Wire it up: A packaged silicon avalanche photodetector (the gray square in the center of the image) is wired to several electrical contacts. The setup lets researchers test the device.
Avalanche photodetectors are based on similar principles. The difference is that a special region of the detector creates additional electron-hole pairs, so a single incoming photon can produce tens or hundreds of electrons, effectively amplifying the signal. Avalanche photodetectors use III-V materials for the amplification, but Intel’s researchers turned to silicon instead. The new detector consists of an absorption region made of germanium and a “multiplication region” made of silicon. Previously it had been tricky to combine these two materials due to the spacing of each material’s atoms. But a special method for laying germanium developed at Intel helped overcome this challenge. “The benefit of doing this in silicon is that it’s a much better multiplication material because of its inherent properties of low noise and crystalline purity,” says Paniccia.
The performance of a detector is measured by a characteristic called gain-bandwidth, which measures in gigahertz its potential speed and sensitivity. Traditional detectors operate with a gain-bandwidth of around 120 gigahertz, but Intel’s detector reaches 340 gigahertz. What this means, explains Paniccia, is that it can produce a larger electrical response, for a given speed, than traditional detectors. Ultimately, this gives engineers the freedom to build network components that operate faster and more efficiently.
“We now have the opportunity to build high-speed optical links in networks and bring low-cost optical communication in and around our PCs,” Paniccia says.
Intel’s work is “a significant development,” says Bahram Jalali, professor of electrical engineering at the University of California, Los Angeles. “Avalanche photodetectors provide the amplification needed to detect very weak optical data,” he says. “Normally, the downside is that the avalanche process that leads to amplification also adds noise to the data,” an undesirable characteristic that must be cleaned up using extra electronics. “However,” he says, “silicon’s material properties lead to very low avalanche noise.”
Paniccia stresses that the new detector is not yet ready to appear in products. “We still have work to do in reducing dark current,” he says, referring to stray current that leaks from the device even when it’s not absorbing photons. But he says that he expects to see commercial silicon photodetectors from Intel in the next couple of years. For one thing, it should be possible to make the device with the equipment used to make other kinds of silicon electronics. “This was fabricated in our production fabs alongside other products,” Paniccia says. “There’s nothing fundamental there that we couldn’t commercialize in the future.”