The behavior of the electron impact‐ionization coefficient &agr;nin In0.53Ga0.47As is measured with unprecedented sensitivity down to very low electric fields. The data are derived from measurements of the multiplication coefficientM−1 in suitably designed heterojunction bipolar transistors. Previously available data are extended by two orders of magnitude in the low field domain, down to &agr;n≊1 cm−1. The experimental behavior of &agr;nat fields below 200 kV/cm is in agreement with the theoretical prediction of a weak field dependence of &agr;nat low electric fields. ©1995 American Institute of Physics.