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.
Architecture
Pixel
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.
Array
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.

The active-pixel sensor is descended from the original MOS active-pixel image sensors, which, like the CCD, were invented in the late 1960s. Noble in 1968 and Chamberlain in 1969 created sensor arrays with active MOS readout amplifiers per pixel, in essentially the modern three-transistor configuration.

Other early MOS image sensors, at about the same time, were passive-pixel sensors; each passive pixel contained a photodiode and an access transistor. Pixels were arrayed in a two-dimensional structure, with access enable wire shared by pixels in the same row, and output wire shared by column. At the end of each column was an amplifier. Passive-pixel sensors suffered from many limitations, 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.

Similar devices to the APS concept were in production as early as 1978 for use in Infrared Focal-Plane Arrays (IRFPA). Although these devices were CMOS/InGaAs or HgCdTe hybrids, the key difference from the later CMOS devices was that the photodiodes were not integrated into the readout substrate.

In 1992, Eric Fossum, et al., published the first extensive article predicting the emergence of APS sensors as the commercial successor of CCDs. Between 1993 and 1995, the Jet Propulsion Laboratory developed a number of prototype devices which validated the key features of the technology. Though primitive, these devices demonstrated good image performance with high readout speed and low power consumption.

In 1995, personnel from JPL founded Photobit Corp., who continued to develop and commercialize APS technology for a number of applications, such as web cams, digital radiography, and endoscopy cameras.

For applications such as large area digital x-ray imaging Thin Film Transistors (TFTs) can also be used in APS architecture. However, because of the larger size and lower transconductance gain of TFTs compared to CMOS transistors, it is necessary to have fewer number of on-pixel TFTs to maintain image resolution and quality at an acceptable level. A two-transistor APS/PPS architecture has been shown to be promising for APS using amorphous silicon TFTs. In the two-transistor APS architecture on the right, T_AMP is used as a switched-amplifer integrating functions of both M_sf and M_sel in the three-transistor APS. This results in reduced transistor counts per pixel, as well as increased pixel transconductance gain.^[8]

Here, C_pix is the pixel storage capacitance, and it is also used to capacitively couple the addressing pulse of the "Read" to the gate of T_AMP for ON-OFF switching. Such pixel readout circuits work best with low capacitance photoconductor detectors such as amorphous selenium.