Charge Coupled Device (CCD) – Definition
CCD was developed in the year 1969 by Willard Boyle and George E. Smith at AT & T Bell Labs. It is a shift register device which can be used for the movement of electrical charge within the device. This movement can be from one area of the device to another and the digital value of the moved charge can be easily found out. When the signals are moved, one at a time from one place to another within the device, the value of the charge can be easily manipulated. There are capacitive bins in the device that allow the movement of charge.
During the invention of CCD there was no means to produce the charge than injecting it. But through repeated experiments, it was later found out that when a sensor like a photoelectric device was connected to it, a charge could be easily produced. This charge could then be given to the CCD for its transfer in the device. This discovery was huge enough as it became the stepping stone to the conversion of ordinary signals into digital signals. The device that is used to capture the images with ordinary cameras and replacing them as a digital storage is called a CCD imager.
To know the difference in working between Charge Coupled Devices (CCD) and a CMOS Active Pixel Sensor (APS), click on the link below.
TAKE A LOOK : CMOS ACTIVE PIXEL SENSOR (APS)
Charge Coupled Device (CCD) – Operation
There are mainly two regions of a CCD. They are
1. Photoactive Region
As told earlier, a CCD is used to convert a electrical signal into a digital signal. The photoactive region mainly consists of a capacitor array. These arrays can be one-dimensional or two-dimensional depending on the type of device that uses the CCD. If a line scan camera is used, it introduces a one-dimensional capacitor array. It is called 1D because it captures the image in 1D form, that is, a single slice of the image. 2D is used mostly in video applications. This device captures the image in 2D form. The photoactive region is made out of an epitaxial layer of silicon. It is made by doping a boron ion on a substrate such as p++. Sometimes CCD’s are also implanted with a phosphorus ion so as to give them an n-doping . This is often carried out in devices consisting of n-channels This is done in some areas of the silicon ion causing the movement of photo generated packets across them.
As soon as the silicon layer and substrates are made, a dielectric in the form of a gas oxide (mostly capacitor) is made to grow on top of them. Thus the separately lying gates will lie in a perpendicular angle to the channels. This is because the poly-silicon gates are undergoing chemical vapour deposition and then photolithography. Then the channel stop region and the charge carrying channel is made, and that too parallel to each other.
2. Transmission Region
After the image is projected onto the capacitor array, the control circuit comes into action. This circuit makes the capacitors send the appropriate signal to a shift register. The shift register converts each signal into a voltage sequence. This is later sampled, digitized and then stored in the memory.
With different modes of operation for the CCD, the type of the device will also differ. There are versions of CCD called frame transfer CCD and also peristaltic CCD. In the case of a frame transfer CCD, the gate clocks are used to bias the diode in the reverse as well as forward direction. This is mainly done by the n-doped and p-doped layers. Thus the CCD across or near the p-n junction will get depleted. Thus the charges situated under he gates and also across the channels will be collected and moved.
A peristaltic CCD generates a huge electric field from one gate to the next by providing an additional implant. This implant helps in blocking the charge from the Si/SiO2 interface. Thus the additional driving force created die to this action helps in faster transfer of charge particles.
Charge Coupled Device (CCD) – Applications
CCD’s are used in astronomy because of their high linearity in outputs. CCD is used in all the astronomical Ultra violet and infrared applications. They are also highly efficient in quantum applications. Though the CCD characteristics may be affected by thermal noise and cosmic rays, astronomers have taken several counter measures to reduce this. One method includes the timing of the CCD shutter. With the shutter closed the number of images taken will determine the random noise. After tking the mage with the shutter closed, the result is then differed with the open-shutter image so as to remove the dead and hot pixels.
Application in astronomy also includes a method called drift scanning. This method is mainly used to follow the motion of the sky. This is done by converting a fixed telescope into a tracking telescope with the application of CCD.
For the use of CCD in cameras, a much more advanced for called 3CCD is used commonly. By using 3CCD devices, the colour separation becomes much improved, thus increasing the quantum efficiency as well. The image resolution of a camera completely depends on the CCD chip. When the photons hit the sensor, the sensor counts their number. So, the brighter the image at a given point on the sensor, the larger the value that is ready for that pixel.