Matsushita Electric Industrial Co., Ltd., best known for its Panasonic brand of consumer electronics and digital communication products has announced the development of the world's first 1/4-inch MOS (Metal Oxide Semiconductor) image sensor with 2 million pixels at ISSCC (International Solid-State Circuits Conference) 2004 held from February 15 to 19. With its compactness, high resolution, and, low power consumption the new image sensor can offer an optimum solution for cameras in mobile phones and other devices.
Panasonic achieved the world's smallest pixel size of 2.25 µm by sharing the signal-detecting transistors in the pixels sensing RGB colors through a new pixel circuit, enabling the resolution of 2 million pixels despite the compact size. One signal-detecting transistor is shared by 4 pixels, reducing the average number of transistors to 1.75 per pixel compared to 4 transistors per pixel required in conventional MOS image sensors.
The aperture ratio of the photodiode in the pixel has increased 25 percent by decreasing the metal-shaded area by the new "parallel common addressing structure". The number of metal lines and electrodes has been reduced to half of the conventional one. The new scheme enables simultaneous signal readout from photodiodes in adjacent lines, compared to the conventional method in which signals are read out pixel by pixel. With less area of obstacles, more light can be gathered to achieve high sensitivity.
The new image sensor is also power-saving, with driving power reduced to 2.5 volts, which decreases the power consumption to a mere one-fifth or less compared to the conventional CCD (Charge Coupled Device) image sensors. The use of a new "asymmetric electric field photodiode structure" enables construction of high and low electric field areas inside the photodiode, leading to charge accumulation at a lower voltage.
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