PHOTOCONDUCTIVE CELLS

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PHOTOCONDUCTIVE CELLS

• Reported by
• Francis Lee B. Mondia

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What is a photoconductive cell?

• The photoconductive cell is a two-terminal
  semiconductor device whose terminal resistance
  will vary (linearly) with the intensity of the
  incident light.
• are light-sensitive resistors in which resistance
  decreases with an increase in light intensity
  when illuminated.
• Frequently called a photoresistive device
• These devices consist of a thin single-crystal or
  polycrystalline film of compound semiconductor
  substances.

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Symbol
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What are they made of?

• The photoconductive materials most frequently
  used are
• Cadmium sulfide (CdS)
• Cadmium selenide (CdSe)

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What are they made of? (cont.)

• Cadmium sulfide
• peak central response occurs at approximately
  5100 A
• Response of CdS units is about 100ms
• Cadmium Selenide
• peak central response occurs at approximately
  6150 A
• Response of CdS units is about 10ms

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What are they made of? (cont.)

• CdS photoconductive cells (CdS cells) are often
  referred to as light dependant resistors (LDR).
  They function within the same general spectral
  range as the human eye, and are therefore widely
  used in applications where this type of spectral
  response is required. 
• The photoconductive cell does not have a junction
  like the photodiode.
• A thin layer of the material connected between
  terminals is simply exposed to the incident light
  energy.

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What are they made of? (cont.)

• Other materials that are less commonly used in
  photoconductive cells include
• lead sulfide (PbS)
• lead selenide (PbSe)
• lead telluride (PbTe)
• they react to infrared light, not the visible
  spectrum.

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How does a photoconductive cell function?

• Photoconductive cells function by receiving light
  energy, which in turn free electrons from their
  valence bonds in semiconductor material.
• As the illumination on the device increases in
  intensity, the energy state of a large number of
  electrons in the structure will also increase,
  because of the increased availability of the
  photon packages of energy.
• The result is an increasing number of relatively
  free electrons in the structure and a decrease
  in the thermal resistance.

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Applications

• Photoconductive cells are generally inexpensive
• their small size and ease of use makes them
  popular in many applications. 
• Some of the many uses include
• making street lights turn on and off
  automatically according to the level of daylight
• in point-of-sale and inventory bar code reading
  devices
• in security devices such as motion sensing lights
  and cameras
• in alarm systems 
• They also are used as light meters in
  photographic applications.

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Photoconductive cell-terminal characteristics (GE
type B425)
R ( log scale )
The sensitivity curve for a typical
photoconductive device
(Average characteristics)
100kO
10kO
1kO
0.1kO
Illumination (footcandles)
0.1
1.0
10
100
1000
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The END