Recombination in silicon and germanium takes place primarily through
energy states or recombination centers within the forbidden gap caused by
lattice defects or impurity atoms. For instance, a conduction band electron
might fall into a recombination center. If a valence band hole then
falls into the same center, a hole-electron pair has been annihilated and
recombination has occured. Sometimes, however, centers exist for which a captured electron is more likely to be emitted to the conduction band
than to recombine with a hole. The electron is said to be trapped and
the center is called a trap. If recombination does not occur, the electron
remains trapped until it is excited to the conduction band. Such
excitation may require much more time than recombination through ordinary
recombination centers. The trapping of holes is analogous to that of
electrons. If minority carriers are trapped, photoconductive decay is
governed by the release of minority carriers from the traps and the result
is an erroneously long t-prj) measurement. Experimental studies of
the phenomenon of trapping [43-46] and mathematical models to describe
the behavior of excess carriers in the presence of trapping [31, 44] are reported in the literature.