Tuesday, October 6, 2009

Charge-Coupled Image Sensors

One half of the 2009 Nobel Prize in Physics is shared by Willard Boyle and George Smith for the charge-coupled device (CCD) image sensor. Smith says they invented the CCD in 1969 at Bell Labs in Murray Hill, New Jersey, to give their semiconductor device organization leverage against a competing organization's magnetic bubble technology. (In a bit of Bell Labs dirty laundry, Gene Gordon says in this 2000 interview that he was originally listed with Boyle and Smith on the patent, and that he "never could understand why Bill Boyle was listed.") Although the magnetic technology never became important, the CCD became the mainstay of digital imaging for decades.

Electronic devices detect light when it liberates electrons, either into vacuum (as in a photomultiplier) or within a semiconductor. A key challenge is that these photogenerated electrons must outnumber the background "dark current." This becomes more difficult when light levels are low, as in many scientific experiments, or when individual pixels are made small so that they receive few photons. The CCD addresses this problem by electrically isolating the light-detecting region of the semiconductor from the detection circuitry, so it can accumulate electrons for seconds or longer if necessary. To read out the accumulated charge, the device uses a "bucket brigade" that efficiently passes the charge from detector to detector along a row. Circuitry at the end of the row measures the charge from each "bucket" in sequence as it arrives.

Since its invention, CCD technology has been used in thousands of scientific experiments, and it helped to establish the consumer digital camera industry. In the past decade or so, however, technologists have developed imagers based on the mainstream CMOS (complementary metal-oxide-semiconductor) technology by improving both the device design and the manufacturing process. These CMOS imagers are cheaper and can be integrated onto a single chip with the processing electronics, for which CMOS is standard. CCD technology is still preferred for the highest quality images and the lowest light levels, such as those in the Hubble space telescope.

Over the years, researchers have considered CCD technology as a replacement for CMOS in low-power electronics, but it has not had wide impact in that application.

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