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.

8. THANK YOU