CHARGE-COUPLED DEVICE

PREPARED BY:JOLLO M. VILLANUEVA

APRIL JEROMEE F. SARMIENTO

A charge-coupled device (CCD) is a device for the movement of electrical charge, usually from within the device to an area

where the charge can be manipulated, for example

conversion into a digital value. This is achieved by "shifting" the

signals between stages within the device one at a time.

CCDs move charge between capacitive bins in the device, with the shift

allowing for the transfer of charge between bins.

The CCD is a major piece of technology in digital imaging. In a

CCD image sensor, pixels are represented by p-doped MOS

capacitors.

Basic operation:The charge packets (electrons, blue) are collected in potential wells (yellow) created by applying positive voltage at the gate electrodes (G). Applying positive voltage to the gate electrode in the correct sequence transfers the charge packets.

In a CCD for capturing images, there is a photoactive region (an epitaxial layer of silicon), and a

transmission region made out of a shift register (the CCD, properly

speaking).

An image is projected through a lens onto the capacitor array (the photoactive region), causing each capacitor to

accumulate an electric charge proportional to the light intensity at that location. A one-dimensional array, used in line-scan cameras, captures a single slice of the image, whereas a two-dimensional

array, used in video and still cameras, captures a two-dimensional picture

corresponding to the scene projected onto the focal plane of the sensor.

Once the array has been exposed to the image, a control circuit causes each

capacitor to transfer its contents to its neighbor (operating as a shift register). The last capacitor in the array dumps its

charge into a charge amplifier, which converts the charge into a voltage. By repeating this process, the controlling

circuit converts the entire contents of the array in the semiconductor to a sequence

of voltages.

In a digital device, these voltages are then sampled, digitized, and usually stored in memory; in an

analog device (such as an analog video camera), they are processed

into a continuous analog signal (e.g. by feeding the output of the charge

amplifier into a low-pass filter), which is then processed and fed out

to other circuits for transmission, recording, or other processing.

The charge packets (electrons, blue) are collected in potential wells(yellow) created by applying

positive voltage at the gate electrodes (G).Applying positive voltage to the gate electrode in

the correct sequence transfers the charge packets.

Fundamentally, a charge coupled device (CCD) is an integrated circuit etched onto a silicon surface forming

light sensitive elements called pixels. Photons incident on this

surface generate charge that can be read by electronics and turned into a

digital copy of the light patterns falling on the device.

CCDs come in a wide variety of sizes and types and are used in

many applications from cell phone cameras to high-end

scientific applications. Shown above are various CCDs, the

largest is mounted on a 6" wafer and is used in some of Spectral

Instrument’s products.

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The function of a CCD can be visualized as an array of buckets

(pixels) collecting rainwater (photons). Each bucket in the array is exposed for the same amount of time to the rain. The buckets fill up with a varying amount of water, and the CCD

is then read one bucket at a time.

This process is initiated by pouring water into the adjacent empty column. The buckets in

this column transfer their ‘water’ down to a final pixel where the electronics of the camera read-

out this pixel (the computer measuring the bucket) and turn

it into a number that can be understood and stored by a

computer.

The substrate of a CCD is made of silicon, but this is not where most of the action occurs. Photons coming from above the gate strike the epitaxial layer – essentially silicon with different elements doped into it – and generate photoelectrons. The gate is held at a positive charge in relation to the rest of the device, which attracts the electrons to it. Because of the insulating layer – essentially a

layer of glass – the electrons can’t make it through to the gate, and are held in place by the positive

charge above them.

The top black trace shows the ‘potential well’ for the electrons that are

represented by the blue color and is low, or downhill, where the potential is high since opposites attract. As the voltage

adjacent to the electron’s pixel is brought high, they begin to migrate in this direction until the voltage in the

preceding gate is then brought to zero, or low, thus effectively transferring all the

electrons into its neighboring pixel.

Now that the electrons are held in place, they need to be moved to where the light signal they represent can be quantified.

Shown at left is how this is accomplished. Electrons are shifted in two directions on

a CCD, called the parallel or serial direction. One parallel shift occurs from the right to the left. The serial shift is

performed from top to bottom and directs the electron packets to the measurement

electronics.

Quantum Efficiency- The percentage of photons striking the CCD that are actually collected.

CCD Properties

Read Noise-The level of noise present in a “no exposure” readout of temperature.- use a zero second “Bias” or “Zero” exposure to measure-3-10 electrons per pixel per read are typical today

Dark Current-Thermal noise (thermal electrons)- strong function of temperature-Essentially zero in research quality CCDs operated near -100 C