Light-emitting diodes — LEDs — are important in many more applications than just illumination. These light sources are useful in 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 on). 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, a measure of 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).
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 an abundant, cheap semiconductor material. 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.
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. So, the research team’s goal was to put all this together in one system. They designed a silicon-based LED with junctions specially engineered 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, the LEDs were manufactured in a commercial foundry right alongside other silicon microelectronic components, including transistors and photon detectors. While this 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 was quite an improvement over past reports,” said lead researcher Jin Xue. 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. The team plans to continue developing the technology. But, Xue says, “it’s already made great progress.”
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 nonideal surfaces, riddled with “dangling bonds” that allow energy to be lost as heat rather than as light. In contrast, silicon forms a cleaner crystal surface. “We can take advantage of those very clean surfaces,” said Professor Rajeev Ram. “It’s useful enough to be competitive for these microscale applications.” It allows silicon integrated circuits to communicate with one another directly with light instead of electric wires. This is somewhat surprising as silicon has an indirect bandgap and does not normally emit light. This advance represents a step toward silicon-based computers that are less reliant on electronic communication. For example, the semiconductor industry has long been dreaming of an optical CPU architecture. The report of silicon-based micro-LEDs shows significant progress in these attempts.