LED developed for direct integration into computer chips

Researchers have fabricated a silicon chip with fully integrated LEDs, bright enough to enable sensor and communication technologies, which could streamline manufacturing and also provide better performance for nanoscale electronics.

By Daniel Ackerman December 16, 2020

Light-emitting diodes — LEDs — can do more than illuminate your living room. These light sources are useful microelectronics, too. Smartphones, for example, can use an LED proximity sensor to determine if you’re holding the phone next to your face (in which case the screen turns off). The LED sends a pulse of light toward your face, and a timer in the phone measures how long it takes that light to reflect back to the phone, a proxy for how close the phone is to your face. LEDs are also handy for distance measurement in autofocus cameras and gesture recognition.

One problem with LEDs: It’s tough to make them from silicon. That means LED sensors must be manufactured separately from their device’s silicon-based processing chip, often at a hefty price. But that could one day change, thanks to new research from MIT’s Research Laboratory of Electronics (RLE). The researchers have fabricated a silicon chip with fully integrated LEDs, bright enough to enable state-of-the-art sensor and communication technologies. The advance could lead to not only streamlined manufacturing, but also better performance for nanoscale electronics.

Silicon is widely used in computer chips because it’s abundant, cheap, and a semiconductor, meaning it can alternately block and allow the flow of electrons. This capacity to switch between “off” and “on” underlies a computer’s ability to perform calculations. But despite silicon’s excellent electronic properties, it doesn’t quite shine when it comes to optical properties — silicon makes for a poor light source. So electrical engineers often turn away from the material when they need to connect LED technologies to a device’s computer chip.

The LED in your smartphone’s proximity sensor, for example, is made from III-V semiconductors, so called because they contain elements from the third and fifth columns of the periodic table. (Silicon is in the fourth column.) These semiconductors are more optically efficient than silicon — they produce more light from a given amount of energy.

MIT researchers have developed a bright, efficient silicon LED, pictured, that can be integrated directly onto computer chips. The advance could reduce cost and improve performance of microelectronics that use LEDs for sensing or communication.
Courtesy: Masssachusetts Institute of Technology (MIT)

And while the proximity sensor is a fraction of the size of the phone’s silicon processor, it adds significantly to the phone’s overall cost. “There’s an entirely different fabrication process that’s needed, and it’s a separate factory that manufactures that one part,” said professor Rajeev Ram. “So the goal would be: Can you put all this together in one system?” Ram’s team did just that.

Jin Xue, a PhD student in RLE, designed a silicon-based LED with specially engineered junctions — the contacts between different zones of the diode — to enhance brightness. This boosted efficiency: The LED operates at low voltage, but it still produces enough light to transmit a signal through 5 meters of fiber optic cable. Plus, Global Foundries manufactured the LEDs right alongside other silicon microelectronic components, including transistors and photon detectors. While Xue’s LED didn’t quite outshine a traditional III-V semiconductor LED, it easily beat out prior attempts at silicon-based LEDs.

“Our optimization process of how to make a better silicon LED had quite an improvement over past reports,” Xue said. He added that the silicon LED could also switch on and off faster than expected. The team used the LED to send signals at frequencies up to 250 megahertz, indicating that the technology could potentially be used not only for sensing applications, but also for efficient data transmission. Xue’s team plans to continue developing the technology. But, he said, “It’s already great progress.”

Ram envisions a day when LED technology can be built right onto a device’s silicon processor — no separate factory needed. “This is designed in a standard microelectronics process,” he said. “It’s a really integrated solution.”

In addition to cheaper manufacturing, the advance could also improve LED performance and efficiency as electronics shrink to ever smaller scales. That’s because, at a microscopic scale, III-V semiconductors have non-ideal surfaces, riddled with “dangling bonds” that allow energy to be lost as heat rather than as light, according to Ram. In contrast, silicon forms a cleaner crystal surface. “We can take advantage of those very clean surfaces,” Ram said. “It’s useful enough to be competitive for these microscale applications.”

Ram is confident that his team can continue fine-tuning the technology, so that one day LEDs will be cheaply and efficiently integrated into silicon chips as the industry standard. “We don’t think we’re anywhere close to the end of the line here,” Ram said. “We have ideas and results pointing to significant improvements.”

– Edited by Chris Vavra, associate editor, Control Engineering, CFE Media and Technology, cvavra@cfemedia.com.

Author Bio: Daniel Ackerman, MIT News Office