GaN has been showing promise in the power industry for years, enabling smaller, lighter power systems for lower-voltage applications. Plessey Semiconductors, however, has been using it to make strides in microdisplays.
Last month, Plessey announced that they had innovated a way to achieve native green LEDs without color conversion. Additionally, last week, Plessey announced a world first with its monolithic microLED array pairing with a silicon backplane from Jasper Display.
To learn more about GaN-on-silicon, AAC spoke with Clive Beech, micro-LED Applications Director at Plessey Semiconductors, about their GaN-on-silicon technology, its use in microLED displays, and the future of LED technology.
GaN-on-silicon allows "native green" LEDs.
The Advent of GaN-on-Silicon Displays
Over the last 15 years, Beech says, displays have moved through many forms, from CRTs to flat screens to plasma displays. Growth in gaming and high-definition programming have continued to push developers into new solutions. LEDs have (so far) emerged as the most effective solution as displays have evolved into 1080, 1080p, and 4K versions—and they look key for the coming 8K displays.
Until recently, engineers used 5630 white LEDs for these displays, but with advancements in the use of gallium nitride (GaN), the approach changed because, as Beech notes, "Gallium nitride is the starting point for making blue LEDs."
The next step comes from a corner of the industry that many don't associate with displays. GaN offers several advantages for microdisplays when built on a silicon wafer, according to Beech: "GaN-on-silicon technology allows us to make microLED displays that actually can't be realized by normal GaN-on-sapphire approach," he notes.
How Does GaN-on-Silicon Work?
Making GaN-on-silicon technology is a complicated process. Starting with a silicon growth wafer, layers of GaN are added in precise layers.
"Over 130 separate layers go down before you end up at the top of the gallium nitride structure," notes Beech. "They're all very thin layers, so the total thickness of those layers is only of the order of three microns. Some [layers] are only a few nanometres thick, but all the different layers serve distinct purposes, in terms of matching the silicon crystal structures, the gallium nitride crystal structure."
The completed wafer is bonded to a CMOS wafer which carries the backplane and serves as the switching matrix for the LEDs.
"We are bonding high-performance LEDs to a high-performance CMOS backplane to make a microdisplay," Beech explains.
When using GaN-on-silicon, the light emitted from the LEDs is blue and must be converted to present an RGB color display. This is done through a technology known as quantum dot, an overlay design that takes the display's sea of blue pixels and groups them into threes. One of these pixels is left blue, one is converted to red, and the other is converted to green.
TVs Grow Larger, Microdisplays Get Sharper
While televisions have grown larger with higher definition, GaN-on-silicon has taken Plessey's components into smaller and smaller form factors, including .7-inch, .35-inch, and .27-inch sizes, with pixel performance equal to that of a large TV.
Unlike a television display, which sends light in a wide dispersion for everyone in the room to see it, microdisplays need the light from the display to be seen only by the user. Plessey uses a technique called collimated display, in which they send the display directly to the lens and eye of the waveguide.
A depiction of collimated light emission. Image used courtesy of Plessey.
Beech further explains, "We can actually take the light from each pixel and instead of having what's called Lambertian emission which is effectively in all directions, we can collimate it and bring that light into almost like a projector output, sending it directly into the lens and optics of the waveguide."
This collimated light is one of the biggest advancements for GaN-on-silicon displays and makes augmented reality (AR) one of the most prominent applications.
With virtual reality (VR) applications, eyepieces typically must wrap around a user's face in order to keep out light, or the application must have tremendous battery life for a bright display. By directing light directly to the waveguide, Plessey aims to provide microdisplays that will allow users to see the display even in daylight. AR is a natural fit for this technology as it places computer-generated visual information overtop a user's surrounding environment.
A visualization of Augmented Reality. Image courtesy Plessey.
Beech uses a warehouse environment as an example. "People will walk around the warehouse and they will be directed to pick up particular boxes or objects because, as they look around the factory, those objects will be highlighted in green or red in the wearer's eye."
Google is one of the companies researching professional AR applications, including its oft-maligned Google Glass project, which has indeed found utility in warehouse environments.
Gaming is another application with many possibilities. In one example, AR glasses pair a microdisplay with forward-looking cameras and can superimpose game elements onto the objects in front of the wearer. "So, it's a table in front of the wearer," says Beech, "and you can [see] lemmings rolling along the table and jumping off the axis of it."
The microdisplay technology is highly specialized and Plessey must custom design each application. As they own the IP and have a complete in-house design, testing floor, and assembly facility, they are able to turn out products rather quickly.
MicroLED Displays with Native Green
Beyond the 8K displays that will be arriving soon, Plessey is also developing a new technology following behind quantum dot. This technology emits green light without the need for color conversion.
Beech explains, "What we have done is to take our technology that generates blue and extended that by engineering those early layers that generate the transitions with silicon to gallium nitride. We've engineered those so that the natural emission is moved from 450 nanometres, which is blue, to 530 nanometers, which is green."
The "native green" display. Image from Plessey
This change brings about big changes in efficiency and luminescence. "When you do quantum dot color conversion, because green is not very far away from blue, the efficiency of conversion from blue to green is quite poor. But if we can generate green light naturally from the GaN then we can get considerably greater efficiency out of the display. In a color display, the color you need most light from is green so you have a small amount of blue a small amount of red, we can use lots of green in a display in terms of light output."
The native green LEDs will increase efficiency, as green requires higher energy to convert. Without these conversion losses, native green from GaN will provide higher luminescence with outstanding wavelength stability.
This technology is currently about six months behind the quantum dot technology, notes Beech.
World-First GaN-on-Silicon Full HD microLED Bonded Display
Last week, Plessey followed up the native green display with successful wafer-level bonding with Jasper Displays' eSP70 high-density CMOS silicon backplane. The result is a microLED display with addressable LEDs.
The microdisplay. Image from Plessey.
The significance of this achievement is that Plessey's wafer-to-wafer bond has now been followed by an electrical and chemical bond, allowing for a functional microLED display. This technology was demonstrated at last week's SID Display Week 2019 in San Jose, CA.
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