Here’s some news to brighten your day: Small display screens, like the ones needed for AR, VR, and other head-up displays, may soon become way more efficient and yield unprecedented resolution, thanks to a new kind of transistor made from vertically oriented gallium nitride nanowires.
GaN micro-LEDs are more desirable than today’s commercial designs in almost every way. For instance, each pixel on the screen of the iPhone X is created by a collection of organic LEDs, which are much bigger in size than nanowire GaN LEDs, resulting in larger pixel sizes.
What’s more, to control the color of each pixel, commercial devices rely on relatively bulky thin-film transistors (TFTs), whereas GaN LEDs can be designed to produce a color directly—and emit that color at 100 to 1,000 times the brightness and with at least double the efficiency.
The inherent properties of GaN also means that electrons can move up to 1,000 times faster than in silicon TFTs, which allows for much quicker on-and-off switching.
GaN LEDs created to date have had a horizontal design, however, with the transistor laid out next to the LED. Matthew Hartensveld, a Ph.D. student at the Rochester Institute of Technology who helped develop the new vertical GaN transistor, says its layout is analogous to building skyscrapers, instead of building multiple houses next to one another.
With the new vertical design, the transistor, or switch, resides directly below the LED, providing both dimming and switching abilities. “Without the area consumed next to the LED, additional LEDs can be placed in close proximity, creating higher pixel densities,” says Hartensveld.
In a recent publication in IEEE Electron Device Letters, Hartensveld and his colleagues describe how they vertically combined a nanowire GaN Static Induction Transistor (SIT) with a nanowire LED. The LEDs use unintentionally doped GaN as a starting layer. While this type of layer is not uncommon in LEDs, here it is used to form the vertical wires that integrate the LED with the transistor. To combine the transistor and LED, the group deposited metal and insulating layers, creating the different electrical connections for the device.[shortcode ieee-pullquote quote=""Without the area consumed next to the LED, additional LEDs can be placed in close proximity, creating higher pixel densities."" float="left" expand=1]
The new nanowire SIT provides a 900-fold greater on-to-off ratio compared with a recently developed GaN fin SIT with a planar layout, leading to increased power savings. The compactness of these new LEDs also allows more of them to be placed within a given area, which ultimately leads to higher resolution. Their minute dimensions reduce pixel sizes by 1,500-fold compared with the iPhone X, says Hartensveld.
One of the main limitations of this design is that SITs require a negative voltage in order to turn the LEDs off, so by default the LEDs are on. Also, Hartensveld notes that commercializing the design will require some expensive photolithography in order to precisely place each nanowire.
Nevertheless, the researchers are pushing forward. The GaN material, which makes up both the LED and transistor components, is optically transparent. That means it could pave the way for the development of transparent displays, perhaps in glasses used for augmented and virtual reality.
“The remaining key component for a head-up display is integrated memory. We are actively working on a novel way to introduce memory on the same platform, and with the same material, in order to complete the device. Once the memory aspect has been performed, all the needed functions would be tied together in one system to create a transparent display,” says Hartensveld.
Michelle Hampson is a freelance writer based in Halifax. She frequently contributes to Spectrum's Journal Watch coverage, which highlights newsworthy studies published in IEEE journals.