Active Pixel Sensors

Active Pixel Sensors have become the dominant solution for most imaging applications. Their popularity is due their of mainstream CMOS technology and high levels of on-chip integration. Their shortcomings compared to Charge Coupled Devices (CCDs) have, in the main, been eliminated. So you can have high-speed, flexible read-out, and multi-sourcing; together with a noise levels, uniformity and dynamic range that have traditionally been associated with CCDs.

The Basics At the heart of all CMOS imagers is the photosensitive element or pixel. Its simplest form is the three-transistor (3T) pixel.

The reset transistor, T1, acts as a switch to reset the photodiode. When the reset transistor is turned on, the photodiode is effectively connected to the power supply, VRST. Any electrons generated by incident photons (or ionising particles) are collected at this photodiode and effectively reduce the voltage across it.

The read-out transistor, T2, acts as a buffer or amplifier, which allows the diode voltage to be read without removing its accumulated charge. Its power supply, VDD, is typically the same as that of the reset transistor. The select transistor, T3, allows a single row of the pixel array to be read by the read-out electronics.

It is the presence of the read-out transistor, T2, which makes the device an Active Pixel Sensor. Many variants of this basic pixel exist.

The pixels form a two-dimensional array, which permit the sequential reading of each pixel to produce two-dimensional images.

The array 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 outputs of each pixel in any given column are tied together. Since only one row is selected at a given time, there is no competition for the output line. Further amplification is typically on a column basis. Again many variations exist.