Optical-isothermal capacitance transient spectroscopy (O-ICTS) was used to characterize prominent midgap carrier traps in Si-dopedn-type GaN grown by metalorganic vapor phase epitaxy. Strong carrier photoionization was observed from two deep levels to the conduction band. The first level photoionizes in the broad range from below 1.8 eV to over 2.3 eV. Seen in alln-type GaN, this level is believed to be defect related and involved in the commonly observed yellow luminescence. The second, more dominant O-ICTS peak develops with incident photon energy of∼2.3 eV.This is a previously unreported, yet unidentified level of impurity nature. The two midgap states can be clearly distinguished by means of the time constant for photoionization of carriers from the deep levels. ©1997 American Institute of Physics.