Light-emitting diodes can do way 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. 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.

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. 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. 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,” says Rajeev Ram, who leads the Physical Optics and Elec­tronics Group. “So the goal would be: Can you put all this together in one system?” Ram’s team did just that.

PhD-student Jin Xue designed a silicon-based LED with specially engi­neered junctions 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 company Global­foundries 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,” says Xue. He adds 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 says, “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 says. “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 nonideal 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,” says Ram. “It’s useful enough to be compe­titive for these microscale appli­cations.”

“This is an important develop­ment,” says Ming Wu, an electrical engineer at the Univer­sity of California at Berkeley, who was not involved with the research. “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.” Silicon “occupies the crown in electronic devices” will continue its reign “without a doubt,” says Chang-Won Lee, an applied optics researcher at Hanbat National University, who also was not involved in the work. However, he agrees with Wu that this advance represents a step toward silicon-based computers that are less reliant on electronic communi­cation. “For example, there is an optical CPU archi­tecture that the semi­conductor industry has been dreaming of. The report of silicon-based micro-LEDs shows signi­ficant progress in these attempts.”

Ram is confident that his team can continue fine­tuning 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,” says Ram. “We have ideas and results pointing to signi­ficant improve­ments.” (Source: MIT)

Reference: J. Xue et al.: Low Voltage, High Brightness CMOS LEDs, IEEE 66th International Electron Devices Meeting IEDM, Montgomery Village, USA

Links: Physical Optics and Electronics Group, Massachusetts Institute of Technology, Cambridge, USA • Globalfoundries, Santa Clara, USA