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.

History

The active pixel sensor is descended from the original MOS image sensors, which, like the CCD, were invented in the late 1960s. The original MOS sensors were passive pixel sensors. Each passive pixel contained a photodiode and an access transistor. Pixels were arrayed in a two dimensional structure, with some circuitry shared by pixels in the same row, and other circuitry by column. At the end of each column was an integrating amplifier. Passive pixel sensors suffered from many problems, such as high noise, slow readout, and lack of scalability. The addition of an amplifier to each pixel addressed these problems, and resulted in the creation of the active pixel sensor. The APS pixel solves the noise, speed, and scalability issues of the passive pixel sensor. APS imagers still suffer from higher fixed pattern noise than CCDs, but active pixel sensors are catching up with respect to noise, dynamic range, and responsivity.

Architecture

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 acts as a switch to reset the device. When this transistor is turned on, the photodiode is effectively connected to the power supply, clearing all integrated charge. The second transistor acts as a buffer (specifically, a source follower), an amplifier which allows the pixel voltage to be observed without destroying the information. The third transistor is called 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.