Penn State Researchers Build Monolithic GaN AC-Powered LED Chip
In this new research, gallium nitride (GaN) LEDs and their power supply are integrated onto the same monolithic chip.
In this new research, gallium nitride (GaN) LEDs and their power supply are integrated onto the same monolithic chip.
GaN light-emitting diodes can now demonstrably be fabricated on the same chip as their power supply circuits—using industry-standard manufacturing techniques, no less.
Researchers from Pennsylvania State University recently published a paper on this milestone, Alternating Current III-Nitride Light-Emitting Diodes With On-Chip Schottky Barrier Diode Rectifiers, in IEEE Transactions on Electron Devices.
Image from IEEE Transactions on Electron Devices
It's important to note first that these researchers did not develop an LED that runs directly off AC power. What they did do is to develop a practical method of integrating a gallium nitride LED and a simple bridge rectifier onto the same chip. The groundbreaking result of this work means that these LEDs can be run directly from the 110 VAC wall outlet, with no separate rectifier ICs required.
Why Now? (And Not Before?)
It seems like a simple idea. Circuitry far more complex than LEDs and rectifiers has been placed on single chips for decades now.
Penn State's Professor Jian Xu, who led the project out of the university's Photonic and Optoelectronic Devices Group, explains the difficulty. “Gallium nitride (GaN) is a fairly new material system,” Xu says. “The technology has become mature only recently; that’s why integration on a single chip is a very new idea.”
The idea of an AC LED has been tried out before. In fact, the principal author on the paper, Jie Liu, has been working with GaN AC LEDs for years. His 2013 thesis, Colloidal Quantum Dot-enabling and Alternating Current Driven Gallium Nitride Based Light Emitting Diode Technology, explored the concept of GaN-based LEDs
Photos of (a) an unlit and (b) a lit microscopic AC LED integrated with a Schottky diode from Dr. Jie Liu's dissertation
Previous attempts, however, required either specialized LED structures, or required complicated non-scalable manufacturing processes or produced inefficient LEDs.
Solving the inefficiency issue was the key to the success of the new project. When building silicon chips, a “wet chemistry” process, such as treatment with hydrofluoric acid, is generally employed. But, as Xu explained GaN is too tough for that to work. Instead, a “dry etching” technique, such as inductively coupled plasma etching, must be employed.
But dry etching by itself left defects on the LED surface that causes the inefficiency. In the end, it was discovered that even though wet etching isn’t strong enough to form the chip, it could be used to remove the defects left by dry etching. Xu and his team eventually developed a sequence of dry etching and wet etching that produced low-defect, high-quality devices.
More Robust LEDs
Typically an LED is made of robust materials such as GaN, but the driver circuit is composed of silicon. Silicon isn't as rugged as GaN, so the driver circuit is usually what causes the device to require replacement. Since these devices are all GaN, they will be GaN tough and resilient through and through.
Xu also pointed out that the integration of the LED’s driver system (or power supply) could also greatly cut manufacturing costs. This is because the driver system is over half the overall cost of an LED bulb.
What’s Inside the Chip?
In the diagram below, the top left portion sketches out the all-GaN circuit. Here, we see four Schottky barrier diodes (SBD) in a bridge rectifier circuit.
A pulsating DC is applied to the LED array on the right. If the number of LEDs is chosen correctly, the voltage drop across each of them will be enough for each of them to work correctly.
Image from IEEE Transactions on Electron Devices
Of course, the output of the bridge is pulsating DC, so the light that the LEDs emit will have a 120-hertz flicker. This will make any product built of this technology more suitable for outdoor deployment. In that sort of application, light quality is secondary to low maintenance costs.
However, it’s easy to imagine an external filter attached to future products to smooth out the pulses, making these devices suitable to more genteel, indoor applications.
Are you working on an application that could benefit from this technology? Tell us about it in the comments below.
