LCD TFT LED-OLED CCD CMOS DISPLAY SYSTEMS AND PHOTOSENSORS (PART 4)

Charged Coupled Device (CCD)

Many image sensors: Infrared, gamma ray, x-rays etc. Focus on sensors for visible light (slightly into infrared and uv light) Michael Thomas, TU Berlin, 2010 Processing Digital Camera Images, WS 2010/2011, Alexa/Eitz 2 CCD and CMOS sensors are normally used for visible light  Have good natural sensitivity in near infrared, usually removes that by filters  Can be specially adapted for other parts of the spectrum e.g. X- rays

The beginnings Video camera tube sensors in the 1930s.1969, George Smith ، First, CCD, Willard Boyl Fairchild's effort, led by ex-Bell researcher Gil Amelio, was the first with commercial devices, and by 1974 had a linear 500-element device and a 2-D 100 x 100 pixel device. The first KH-11 KENNAN reconnaissance satellite equipped with charge- coupled device array technology for imaging was launched in December 1976.[6] Under the leadership of Kazuo Iwama, Sony also started a big development effort on CCDs involving a significant investment. Eventually, Sony managed to mass produce CCDs for their camcorders. Before this happened, Iwama died in August 1982; subsequently, a CCD chip was placed on his tombstone to acknowledge his contribution.[7] Michael Thomas, TU Berlin, 2010 Processing Digital Camera Images, WS 2010/2011, Alexa/Eitz 3

How to convert light to electric charge? Inner photoelectric-effect at a photodiode: Photon excites electron creating a free electron and a hole The hole moves towards the anode, the electron towards the cathode Now we have our charge! Michael Thomas, TU Berlin, 2010 Processing Digital Camera Images, WS 2010/2011, Alexa/Eitz 4

Charge-Coupled Device (CCD) Integrated circuit Array of connected capacitors (Shift register) Charge of capacitor is transfered to neighbour capacitor At the end of chain, charge is converted into voltage by charge amplifier Transfer stepped by Clock-Signal CCD has photosites, arranged in a matrix. Each comprises a photodiode which converts light into charge and a charge holding region The charges are shifted out of the sensor as a bucket brigade The A/D conversion is done at the edge of the circuit Serial charge processing Michael Thomas, TU Berlin, 2010 Processing Digital Camera Images, WS 2010/2011, Alexa/Eitz 5

Each capacitor is coupled with a photodiode All capacitors are charged parallelly Charges are transferred serially

CCD-Sensor But how to prevent light to charge up the capacitors while transferring? Mechanical shutter Buffer of capacitors that store the charge until it is transferred Loss of resolution or larger sensor → more expensive

One-or three-chip camera three-chip is usually at least 3 times as expensive  The color filter matrix for one-chip, usually ”Bayer mosaic” Reduces color resolution to about half Also reduces light collection efficiency Anisotropic in x and y A new method invented by Foveon uses “vertical filters” with less resolution loss

What happens, if too much light hits the sensor ? Capacitors overload Charge “spills“ over to neighbor capacitors Blooming effect occurs

CCD: Pro: No rolling shutter Lower noise Good image quality Con: Analog device! Blooming effect Michael Thomas, TU Berlin, 2010 Processing Digital Camera Images, WS 2010/2011, Alexa/Eitz 12 CCD vs. CMOS CMOS: Pro: No blooming Cheaper Lower power consumption Faster readout Con: Rolling shutter Variations in brightness (per Pixel)

CMOS IMAGE SENSORS Presenter: Alireza eyvazzadeh

Complementary metal–oxide–semiconductor a technology for constructing integrated circuits CMOS technology is used in Microprocessors Microcontrollers static RAM other digital logic circuits CMOS technology is also used for several analog circuits such as image sensors data converters

Two most common types of sensors used in digital cameras : CCD - Charge Coupled Device CMOS - Complementary Metal Oxide Semiconductor With the CMOS imager both the 'Photon-to-Electron' conversion and the 'Electron- to-Voltage' conversion is done within the pixel CMOS image sensors All CCD and CMOS image sensors operate by exploiting the photoelectric effect to convert light into electricity

Some drawbacks of CCDs complex clocking requirements high power consumption difficulty of on-chip integration of circuitry limited frame rate CMOS image sensors address these drawbacks by using the same technology as microprocessors and memory chips

Advantages and drawbacks of the CMOS image sensors Low Power Consumption Also they are still too noisy and less sensitive than CCDs one-third to more than 100 times less than that of CCDs Lower cost compared to CCD’s technology On chip functionality A sensor can integrate various signal and image processing blocks such as amplifiers, ADCs, circuits for color processing and data compression, etc. on the same chip Miniaturization Random access of image data Selective read-out mechanism High-speed imaging

internet camera digital still camera machine vision Automotive children’s toy medicine and dentistry fingerprint ID surveillance aerospace motion analysis industrial inspection quality control process control target tracking spectroscopy present applications of CMOS image sensors

Overall architecture CMOS imagers architecture can be divided into four main blocks: 1. Pixel Array 2. Analog Signal Processors 3. Row and Column Selector 4. Timing and Control

Each pixel contains a photodetector and some transistors. This area is the heart of an image sensor and the imaging quality is largely determined by the performance of this area. active pixels (APS) passive pixels (PPS) APS are sensors that implement a buffer per pixel. Currently, APS are the predominant devices, although in some cases PPS are also used. Basic pixel structures An APS has three transistors in a pixel, while a PPS has only one transistor. To achieve further improvement, an advanced APS that has four transistors in a pixel, the so-called 4T-APS, has been developed. Pixel circuits The in-pixel amplifier in APS enables non-destructive read of the photodiode charge at a faster speed and a generally higher signal-to-noise ratio (SNR) than PPS

Operation of a photodetector comprises: (a)generation of free electron-hole pairs due to impinging light (b)separation and collection of electrons and holes (c) production of an output signal through interaction with other components Several popular silicon-based photosensing devices are Photoconductors PN and PIN photodiodes Phototransistors photo gates

PN junction photodiode an important sensor for digital imaging The potential voltage decreases when electrons accumulate. By measuring the voltage drop, the total amount of light power can be obtained.

ACTIVE PIXEL SENSOR, 3T-APS First, the reset transistor M RS is turned on PD is reset to the value V dd −V th V th is the threshold voltage of transistor M RS M RS is turned off and the PD is electrically floated The accumulated charge changes the potential in the PD; the voltage of the PD,V PD,decreases according to the input light intensity After an accumulation time, the select transistor M SEL is turned on and the output signal in the pixel is read out in the vertical output line. When the read-out process is finished M SEL is turned off and M RS is again turned on to repeat the above process.

Sensor peripherals Addressing In CMOS image sensors, to address each pixel, a scanner or a decoder is used Readout circuits The voltage of a PD is read with a source follower (SF) a follower transistor M SF is placed in a pixel and a current load M b is placed in each column

The End