Optimising heat flow in wide and ultra-wide bandgap devices
: The applications of diamond

  • Callum Middleton

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Wide and ultra-wide bandgap materials are able to give the high performance necessary for applications such as LIDAR and wide bandwidth communications. These applications result in high power densities that must be managed, making innovative heat extraction techniques vital. In this work, heat flow and diamond heat spreading solutions are studied in novel device designs fabricated in GaN, AlGaN, and β−Ga2O3 to ensure that these materials can be utilised to their full potential. GaN-on-diamond material properties are studied through nanoparticle assisted Raman thermography and transient thermoreflectance. Seeding and growth conditions are shown to
impact the thermal properties, with an optimal seed size suggested to be
≈ 50 nm. The impact of GaN buffer thickness on GaN-on-diamond device operating temperature is assessed using photoluminescence. Excellent electrical performance and thermal resistances as low as 9±1 K/(W/mm) are demonstrated in devices with a buffer thickness of 354 nm. Superlattice castellated field effect transistors are efficient RF switches, but power density
is localised periodically along the gate. The operating temperature of these devices was found through micro-Raman thermography, gate resistance thermometry, and finite element simulations. The 3D gate was found to aid heat flow by providing a high thermal conductivity heat pipe; the devices have a thermal resistance of 19.1 ± 0.7 K/(W/mm) which is similar to that of
GaN-on-SiC devices. The first thermal study of β−Ga2O3 devices is presented, with a thermal resistance of 88±3 K/(W/mm) found, as well as the first study of it’s phonon lifetimes. Devices fabricated in low thermal conductivity materials such as β−Ga2O3 will need innovative thermal management.
This is explored using simulations, with a diamond passivation layer recommended.
Date of Award7 May 2019
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorMartin H H Kuball (Supervisor)

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