Corrected 8 October 2009.
7 October 2009—Willard Boyle and George Smith, formerly of Bell Telephone Laboratories, in Murray Hill, N.J., will share half of this year’s Nobel Prize in Physics "for the invention of an imaging semiconductor circuit-the CCD," the basis for digital imagery in everything from pocket cameras to the Hubble Space Telescope. (The "imaging" part of the citation is in dispute, as the first imaging CCD was developed by IEEE Fellow Michael F. Tompsett, a colleague of Boyle and Smith.) In announcing the awards, the Royal Swedish Academy of Sciences called Boyle and Smith “masters of light” and said that, with fellow winner and optical-fiber pioneer Charles Kuen Kao, they “helped to shape the foundations of today’s networked societies.”
Boyle and Smith came up with the idea for the CCD during a brief meeting in 1969. The two were working on semiconductor integrated circuits, and Smith had been involved with trying to create an imaging chip for the Picturephone, which consisted of an array of silicon diodes. At the time, Bell Labs was also working on a new type of computer memory that relied on tiny bubbles of magnetism. As the two recalled in a 1976 article in IEEE Transactions on Electronic Devices , their boss, Jack Morton, urged them to look at whether it was possible to make a form of bubble memory using semiconductors.
The idea is fairly simple. You start with a layer of silicon, doped so that it’s deficient in electrons and oxidized at the surface. Atop the oxide, add an array of metal electrodes as gates, creating capacitors that can store charge. Then apply a voltage to the gate, which repels the silicon’s positive carriers—the holes—and creates a potential well at the surface of the silicon. When a photon strikes the silicon, it creates an electron-hole pair, and the electron moves toward an electrode into the well. Electrons accumulate in the interface between the silicon and the oxide. When you apply a sequence of high and low voltages to adjacent gates, the electrons move from one gate to the next, like water being poured from one bucket to another, until they reach the edge of the chip, where the level of charge can be read as a measure of light intensity.
“Essentially, this was the invention,” Smith recalled in a 2001 interview with the IEEE History Center. “Let’s not have any circuit in between. Let’s just put these things close together. If charge is stored here, just put a voltage over here that’s bigger than the voltage over there, and the charge will fall over. This voltage is then reset, and you repeat the process.”
Though their aim was to invent memory, Smith says it was obvious that the technology would work for imaging as well. And when random access memory (RAM) came along, attempts to create bubble memory were abandoned.
James Janesick, director of sensor development at Sarnoff Corp., says he’s long thought the two should receive a Nobel. “Besides seeing the edge of the universe, which is how I applied the technology at the time, it made billions and billions of dollars.”
Janesick was a young researcher at NASA’s Jet Propulsion Laboratory, in Pasadena, Calif., in the early 1970s, where he worked on the imaging system for the Hubble. At the time, engineers were actually considering putting film in the telescope’s cameras and sending astronauts to retrieve it, or using old-fashioned vidicon camera tubes. The fact that CCDs were 100 times as sensitive as either film or camera tubes, Janesick says, led them to quickly sweep through the field of astronomy and completely dominate scientific imaging today.
Carlo Sequin, a professor of electrical engineering and computer science at the University of California, Berkeley, joined Smith’s group to work on CCDs after the initial invention. He helped build the first CCDs that were compatible with the U.S. television format, thus reducing the size of TV cameras. “It was very exciting, because there was this very new principle that at the beginning seemed to be very simple,” Sequin says. In practice, he says, there were a lot of design issues to solve to make the device practical.
“What used to be this incredibly simple principle is one of the most complex devices being built today, yet for $100 you find it in every camera,” Sequin says.
Imagers based on complementary metal-oxide-semiconductor (CMOS) technology are starting to take over in some areas, particularly in low-end cameras, where price is a greater issue than performance. These sensors are still not quite as sensitive as CCDs, but the same advances in lithography that have propelled microprocessors are improving CMOS sensors, and it’s widely agreed they’ll eventually displace CCDs.