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Professor Martin Kuball

Center for Device Thermography and Reliability

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 bandgap materials, such as GaN, SiC and diamond. 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.

Please visit the CDTR website for more information:

I am Fellow of IET (Institute of Engineering and Technology) and IoP (Institute of Physics), and Royal Society Merit Award Holder.

We are presently looking for PhD students to join our group in the following areas:

Diamond related materials for GaN high power microwave devices – Thermal properties (M. Kuball, J.W. Pomeroy)
This research project focuses on diamond materials, in particular interfaces of diamond to metals and GaN devices. Bristol leads a £5M UK research programme on GaN-on-diamond to enable ultra-high power electronic microwave devices. Diamond enables the efficient heat extraction from the GaN devices which reduces channel temperature, to enable reliable long lifetime GaN devices. For this it is critically important to understand phonon transport across interfaces. The project aims to understand the heat transfer from the GaN into the diamond, the impact of the 30-50nm thin dielectric interface layer typically present at this interface, and of the diamond grain structure on thermal conductivity. The project takes advantage of thermal characterization techniques pioneered in Bristol (Raman thermography, transient reflectance, etc.), and recently established thermal AFM facilities.

Interface nano-mechanical properties of semiconductor interfaces (M. Kuball)
For the past 50 years, microwave and power devices have relied on traditional semiconductor materials such as Silicon (Si) and Gallium Arsenide (GaAs). However, they have now reached their limits. There is now a huge drive to develop new semiconductor devices that utilise new materials, in particular integrate different new semiconductor materials to benefit from the best properties of each of the different semiconductors. This however will introduce a multitude of interfaces with its challenges and opportunities. This project investigates the nano-mechanical properties of interfaces such as of GaN-on-diamond, bonded semiconductor interfaces and 2D material mechanical properties, to enable stable material structures and devices. The project takes advantage of our recently established nano-indentor and nano-mechanical testing facility.

GaN buffer design for microwave and power electronic applications (M Kuball, MJ Uren)
GaN devices suffer from current-collapse / dynamic Ron. This is the difference in current between DC and pulsed / RF device operation. The doping and point defect density in the GaN buffer of the devices plays a critical role to minimize these effects, and ep itaxial design of the buffers is a key enabler of new GaN device physics and device technology. We will take advantage of the Bristol pioneered buffer back-biasing technique and develop new electrical techniques to gain insight into point defect densities in the context of their impact on RF and power devices.

2D Materials beyond graphene (M Kuball, MJ Uren, JW Pomeroy)
Graphene has been pioneered as a next generation electronic material, however due to its metallic nature it has limited active electronic application. In this project we exploit other 2D materials including GaTe and others for new devices. The project involves device fabrication, and electrical characterization, as well as device modelling to fully exploit the potential of these new materials for electronic applications.

Research keywords

  • Raman
  • photoluminescence
  • thermography
  • temperature
  • stress
  • electrical device analysis
  • electronic traps
  • reliability
  • failure
  • semiconductor
  • transistors
  • Gallium nitride (GaN)
  • Silicon carbide (SiC)
  • diamond
  • 2D materials (MoS2
  • GaTe
  • graphene
  • etc)