Charge trapping and interface state generation in very thin nitride/oxide (4-nm Si3N4 + 8-nm SiO2) composite gate insulators are studied as a function of gate electrode work function and bottom oxide thickness. The behavior of the trapped positive charge under bias-temperature stress after avalanche electron injection (AEI) is investigated. Evidence is presented that secondary hole injection from the anode (gate/Si3N4 interface) and subsequent trapping near the SiO2/Si interface results in a turnaround of the flatband voltage shift during AEI from the substrate. Just like thermal oxides on Si, slow-state generation near the SiO2/Si interface and boron acceptor passivation in the surface-space charge layer of the Si substrate are also observed after AEI in these nitride/oxide capacitors, and they are found to be strongly related to the secondary hole injection and trapping. Finally, interface state generation can take place with little secondary anode hole injection and is enhanced by the occurrence of hole trapping.