CMOS vs CCD Imagers on Cellphone
CMOS (Complementary Metal Oxide Semiconductor) and CCD (Charge Coupled Device) are two different technologies for capturing images digitally. They have their unique strengths and weaknesses. Where CMOS technology is most suited to the mobile phone market due to its high integration capability and low power consumption, CCD imagers are traditionally of higher quality. CMOS sensors use between one-third and one-quarter as much power as CCDs, making them a better choice for battery-powered devices such as mobile phones, PDAs and digital cameras. Both image sensors are pixilated metal oxide semiconductors; they accumulate signal charge in each pixel, proportional to the local illumination intensity.
The CCD imager transfers the charge from the photodiode, created by photoelectric conversion, through a "bucket relay" transfer to the imager output stage. The charge transfer is almost complete, which means that noise is rare, but a high voltage differential is required to improve transfer efficiency. This high voltage differential increases power consumption.
A CMOS sensor converts the charge to voltage within each pixel. This difference in readout technique implies significant differences in sensor architecture, capabilities and limitations. CMOS sensors use an array of photodiodes to convert light into electronic signals. The electronic charge that is generated by the photodiode is too weak and needs amplifying to a usable level. For this purpose, each pixel in a CMOS sensor has its own amplifier circuit, to perform pre-scan signal amplification. The resulting signal is strong enough to be used without any further processing. CMOS sensors often contain additional image processing circuitry - including analog-to-digital converters and digital image signal processors (ISPs) - on the chip itself, making it easier and faster to retrieve and process picture information. This results in a lower chip count, increased reliability, reduced power consumption, and a more compact design.
When packaged as a camera module for mobile phones, CCD based camera modules, which use at least two chips, require larger packages, a critical factor when trying to incorporate camera modules into ultra thin clamshell phone designs.