A trapped plasma, dense electron‐hole pairs trapped in a semiconductorp+‐n‐n+structure, conventionally has been triggered by applying overdriven electrical fields to initiate impact ionization. In this letter it is shown that the trapped plasma also can be triggered by injecting electrons while electrical fields are held just below the critical level. In this new triggering mode, the injected electrons multiplied by impact ionization locally raise the electrical field, which thus further excites the impact ionization.