Active Pixel Sensors

Image sensors are miniature devices or circuits arranged on silicon chips, which are used for image capturing and sensing applications. Although there are different semiconductor substrates for use in the development of these sensors, the implementation of complementary metal oxide semiconductor (CMOS) technology has enabled continuous improvements in design and performance of the image sensors.

There are two basic types of CMOS image sensors, namely, passive and active. Passive pixel sensors employ photosites to convert photons into a voltage. However, the generation of noise during image development is one of the main drawbacks to this type of sensor technology.

Active pixel sensors, on the other hand, consist of an active circuitry that includes pixel sensors with photodiodes and an amplifier. They eliminate the generation of noise due to the pixel. They offer images at high resolution and exhibit performance similar to that of charged couple devices.

Image credit: Photos.com.

Types of Active Pixel Sensors

There are three major types of active pixel sensors which include the following:

Photodiode Active Pixel Sensors

Photodiode sensors consist of a photodiode, a buffer transistor, a row select transistor and a reset transistor. The beam of light emitted from the photodiode is converted into a charge that is in turn converted into a voltage at a sensing node capacitor when the reset transistor is turned off.

A reference voltage is set when the reset transistor is turned on to eliminate the dark current in the circuit. The converted voltage is then transmitted to the buffer transistor to adjust the pixel and maintain fill factor. Finally, the data is obtained upon turning on the row select transistor. This type of sensor is noise-free, and has high dynamic range and fault tolerance capacity.

Photogate Active Pixel Sensors

With photogate sensors, the charge generated by the emission of photons is stored below a photogate. As a result of photogate activation, a potential well is developed for storing charge. The pulse signal of the photogate is then transmitted to the buffer transistor. A reference voltage signal is set during the reset mode. The output signal is read by calculating the difference between the signals in integration and reset modes.

Pinned Photodiode Active Pixel Sensors

Similar to photogate sensors, the charge generated due to photon emission is combined beneath the pinned diode creating the potential well within a buried n-type layer. The signal output is then transmitted to the buffer transistor where the output signal is recorded with respect to the reference voltage signal.

Advantages of Active Pixel Sensors

The key benefits of active pixel sensors over charged couple devices include the following:

• Low power consumption
• High readout speed and random access
• Large array sizes
• High sensitivity and blooming threshold
• High fill factor range of about 50 to 70%
• Low cost.

Applications of Active Pixel Sensors

The following are the major applications of active pixel sensors:

• Digital cameras and webcams
• Automobiles
• Computer-based video games
• Wireless hand-held devices
• Security systems
• Medical diagnostic devices.

References

• Palakodety A, Cmos active pixel sensors for digital cameras: Current State-of-the-art, University of North Texas, 2007
• Active Pixel Sensor – European Space Agency