06/02/2008CCDs1 Charge Coupled Device M.Umar Javed M.Umar Javed

06/02/2008CCDs2 Outlines  Basics  Photodiodes  Photodiode Arrays  Charge Coupled Device (CCD)  History and Principle of working  Characteristics  Applications  Advantages and Disadvantages  References

06/02/2008CCDs3 Photodiode   A photodiode is a PN junction or   PIN structure.

06/02/2008CCDs4 Operation Modes   Forward Bias

06/02/2008CCDs5   Reverse Bias

06/02/2008CCDs6 Characteristics of PN Junction

06/02/2008CCDs7 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.

06/02/2008CCDs8   The photodiode array is a multichannel detector.   They are useful in recording UV-Vis absorption spectra of samples.

06/02/2008CCDs9 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.

06/02/2008CCDs10   History The Charge Coupled Device was conceived in 1970 at Bell Labs by W.Boyle and G.Smith.   Working Principle 1Generate Charge  Photoelectric Effect 2Collect 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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06/02/2008CCDs13 The voltages supplied to the electrodes change, and the electron packets move in response.

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06/02/2008CCDs15 CCD Characteristics  Quantum Efficiency (%)= It is the ratio between photogenerated carriers to incident photons per pixel.

06/02/2008CCDs16 Typical peak values  Photographic plate= 1-2%  Eye =1-2%  Photomultiplier tube=20-30%  CCD= 70-90%  (HgCdTe)=30-50%

06/02/2008CCDs17  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.

06/02/2008CCDs18  Dynamic Range D = well capacity / dark current

06/02/2008CCDs19 Dark Current It is produced by thermally generated carriers in depletion region. Total Noise

06/02/2008CCDs20  Signal To Noise Ratio (SNR) For visible region

06/02/2008CCDs21 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.

06/02/2008CCDs22 Advantages of CCD   Quantum efficiency (QE) ~ 80 %   Low noise.   High dynamic range.   High photometric precision.   Very linear behavior.   Immediate digital conversion of data.

06/02/2008CCDs23   Low voltages required (5V-15V)   Geomatrically stable (Good for astronomy).   Rapid clocking. Disadvantages of CCD  Limited exposure time.  Cooling required to reduce noise.

06/02/2008CCDs24  Blooming or bleeding in columns due to bright sources.

06/02/2008CCDs25References  Spectral Imaging of the Atmosphere (Gordon G.shepherd),June1999  1-f03.html 1-f03.html 1-f03.html   spiff.rit.edu/.../ lectures/ccd1/ccd1.html  Semiconductor Radiation Detectors by Gerhard Lutz,1999.  Single Particle Detection and Measurement by R.Gilmore,1992. 

06/02/2008CCDs26 Thanks for your attention.