Fission fragment irradiation of a number of semiconducting materials establishes that permanent tracks are formed only when the conductivity is ≲2 × 103(ohm-cm)−1. The intermittent nature of particle tracks is a characteristic of layer structures and further analysis confirms that there is one ‘defect’ per sandwich plane crossed by the fission particle. The intermittency is ascribed to the reduced energy loss of the fission fragment when it travels in the interplanar channel.