The electron impact ionization coefficient αn(E) in InP is determined from electron multiplication measurements in NpN InP/GaAsSb/InP double heterojunction bipolar transistors (DHBTs). The staggered (“type-II”) band lineup between the p+ GaAs0.51Sb0.49 base and the InP collector layer allows the injection of a pure electron initiating current directly into the InP collector across an abrupt base/collector heterojunction, without the ambiguities associated with the compositional grading otherwise required to overcome the blocking band discontinuity between Ga0.47In0.53As and InP. InP/GaAsSb/InP DHBTs thus enable us to characterize αn(E) in InP in a low-field regime. Whereas the αn(E) values we extract at higher fields are in good agreement with previously reported photodiode based measurements, our data reveal a clear low-field “tail” for αn(E)≪102 cm-1, indicating that InP has a soft impact ionization threshold similar to that observed for electrons in Ga0.47In0.53As. This finding can be understood in terms of the similarities between the conduction band structures of InP and Ga0.47In0.53As. © 2003 American Institute of Physics.