Impact ionization coefficients have been deduced from photomultiplication measurements performed onGa0.52In0.48Pp–i–ndiodes with nominal intrinsic region thicknesses of 1 .0, 0.7, and 0.2 &mgr;m. The results indicate that &bgr;, the hole ionization coefficient, is slightly greater than &agr;, the electron ionization coefficient at low fields, and they both become effectively equal at high fields. &agr; and &bgr; are also found to be significantly lower than GaAs across the range of electric fields studied, with correspondingly higher breakdown voltages. ©1997 American Institute of Physics.