Abstract
Light-emitting diodes (LEDs) are solid-state devices that are highly energy efficient and considerably more durable compared with other forms of lighting, such as compact fluorescent lights (CFLs) and traditional incandescents. LED technology has improved steadily over the years: every decade, the cost per lumen falls by a factor of 10 and the amount of light generated per LED package increases by a factor of 20 for a given wavelength of light. This has resulted in significant growth in the LED industry. Besides illumination, LED technology has compelling use cases in Visible Light Communication (VLC) and life science research. With its characteristic high switching speed, LEDs can transmit data whilst providing indoor illumination simultaneously. Moreover, the ability to precisely control the wavelength of light emitted allows LEDs to find applications in biology and life science research. However, in order to enable the full potential of next generation smart lighting, there are several crucial parameters that needs to be considered and optimised in LED driver design. These parameters include efficiency, size, and cost of LED driver designs. It is therefore crucial to explore methods that could optimise these parameters without much trade off. A lot of research has tried to improve the power efficiency of LED driver design with various driver topology and control schemes. However, these works integrate the gallium nitride (GaN) elements on the printed circuit board level with the complementary metal oxide semiconductor (CMOS) circuitry. The fully on-board integration of GaN elements results in additional board space required on the printed circuit board (PCB) and could subsequently result in a higher cost required to manufacture the LED driver. Other research targeting to reduce the size of LED drivers has tried to increase the integration level of LED drivers by moving conventionally on-board components, like power field effect transistors (FETs) and LEDs, to be integrated on-chip instead. However, the on-chip and on-board efficiency comparisons of LED driver designs were not reported in these works. This means that there is a lack of empirical iv data to affirm if the fully on-chip design could provide higher efficiency on top of the reduced board area afforded by the heterogeneous integration of GaN elements onchip. Moreover, on-wafer integration of GaN elements might result in process changes required during standard back end of line (BEOL) processing in the foundry. For approaches that require heterogeneous integration happening before or during the BEOL processes, extensive process change and budgets would be required in the foundry, posing difficulties to process adoption. In this dissertation, we explore a three-pronged approach to develop an LED driver for an integrated LED that (1) has a small form factor owing to the heterogeneous integration concept enabled by GaN2BCDTM technology, (2) is energy efficient by utilising improved resonance topologies with GaN technology, and (3) is customised to accommodate the appropriate manufacturing required for achieving higher energy efficiency. In the second half of the thesis, the heterogeneous integration of both the GaN LED die and GaN transistor on top of the Bipolar CMOS DMOS (BCD) driver integrated circuit (IC) is first reported. The integrated LED lighting prototype serves as a proof-ofconcept that both GaN LED and GaN FET can be integrated on the BCD wafer without additional process changes required during standard BEOL processing in the foundry. Resonance techniques were employed to improve efficiency and performance of the proposed on-chip design has been compared with the conventional fully on-board integration of power devices with the LED driver IC. Experimental results confirm that the fully on-chip integrated LED driver achieved a consistently higher efficiency value compared with the fully on-board design within the input voltage range of 4.5 - 5.5 V. As a subset of LED display technology, micro light emitting diodes (µLEDs) driver designs were also explored. The µLEDs matrix is a display technology that is based on microscopic LED devices that are used to directly create color pixels. µLEDs have been shown to have higher luminous efficacy, higher contrast ratio, and higher energy efficiency compared to existing mainstream technologies based on liquid crystals or organic light-emitting diode (OLED). Portable display applications such as wearable devices and head-up display are some of the interesting application of µLEDs v isplays. However, this technology has not yet been mass produced for commercial devices due to process yields, costs and manufacturability issues. In order to improve manufacturability, we eliminated the need for a dedicated bond stack in the bumpbonding process. To validate the concept, custom high performance two dimensional (2D) arrays of parallel-addressed GaN blue micro-light-emitting diodes (µLEDs) matrices were fabricated. The heterogeneous bonding of the µLED matrix with individual µLED pixel diameter of 20 µm in this thesis is the smallest reported for active matrix addressing without the need for a dedicated bond stack at each pixel for the bumpbonding process at the time of which this dissertation is undertaken.