We are always looking for enthusiastic PhD students, Summer students and outstanding Postdoctoral researchers to join our group to perform cutting edge research. If you are interested in joining the CDTR, please send an expression of interest and your CV to Professor Martin Kuball.
For financial support we recommend you look at the following opportunities:
- Royal Commission for the Exhibition 1851
- Newton International Fellowship
- Commonwealth Scholarship
- China Scholarship Council Partnership
Currently the following opportunities are available in our group:
Research Associate in in GaN-on-Diamond Ultra-High Power RF Materials/Devices
Further details are available here: University of Bristol Jobs
We support fellowship applications - please contact us for more details.
PhD projects currently available:
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.