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Characterization of Detectors

Characterization of Detectors. NEP= noise equivalent power = noise current (A/ Hz)/Radiant sensitivity (A/W) D = detectivity = area/NEP IR cut-off maximum current maximum reverse voltage Field of view Junction capacitance. Photomultipliers. e. e. e. e. hf. e. e.

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Characterization of Detectors

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  1. Characterization of Detectors • NEP= noise equivalent power = noise current (A/Hz)/Radiant sensitivity (A/W) • D = detectivity = area/NEP • IR cut-off • maximum current • maximum reverse voltage • Field of view • Junction capacitance

  2. Photomultipliers e e e e hf e e PE effect Secondary electron emission Electron multiplication

  3. Photomultiplier tube hf • Combines PE effect with electron multiplication to provide very high detection sensitivity • Can detect single photons. e- Anode Dynode chain Cathode -V

  4. Microchannel plates • The principle of the photomultiplier tube can be extended to an array of photomultipliers • This way one can obtain spatial resolution • Biggest application is in night vision goggles for military and civilian use

  5. Microchannel plates • MCPs consist of arrays of tiny tubes • Each tube is coated with a photomultiplying film • The tubes are about 10 microns wide http://hea-www.harvard.edu/HRC/mcp/mcp.html

  6. MCP array structure http://hea-www.harvard.edu/HRC/mcp/mcp.html

  7. MCP fabrication http://hea-www.harvard.edu/HRC/mcp/mcp.html

  8. Disadvantages of Photomultiplers as sensors • Need expensive and fiddly high vacuum equipment • Expensive • Fragile • Bulky

  9. Photoconductors • As well as liberating electrons from the surface of materials, we can excite mobile electrons inside materials • The most useful class of materials to do this are semiconductors • The mobile electrons can be measured as a current proportional to the intensity of the incident radiation • Need to understand semiconductors….

  10. Evac Ec Ef Ev Metal Photoelectric effect with Energy Bands Evac Ef Semiconductor Band gap: Eg=Ec-Ev

  11. To amplifier e Ec Evac Ef Ev Semiconductor Photoconductivity

  12. Photoconductors • Eg (~1 eV) can be made smaller than metal work functions f (~5 eV) • Only photons with Energy E=hf>Eg are detected • This puts a lower limit on the frequency detected • Broadly speaking, metals work with UV, semiconductors with optical

  13. Band gap Engineering • Semiconductors can be made with a band gap tailored for a particular frequency, depending on the application. • Wide band gap semiconductors good for UV light • III-V semiconductors promising new materials

  14. 5m Example: A GaN based UV detector This is a photoconductor

  15. Response Function of UV detector

  16. Choose the material for the photon energy required. • Band-Gap adjustable by adding Al from 3.4 to 6.2 eV • Band gap is direct (= efficient) • Material is robust

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