Active-pixel sensor

An active-pixel sensor (APS) is an image sensor made of very small parts called pixels. Each pixel has a special part called a photodetector, usually a pinned photodiode, and tiny electrical switches called transistors that help it work. These sensors are used in many devices we use every day, like the cameras in our cell phones, web cameras, small digital cameras, and bigger cameras such as digital single-lens reflex cameras and mirrorless interchangeable-lens cameras.

In most APS sensors today, something called MOS field-effect transistors, or MOSFETs, are used to make the signals stronger. There are different kinds of APS sensors. One of the earliest types used NMOS technology, but now the most common type uses something called complementary MOS, or CMOS. This type is often just called a CMOS sensor. CMOS sensors became popular because they are a good alternative to another kind of sensor called a charge-coupled device, or CCD. By the middle of the 2000s, CMOS sensors were used more than CCD sensors.

Sometimes the term active pixel sensor refers to just one of these tiny pixel parts, not the whole group of them together. When we talk about the whole group, we might call it an active pixel sensor imager or active-pixel image sensor. These sensors help us capture clear pictures and videos in many of the devices we use.

History

Further information: Image sensor § History

While studying a special kind of technology called metal-oxide-semiconductor (MOS), researchers Willard Boyle and George E. Smith discovered that tiny bits of electric charge could be stored. This led to the invention of a device called the charge-coupled device (CCD) in 1969.

One challenge with CCD technology was that it needed perfect charge transfer to read the image. This made it hard to use in low light or to create large images.

Later, scientists created a new type of image sensor called the pinned photodiode (PPD). This improved many problems with earlier sensors and became common in consumer video and digital cameras.

The active-pixel sensor (APS) was developed to solve problems with older sensors. Each pixel in an APS contains tiny amplifiers that help make the image clearer. This idea was first suggested in 1968 and became practical in the 1980s.

By the early 1990s, a new type called the CMOS sensor was created. This used a different kind of technology and became very popular because it used less power. Today, CMOS sensors are used in most digital cameras, phone cameras, and computer webcams. They changed how many people communicate and express themselves.

Comparison to CCDs

Active-pixel sensors (APS) fix problems with older sensors. They use less power, show clearer images, and are easier to make than older sensors called CCDs. APS sensors can capture and process images all on one small chip. This makes them very popular in devices like camera phones, security cameras, and optical mice. Companies such as Canon, Samsung, and Sony make these sensors.

One big advantage of APS sensors, especially those made with CMOS technology, is that they cost less to make. They also handle bright spots better, stopping colors from spreading. However, they can sometimes make moving scenes look a little skewed. Even so, modern APS sensors are just as good as CCD sensors at reducing unwanted noise in pictures.

Architecture

The active-pixel sensor (APS) is a special type of image sensor used in many cameras and phones. Each tiny part of the sensor, called a pixel, catches light and helps read the light signal.

These sensors can be made in different ways. Some have parts for catching light next to parts for reading the signal. Others stack these parts, which can make the sensor smaller. Some APS sensors can even take pictures using X-rays.

Design variants

Many designs for pixel sensors exist. A simple design uses fewer wires and parts, leaving more space to sense light. Too many parts can make less space for light sensing.

Designers need to balance pixel size with things like reducing noise (which changes how well light is measured) and stopping image lag (when old picture parts stay in a new picture).

Some designs use a “hard reset” way to stop image lag but this can add noise. Others mix hard and soft reset ways to balance noise and lag. Advanced designs, like the active-reset pixel, can lower noise more but need more parts or bigger pixels.