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.
Sponsored links :