Photoconductivity.pptx

Semiconductor Physics

1. Presented By: Abdul Qadir Roll No: F21BPHYS3E01021 Dr Faisal Iqbal Topics: Photoconductivity Department of physics

2. PHOTOCONDUCTIVITY The term photoconductivity is derived from two words i.e. ‘Photo’ and ‘Conductivity’. “Photo” means “photon of light or X-ray” and “Conductivity” means “movement of current of charge carrier in semiconductor”. So, the whole term “photoconductivity” means “the variations or changes of current due to incident light”. Photoconductivity is the optical property of light.

3. Continued……… Photoconductivity is defined as: “The process in which electrical conductivity of semiconductor material increases when exposed to light”. A device through which the electrical conductivity of semiconductor increases due to incident of light is called “Photoconductor”. When light is incident on semiconductor material then valence electrons of atoms of semiconductor become excited.

4. Continued……. These excited electrons are moved from valence band to conduction band and take part in conduction. Due to these conduction electrons the electrical conductivity of semiconductor increases. Excitation is only possible if the energy of incident photon is greater than the gap energy. For semiconductor the bang gap energy is approximately equal to 1ev. Certain crystalline semiconductor such as germanium, lead sulfide, and cadmium sulfide are strongly photoconductive.

5. Continued …. ADVANTAGES OF PHOTOCONDUCTIVE MATERIALS The advantages of photoconductive materials are: They mainly depend on light so their resistance will be decrease when light drops on them. They are cheap and available in different sizes and shapes. They need less power and voltage for their operation. They are used to make simple circuit.

6. Continued….. DISADVANTAGES OF PHOTOCONDUCTIVE MATERIALS The disadvantages of photoconductive materials are: These are less responsive as compared to phototransistors and photodiodes. When the cadmium sulfide is used in the construction of a photo resistors, then it is dangerous to the atmosphere. Their respond time is very slow i.e. 10sec or 100 milisec. They are sensitive to temperature. Their characteristics are non linear .

7. Continued….. USES OF PHOTOCONDUCTIVE MATERIALS Photoconductors are used for X-ray image detectors. They are used for relay control. They are used to switch ON and OFF transistors. They are used to control the level of current flow in LED. Photoconductivity is used in the process of Xerography or Photocopying.

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