A simple analytical theory is developed to describe the carrier distribution and the current flow in a punch‐through structure under the diffusion‐limited injection condition and negligible generation recombination effects. The current expression determined is similar to the diode current expression with the diffusion length replaced by the injection length. According to the nature of the injection length, the current flow process is found to be intimately related to the doping concentration and carrier mobility in the base. Recombination generation effects are next included in the theory to account for the photogain observed in punch‐through structures. Finally the model is applied to the gate‐field‐controlled barrier‐injection transit‐time transistor and the field and light injection‐controlled punch‐through transistor.