Active Pixel Sensor

An Active Pixel Sensor (APS, dt active pixel sensor ) is a semiconductor detector, for measuring light, which is produced in CMOS technology, and is therefore often referred to as a CMOS sensor. In contrast to the also made ​​in CMOS technology passive pixel sensor, each pixel includes an amplifier circuit for signal readout.

By the use of CMOS technology, it is possible to incorporate other features into the sensor chip, such as exposure control, the contrast correction or the analog-to- digital conversion.

Principle of operation

The simplest implementation of an integrating APS pixel consists of a photodiode which is operated in the reverse direction, as a photosensitive member, and three n-channel MOSFET ( field effect transistors). At the start of metering the voltage across the photodiode is set by means of the reset transistor on a defined initial value, here, the threshold voltage of the transistor. During the following photometry, the junction capacitance of the photodiode is discharged by the photocurrent. The voltage across the photodiode is reduced proportional to the irradiance and the exposure time. After expiry of the exposure time of the voltage value is read out and supplied to an analogue post-processing immediately or an analog-digital converter. For each pixel has an amplifier transistor, which is mostly in columns connected to a common for all pixels of one line read line by means of the selection transistor.

Compared with CCD sensors consists of the advantage that the electronics can directly read the voltage signal of each pixel, without having to move the charges, resulting in a significantly lower tendency to Blooming result. The disadvantage is that there is between the light-sensitive photodiodes lot of electronics, which is not sensitive to light itself, which originally led to a smaller relative to the CCD technology light sensitivity with the same chip area. Since the necessary integration density in order to be competitive with CCD, had not been reached, this technique was meaningless in the 1970s and 1980s.

History

Because of the initially poorly reducible readout electronics was the filling factor, ie the ratio of the light-sensitive area of ​​the total of a pixel, only 30 percent, that is, the charge yield was low ( hence also the attainable signal strength), resulting in a poor signal -to-noise ratio led, and manifested as a strong image noise in low light sensitivity. These disadvantages were later reduced by intensive development in the miniaturization of the CMOS technology and through the use of micro-lenses over each picture element, the direct all of the incident light on the photosensitive member.

Areas of application

AP sensors are used as image sensors in digital cameras and video cameras. Today they come in different digital SLR cameras are used. In mobile phones with a camera function are practically only those sensors.

In camcorders CCD sensors are currently used almost exclusively, but Sony released the HDR -HC1 in 2005, a high definition video camcorder that uses an AP sensor. AP sensors are also used in many industrial cameras. The Munich-based company brought in 2004 with the ARRI D-20, a video camera out, the AP uses a sensor with an image resolution of 2880 × 1620 pixels. Its size corresponds to the active picture area of ​​a 35 mm film, which is to allow the use of generic film camera lenses and adjust the depth of field of the images to the film. Sometimes a separate CMOS sensor for each primary color installed (so-called 3MOS sensor ), so even at lower brightness greater color saturation is reached.

A special form of CMOS image sensors represent the photodiode arrays, which is an n × 1 are quasi - CMOS image sensor. They are typically used in embedded applications, i.e., applications where the image is not considered, or evaluated by people. Examples include barcode readers and angle sensors.

Color image sensors

For receiving a color image of three wavelength regions of light are at least recorded separately, usually associated with the primary colors red, green and blue. This is done by using a sensor frequently by the pixels superimposed color filter mosaic, such as the Bayer pattern (English " Bayer pattern "). In contrast to CCD sensors, may be carried out with CMOS sensors, this color separation even in the same pixel by three photodiodes are stacked, which can be achieved by different colors due to differences in depth of the various wavelengths of light. Commercially, such sensors from Sigma under the name Foveon X3 are used in digital cameras. An alternative design called Transverse Field Detector is explored.

Differences to CCD sensors

Initially, a low production was hoped for a larger production volume, assuming that the sensors could be produced on the designed for high volume production lines without modification and so would cause a lower manufacturing cost per chip. This has not been confirmed (as of 2001). However, and often parts of the peripheral circuit, such as analog-to- digital converter clock generation, timing control and power level adjustment, integrated in chips with active- pixel sensors, which allows more compact and cost-effective overall systems.

A principal advantage of APS is in the existing amplifier in each pixel, so that no individual amplifier as used in CCD must be used for multiple pixels. Can thereby be operated at a given pixel rate, each amplifier having a lower bandwidth and therefore lower noise floor. To reach 2013 AP sensors, an input noise of 1-2 photons at an image scanning of over four hundred megapixels per second, wherein the sensors comprise 4-10 megapixels and having a quantum efficiency of over 70%. About Weighs only one aspect, but can be advantageous CCD To detect fewer photons with very low noise EMCCD be used; CCD can with quantum efficiencies close to one hundred percent in a limited spectral range are produced, and their low dark current results in a small image noise at very long exposure times.

CMOS image sensors often have a higher sensitivity in the NIR region (german near infra -red, short-wave infrared radiation) than CCD sensors. Many of the CMOS sensor is the maximum sensitivity in the NIR region ( > 650 nm ), while CCD sensors, the maximum in the visible range (green light, 550 nm) possess.

The following list of advantages and disadvantages of CMOS sensors compared to CCD sensors refers to general statements of standard components. Specialized sensors can be used in both techniques have significantly different properties.

Benefits ( CMOS sensors):

• Lower power consumption
• Lower (device ) Frame size, by integrating the evaluation logic on the same chip ( System on a Chip )
• Some processing steps can be performed in the same pixel amplifier, such as the logarithm HDRC sensor ( engl. high dynamic range CMOS).
• By separate processing of each pixel (conversion of charges into voltages ): Extremely high frame rate compared to a CCD size ( quick preview view video function )
• Read Flexible by direct addressing ( binning, multiple reading, simultaneous reading of several pixels )
• Strongly limited blooming effect

Cons:

• Separate conversion of the charge to voltage for each pixel and integration of the evaluation logic leads to: greater sensitivity differences between the pixels ( uniformity ) by manufacturing tolerances, which at Bayer sensors leads to a stronger color noise, and
• A lower fill factor ( ratio of the light-sensitive to the total pixel area ), resulting in a total low light sensitivity.