Jobs & Opportunities
We are always looking for enthusiastic PhD students, summer students and outstanding postdoctoral researchers to join our group to perform cutting-edge research.
We have a PhD Studentship available in thermal management research of ultra-wide band gap semiconductors, associated with the DOE ULTRA centre we are involved in - if you are interested please get in touch: Martin.Kuball@bristol.ac.uk.
Listed below are a range of possible PhD topics that individuals can undertake with the Center for Device Thermography and Reliability. In the funding box to the side we've listed potential funders and funding opportunities.
Ultra-wide bandgap (UWB) semiconductor devices
Almost all low-carbon technology presently relies on Silicon (Si) based power electronic devices. However, some is starting to be replaced with wide bandgap semiconductors based on Gallium Nitride (GaN) and Silicon Carbide (SiC) to enable our daily lives, from charging our phones and laptops, to feeding in wind or solar energy into the national grid. While these devices are serving us well, the unequivocal need to accelerate the reduction of our carbon footprint and to reduce the ongoing climate changes demonstrates the clear urgency to do much more. This project focuses on a new class of materials and devices with high versatility - ultra-wide bandgap semiconductors - defined as materials with a bandgap of greater than 4eV, including the new materials of Gallium Oxide (Ga2O3) and Boron Nitride. This project aims to deliver the fundamental UWB device technologies promising a >10x reduction in on-state energy losses compared to the incumbent devices, and energy savings in excess of 10% across the globe. This project involve device fabrication and detailed characterization of the limiting factors or their performance and reliability.
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.
MOCVD growth of Ga2O3 and related materials
As fundamental building block for Ga2O3 devices understanding its growth on the pathway of innovative device structures is critically important. We recently invested into an MOCVD Ga2O3 growth system; this project will investigate Ga2O3 growth on bulk Ga2O3 substrate, doping, heterostructures; growth on high thermal conductivity substrates will also be explored, as good heat sinking in present Ga2O3 devices is lacking due its low thermal conductivity. New material structures for next generation power devices will be developed and implemented. Involvement in device fabrication is possible.
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!
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.
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.
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.