These displays could play crucial roles in ultra-portable products such as next-generation pico-projectors and in emerging fields such as biophotonics and optogenetics.
Texas Tech University and III-N Technology, Inc., Lubbock, TX, and the US Army RDECOM CERDEC Night Vision and Electronic Sensors Directorate, Ft. Belvoir, VA
In the last 20 years, high-brightness LEDs based on III-nitride semiconductors have achieved dramatic advances alongside developments in indicators and solid-state lighting. For example, InGaN-based white emitters have achieved a luminous efficacy of more than 150 lm/W, which is much higher than those of other self-emissive devices, such as organic LEDs (OLEDs) and electroluminescent emitters. With the incorporation of multiple quantum wells (MQWs) as the active region, LEDs have a narrow emission band of about 25nm, providing a basis for high color purity and chromatic fidelity. With their intrinsic material properties and low-voltage operation characteristics, LEDs have a much longer operational lifetime (>100,000 hours), and can be operated at extreme conditions such as high or low temperatures (-100 to 120 °C) and humidity. All of these intrinsic properties make LEDs an ideal candidate for many applications where performance, reliability, and lifetime are critical.
Since their inception, μLED arrays based on III-nitride semiconductors have emerged as a promising technology for applications including self-emissive microdisplays, and single- chip, high-voltage ACLEDs for solid-state lighting. The InGaN-based μLED array has opened a new avenue for the multi-site photostimulation of neuron cells, and offers the opportunity to probe biological neuron networks at the network level. In particular, III-nitride μLED arrays provide high brightness/contrast/resolution/reliability, long life, compactness, operation under harsh conditions and under bright daylight — properties that cannot be matched by more conventional liquid crystal display (LCD), OLED, and digital light processing (DLP)-based microdisplay technologies.
III-nitride μLED array integration with Si CMOS accomplishes a high-resolution, solid-state, self-emissive micro - display operating in an active driving scheme. The fabricated blue or green video graphics array (VGA) microdisplays (640 x 480 pixels) have a pixel size of 12 μm, a pitch distance of 15 μm, and are capable of delivering real-time video graphics images. An energy-efficient active driving scheme is accomplished by integrating micro-emitter arrays with CMOS active matrix drivers that are flip-chip-bonded together via indium metal bumps. This success means that InGaN μLED arrays could play crucial roles in emerging fields such as biophotonics and optogenetics, as well as ultra-portable products such as next-generation pico-projectors.
In a monolithic μLED microdisplay, the μLEDs themselves and the interconnection between these μLEDs (the signal transmission paths, including all the metal lines for n- and p-type contacts) are all integrated on the same GaN wafer. This monolithic integration has the merits of easy and quick demonstration and characterization, since the microdisplay itself is an independent package, and the driving circuit can be designed using an off-the-shelf CMOS integrated circuit (IC) chip. However, to achieve a full-scale microdisplay in a monolithic μLED array, connecting the huge amount of control signals from a separate driving circuit to the microdisplay within a limited space is a very difficult, if not impossible, task.