Transcript Radiation Detectors - pep

Charge Coupled Device
M.Umar Javed
06/02/2008
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Outlines
 Basics
 Photodiodes
 Photodiode Arrays
 Charge Coupled Device (CCD)
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History and Principle of working
Characteristics
Applications
Advantages and Disadvantages
 References
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Photodiode
 A photodiode is a PN
junction or
 PIN structure.
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Operation Modes
 Forward Bias
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 Reverse Bias
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Characteristics of PN Junction
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Photodiode Arrays
 A photodiode array is
a linear array of
discrete photodiodes
on an integrated
circuit chip.
 It works on the same
principle as simple
photovoltaic detector.
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 The photodiode array is a multichannel detector.
 They are useful in recording UV-Vis absorption
spectra of samples.
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Charge Coupled Device (CCD)
 An instrument whose
semiconductors are
connected in such a
way so that the
output of one serves
as the input of the
next.
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 History
The Charge Coupled Device was conceived
in 1970 at Bell Labs by W.Boyle and G.Smith.
 Working Principle
1 Generate Charge  Photoelectric Effect
2 Collect Charge Pixels (gates)
3 Transfer Charge Apply a differential voltage
across gates. Signal electrons move down,
vertical registers to horizontal register.
4 Detect Charge Individual charge packets are
converted to an output voltage.
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The voltages supplied to the electrodes change, and the
electron packets move in response.
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CCD Characteristics
 Quantum Efficiency (%)=It is the ratio between
photogenerated carriers to incident photons per pixel.
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Typical peak values
 Photographic
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plate= 1-2%
Eye =1-2%
Photomultiplier
tube=20-30%
CCD= 70-90%
(HgCdTe)=30-50%
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 Charge Transfer Efficiency (CTE)
The fraction of electrons that are moved
from one pixel to another during read-out
is described by the charge transfer
efficiency (CTE).
 Pixel to Pixel Variation
This is fixed pattern noise because of the
cell to cell non-uniformity.
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 Dynamic Range
D = well capacity / dark current
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Dark Current
It is produced by thermally generated carriers in
depletion region.
B
I  Ae
. kT
Total Noise
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 Signal To Noise Ratio (SNR)
SNR 
S .B.t
 B.t (S B  S ) 
For visible region
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 B.  N D    N R 
2
2
SB  0
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Applications
 CCD imaging systems in astronomy.
 The acquisition, guiding and wave front sensing
applications in astronomy.
 Fabry-Perot CCD annular-summing spectroscopy.
 Electron-bombarded CCD detectors for ultraviolet
atmospheric remote sensing.
 MAXDOAS instrument at Bremen.
 To retrieve the 2-dimensional distribution of the
intensity.
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Advantages of CCD
 Quantum efficiency (QE) ~ 80 %
 Low noise.
 High dynamic range.
 High photometric precision.
 Very linear behavior.
 Immediate digital conversion of data.
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 Low voltages required (5V-15V)
 Geomatrically stable (Good for
astronomy).
 Rapid clocking.
Disadvantages of CCD
 Limited exposure time.
 Cooling required to reduce noise.
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 Blooming or bleeding in columns due to
bright sources.
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References
 Spectral Imaging of the Atmosphere (Gordon
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G.shepherd),June1999
http://www.astro.virginia.edu/class/oconnell/astr511/lec11
-f03.html
http://chemistry.hull.ac.uk/lectures/adw/06
spiff.rit.edu/.../ lectures/ccd1/ccd1.html
Semiconductor Radiation Detectors by Gerhard
Lutz,1999.
Single Particle Detection and Measurement by
R.Gilmore,1992.
http://www.iup.uni-bremen.de/doas/doas_glossary.htm
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Thanks for your attention.
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