The properties of electron traps in nominally undopedIn1−xGaxAsyP1−ylayers grown onGaAs0.61P0.39substrates by liquid phase epitaxy were investigated by deep level transient spectroscopy measurements. Only one kind of electron trap with the activation energy of 0.38 eV was observed inIn0.32Ga0.68P. A new kind of electron trap with the activation energy of 0.15 eV began to appear and its concentration increased as the As composition was increased. But this trap was not detected inGaAs0.61P0.39.This 0.15 eV trap was found to have a capture barrier causing the persistent photoconductivity phenomenon inIn1−xGaxAsyP1−yat low temperatures. This defect was also found to have acharge-state controlled stability. ©1997 American Institute of Physics.