As IEEE Fellow Kei May Lau sees it, the problem with conventional LEDs, which are current controlled devices, is that turning them on and off rapidly to control brightness or using them for Li-Fi takes careful engineering and a bunch of circuitry.
“Most IC designers would rather work with voltage control device, but LEDs are current controlled devices,” says Lau. The combination of an LED’s high current and low voltage requirements makes designing drivers for them troublesome.
So she and her students invented a device, the HEMT-LED, that makes it much easier. The HEMT-LED, which is a bit like a light emitting transistor, lets you switch light emission on and off and control brightness with voltage signals.
The HEMT-LED, which is made from gallium nitride, integrates a high-electron mobility transistor (HEMT) and an LED so tightly that they merge into a single device.
HEMTs are capable of fast switching because they confine the flow of current through a very-thin, high conductance layer called a two-dimensional electron gas. In the HEMT LED, the LED becomes part of the transistor structure. Current flows through the LED portion of the device and then funnels into the electron-gas layer.
Simply connecting a HEMT and an LED won’t produce the same results. And neither will driving gallium nitride LEDs bonded to layer of silicon CMOS transistors. In results reported this month in IEEE Electron Device Letters, Lau’s team compared its HEMT-LEDs to the latter system and found that it could switch 5 MHz faster despite driving a light-emitting area that was more than 200 times larger. (Larger devices add capacitance; capacitance slows signals down.)
The HEMT-LED is a single three-terminal device that control light emission via voltage signals.Image: Hong Kong University of Science and Technology
The device’s ability to switch quickly while controlling its current is one of its key features. Modifying the width of the voltage pulse in a 500 Hz signal on the HEMT-LED’s gate made the LED dimmable from 100 percent of its light output to 0.1 percent without the LED changing color as can happen if current isn’t properly controlled. Such a broad range of intensity is something that previously could only be achieved with fairly complex circuitry. But for the HEMT-LED the dimming capability is an inherent characteristic of the device. Such smooth fast dimming might seem like overkill for home lighting, but it’s key to getting a good contrast ratio for LED-backlit LCDs.
Lau noted the device’s potential for visible light communications. Her team measured the HEMT-LED transmitting at just over 16 megabits per second. However, whether it could be used for Li-Fi, vehicle-to-vehicle communications via headlights, or another scheme will require further investigation. She notes that there are important trade-offs that come into play depending on how far the signal is meant to travel. Longer distances need a bigger, brighter LED, but bigger LEDs switch slower, reducing the data rate.
Her lab has instead been focusing on making arrays of smaller HEMT-LEDs, with the hope of incorporating them in microLED displays. These still-in-development displays are in the works by tech giants, such as Apple and Facebook, and startups such as Aledia alike.