| Until recently, almost all digital camera image sensors were CCDs. The disadvantages of CCDs are that they require a lot of electrical power, and conversion of images to digital data is slow. This is why Canon began work on CMOS sensors, which have the same kind of structure as computer microprocessors and CMOS memory chips. Such chips contain large arrays of transistors, which in CMOS sensors are each composed of a photodiode and amplifier. The photodiodes accumulate electrical charge when exposed to light, and those charges are then converted to voltage, amplified and transmitted as electrical signals. In CCDs, the gate structure used to transfer electrical charges to the edge of the sensor requires a separate power source, which means more electrical power is needed. However, CMOS sensors require only a single power source, and consume very little electrical power. They can also read off the electrical charges much more rapidly than CCDs.
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