Editor's Note: This is part of our ongoing coverage of the 2009 Nobel Prize in Physics. Read more about the Nobel Prize winners themselves, the Bell Labs engineer who patented the CCD imager, and the illustrious history of Bell Labs.
So who deserves the accolades for inventing the charge-coupled device? "It depends on what you're celebrating," says Carlo Sequin, who joined the team at Bell Labs developing the CCD a few months after the project began.
"My initial assumption was the Nobel in physics goes to fundamental concepts," says Sequin, now a professor of electrical engineering and computer science at the University of California, Berkeley. "If the fundamental concept was the charge transfer principle, then that goes to [Willard] Boyle and [George] Smith, and maybe Gene Gordon."
But while Boyle and Smith, who were initially trying to design something analogous to magnetic bubble memory for computers in silicon, sketched out the charge transfer concept, they were not the ones who actually built the CCD, Sequin says.
"If we try to find out who made the first practical image sensor, credit would go to Mike Tompsett, possibly [Gilbert] Amelio," he says.
The concept for the CCD was that one could build a potential well in silicon by creating a capacitor out of silicon, silicon oxide, and a metal electrode. Light striking the silicon would be absorbed and would create an electron, which would travel to the well. By applying alternating voltages, you could then moves the accumulated charges from one well to the next until they reached the edge of the chip, where the amount of the charge would reveal the intensity of the light striking each well.
In the first design, as Sequin describes it, rows of pixels were lined up in columns, and the charges would move down the columns, from one row to the next, until they were read out in a register at the bottom. This design meant the device would take an exposure, close a shutter, move the charges down one row, and then open the shutter for the next exposure. That meant that every other frame would be spent moving the charge.
Tompsett solved the issue with the frame transfer principle. "He came up with the idea, why not double the size of that image sensor," Sequin says. Only half of the sensor would be exposed, and then the next frame of exposure would be taken while the charge was being moved. "As far as I know, he has the patent on it and he is the sole author of it, and it was his idea," Sequin says.
He says others, including himself, contributed to the development of the CCD. Amelio made the first 8 x 8 pixel chip. Sequin was heavily involved in turning that into a 128 x 128 pixel array. Walter Bertram came up with the idea of building the chip with three layers of polysilicon replacing metal as the electrodes. Since they're transparent, they could be deposited on top of the chip without blocking the incoming light. Putting them in different layers prevented them from shorting out in a way that would destroy the whole device; at most you'd lose a pixel or a column.
Sequin says two technical associates, Edward Zimany and William McNamara, were also heavily involved in developing a practical CCD. "It was six engineers who took a very fundamental principle and really made a practical thing of it." He likens Boyle and Smith to sperm donors, providing the seed of the idea, while the others were midwives and mothers who nurtured it into reality.
But the credit for the ubiquity of CCD-based cameras may belong overseas, he says. Bell Labs, which had been focused on creating a picture phone, eventually dropped the idea as impractical and stopped developing CCDs, Sequin says. Fairchild Semiconductor took up the challenge for a while, but also let it go. "Everything lay kind of dormant for another 10 years, until the Japanese picked it up," Sequin says. It took Japanese researchers at Sony and other places another five years to perfect the device. "The Americans dropped the ball," he says.