Module 6 : PHYSICS OF SEMICONDUCTOR DEVICES
Lecture 35 : Photoconductivity
 
 
   
  If we define $ \eta$ as the quantum efficiency , i.e. the fraction of absorbed photons that produce electron-hole pairs, the number of pairs produced per unit time is given by
 
$\displaystyle \Delta n = \Delta p = \frac{\eta\alpha I}{h\nu}$
  In principle, the process of illumination will lead to a continued increase in the number of carriers as the amount of energy absorbed (and hence $ \Delta n$ and $ \Delta p$) will increase linearly with time. However, the excited pairs have a finite life time ( $ \sim 10^{-7}$ to $ 10^{-2}$ s). This results in recombination of the pairs. The relevant life time is that of minority carriers as a pair is required in the process. Recombination ensures that the number of excess carriers does not increase indefinitely but saturates.
   
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