The effect of the transition energy of three-donor clusters on far infrared absorption in n-type semiconductor materials has been investigated by a multiconfigurational self-consistent-field model calculation and applied to GaAs and GaN systems. We show that it is crucial to consider the many-particle correlation effects within three-donor clusters. With electron correlation taken into account, the present results support the interpretation of a very recent unidentified peak energy observed in absorption measurement of GaN as due to electronic transitions in these clusters. We also corroborate the suggestion that the X line in GaAs arises from such transitions. © 2002 American Institute of Physics.