Sunday, May 20, 2012

Excess Carriers

       In the discussion of drift and diffusion currents, it is assumed that the temperature of the semiconductor is constant and it is in thermal equilibrium.
       But practically as the current flows through the semiconductor, its temperature changes and its thermal equilibrium gets disturbed.
       Under this condition if a semiconductor is exposed to the light energy then high energy photons incident on the semiconductor give energy to the valence electrons. This energy is sufficient for the valence electrons to break the covalent bonds and become free electrons. Due to this, more electron-hole pairs are generated. These additional electrons and holes are called excess charge carriers. Due to this, concentration of electrons and holes increases above their values at thermal equilibrium.
       Let            δn = Excess electron concentration 
                         n = Electron concentration at thermal equilibrium
                         δp = Excess hole concentration 
                          P = Hole concentration at thermal equilibrium
       Due to thermal nonequilibrium, the new concentrations of electrons and holes are n' and p' respectively. Then,
                n' = n + δn                                         ............. (1)
                p' = p + δp                                        ...............(2)
1.1 Recombination
        It is seen that when a free electron falls into a hole, the recombination takes place and both the charge carriers disappear. Thus after generation of excess charge carriers, the recombination takes place. Finally the concentrations of electrons and holes attain a constant value and do not increase indefinitely. This is steady state of the semiconductor. The excess concentration attains a steady state value due to recombination.
       The mean time for which an excess electron and hole exist before they recombine and disappear, is called the excess carrier lifetime.

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