15 Jun 2022, In: Applied Physics Letters. 120, 24, 6 p., 243502.
Research output: Contribution to journal › Article (Academic Journal) › peer-reviewOpen AccessFile21 Downloads (Pure)
Self-assembled microstructures with localized graphene domains in an epoxy blend and their related propertiesHe, S., Stadler, H., Zheng, X., Huang, X., Yuan, G., Kuball, M. H. H., Unger, M., Ward, C. & Hamerton, I., 22 Sep 2022, (E-pub ahead of print) In: Applied Surface Science. 607, 154925.
Research output: Contribution to journal › Article (Academic Journal) › peer-reviewOpen AccessFile2 Downloads (Pure)
Yang, F., Singh, M., Uren, M. J., Martin, T., Hirshy, H., Casbon, M. A., Tasker, P. J. & Kuball, M. H. H., 1 Feb 2022, In: IEEE Transactions on Electron Devices. 69, 2, p. 525-530 6 p.
Research output: Contribution to journal › Article (Academic Journal) › peer-reviewOpen AccessFile1 Citation (Scopus)49 Downloads (Pure)
Manikant, M. (Creator) & Kuball, M. H. H. (Data Manager), University of Bristol, 21 Aug 2018