Active pixel sensor

An active pixel sensor (APS) is an image sensor consisting of an integrated circuit containing an array of pixels, each containing a photodetector as well as three or more transistors. Since it can be produced by an ordinary CMOS process, APS is emerging as an inexpensive alternative to CCDs.
The standard CMOS APS pixel consists of three transistors as well as a photodetector.
The photodetector is usually a photodiode, though photogate detectors are used in some devices and can offer lower noise through the use of correlated double sampling. Light causes an accumulation, or integration of charge on the 'parasitic' capacitance of the photodiode, creating a voltage change related to the incident light.
One transistor, Mrst, acts as a switch to reset the device. When this transistor is turned on, the photodiode is effectively connected to the power supply, VRST, clearing all integrated charge. Since the reset transistor is n-type, the pixel operates in soft reset.
The second transistor, Msf, acts as a buffer (specifically, a source follower), an amplifier which allows the pixel voltage to be observed without removing the accumulated charge. Its power supply, VDD, is typically tied to the power supply of the reset transistor.
The third transistor, Msel, is the row-select transistor. It is a switch that allows a single row of the pixel array to be read by the read-out electronics.
A typical two-dimensional array of pixels is organized into rows and columns. Pixels in a given row share reset lines, so that a whole row is reset at a time. The row select lines of each pixel in a row are tied together as well. The outputs of each pixel in any given column are tied together. Since only one row is selected at a given time, no competition for the output line occurs. Further amplifier circuitry is typically on a column basis.