Vertical GaN Transistors on Silicon Substrates

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Gallium nitride (GaN) is a wide bandgap semiconductor material with strong potential in power electronics, with lateral GaN transistors becoming dominant in certain low-voltage applications in the last few years. Vertical GaN transistors demonstrate even better efficiency and off-state blocking capability than lateral devices and are ideally suited for high voltage applications, although are currently confined to expensive and unsustainable native GaN substrates, preventing commercialization. Implementation of vertical GaN transistors on silicon (Si) substrates (GaN-on-Si) will enable reduced carbon emissions due to inefficiency across a range of mass market sectors, although significant technical challenges must first be overcome. Optimisation of device performance and advanced understanding of underlying physics of vertical GaN-on-Si power transistors will be achieved through the fabrication and characterization of devices using bespoke designed wafer material and carefully considered device designs. Multiple designs will be produced using TCAD simulation, with initial results already showing the importance of using a fully-vertical (i.e. not quasi-vertical) design. Devices will be fully-vertical, achieved through back-end contact to the 'buried contact layer' using local substrate removal and subsequent etching of required epitaxial layers (i.e. nucleation, strain relief and sub-drift layers), to be developed using the material from the pump-priming project. Device designs will be selected to maximize usage of the wafer material so as to provide the most valuable information for refining subsequent simulations. Devices will be fabricated in the University of Bristol cleanroom, and tested in the electrical characterization lab. Test structures will be implemented to ascertain the effectiveness of back-end contact techniques, and the tolerance of the associated fabrication processes, to be analysed using contact profilometry and scanning electron microscopy. Electrical characterization of fully-vertical Schottky diodes will be used to compare leakage, power density and breakdown characteristics. An optical technique developed at the University of Bristol (EFISHG) will be used to measure internal electric fields within the device, combining with electrical measurement and TCAD simulations to provide a holistic model of the device.