A new experimental technique is described, which measures the injection and propagation of charge in the Si3N4 film of an MNOS structure caused by a pulsed electric field. Contrary to the widely held contention that the electrons injected from the Si are trapped in and very near the SiO2Single Bond signSi3N4 interface, a contention resulting from the assumption that the trap density near this interface usually far exceeds the number of injected electrons, this work finds that the spatial distribution of the trapped electrons extends deep into the Si3N4 film. In fact, the centroid of the spatial distribution of the trapped electrons in a Si3N4 film ≃500 Å thick can be as much as 150 Å or more from the SiO 2Single Bond signSi3N4 interface, indicating that the span of the distribution extends across almost the entire thickness of the film with an average density of trapped electrons in the order of 3 × 1018/cm3. It is shown that, in general, the centroid of the trapped charge distribution in the Si3N4 film of a given device structure is a function of the applied field and its duration. © 1974 American Institute of Physics.