Can electrons donated by donor impurity participate in recombination...???

A semiconductor doped with impurities which are ionized (meaning that the impurity atoms either have donated or accepted an electron) will therefore contain free carriers. Shallow impurities are impurities which require little energy - typically around the thermal energy or less - to ionize. Deep impurities require energies larger than the thermal energy to ionize so that only a fraction of the impurities present in the semiconductor contribute to free carriers. Deep impurities which are more than five times the thermal energy away from either band edge are very unlikely to ionize. Such impurities can be effective recombination centers in which electrons and holes fall and annihilate each other. Such deep impurities are also called traps.

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