Module 6 : PHYSICS OF SEMICONDUCTOR DEVICES
Lecture 35 : Photoconductivity
Photoconductivity
A consequence of small band gap ( $ \Delta$) in semiconductors is that it is possible to generate additional carriers by illuminating a sample of semiconductor by a light of frequency greater than $ \Delta/h$. This leads to an increased conductivity in the sample and the phenomenon is known as photoconductivity . The effect is not very pronounced at high temperatures except when the illumination is by an intense beam of light. At low temperatures, illumination results in excitation of localized carriers to conduction or valence band.
  Consider a thin slab of semiconductor which is illuminated by a beam of light propagating along the direction of its length (x-direction). Let $ I$ be the radiation intensity (in watts/m $ ^2$) at a position $ x$ from one end of the semiconductor. If $ \alpha=$ absorption coefficient per unit length, the power absorbed per unit length is $ \alpha I$. The change in the intensity with distance along the sample length is given by
 
$\displaystyle \frac{dI}{dx} = -\alpha I$
  which has solution
 
$\displaystyle I = I_0e^{-\alpha x}$
   
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