Exploration of GaN heterogeneous integration systems

Abstract

Compound semiconductor materials have attracted much research interest for decades, due to their ability to tune the band gap structure. Among them, the Gallium Nitride (GaN)-based material system has been a major focus due to its wide bandgap energy range, which allows emission wavelengths ranging from ultraviolet to infrared. This feature makes GaN-based materials ideally suited for fabricating optoelectronic devices, including light-emitting diodes, laser diodes, photodetectors, etc. Thanks to their wide bandgaps, these materials are also suitable for applications in power electronics. Although having advantageous characteristics, these GaN-based devices still require other circuit components to augment their functionalities. Integrating these GaN-based devices and components into one platform is preferable to obtain high system compactness and robustness, and even improved performances. Monolithic integration approaches, in which all components are fabricated on a single wafer can deliver system compactness to the highest extent, and some successfully integrated systems have already been reported. However, the fabrication processes, especially for more complex circuits, can be extremely complicated or even barely feasible. On the other hand, heterogeneous integration approaches, which separate the fabrication processes of each device, allow a better compromise between feasibility, cost-effectiveness, and system compactness. The proposed lateral heterogeneous integration systems using the wire bonding technique will bring the simplicity and feasibility of these devices to a new level. In this thesis, two heterogeneous integration systems were proposed, and the results were presented in the form of products and their characterization. The first system was the heterogeneous integration between a monolithic GaN-based micro-LED (μLED) display and three commercially available 74HC595 shift register bare dies, that served as the control units of the display. The monolithic GaN-based μLED display consisted of 50 μm × 50 μm pixels was fabricated in-house. The size of the display itself was 2.80 mm × 3.11 mm, and the overall footprint of the integrated system was 11 mm × 7 mm, which is comparable to the size of a single packaged 74HC595 shift register. This display system was able to deliver a luminance of over 5000 cd/m2. Moreover, the total number of pinouts was reduced from 26 to 11, making it easier to further combine with other systems. The second system was the heterogeneous integration systems of LED drivers using various driving schemes, including linear regulated power supplies and switch mode power supplies. The first two devices were driving circuits for 10 LEDs implementing a TPS71530 lowdropout regulator as the linear regulated power supply, with different platform materials. The other two devices were driving circuits for 10 LEDs using HV9921s as the switch mode power supply, with slightly different circuit configurations. The performances of these heterogeneously integrated circuits were verified by comparing them to their conventionally integrated counterparts and simulations. The system compactness was improved with a dimensional reduction of up to 78%.