A new switching phenomenon in metal‐oxide semiconductor tunnel junction has been discovered. With a sufficiently large negative bias applied to the electrode, incident visible light of intensity greater than about 1 &mgr;W/cm2causes the reverse‐biased junction to switch from a low‐current to a high‐current state. It is believed that hot‐electron‐induced impact ionization provides the positive feedback necessary for switching, and causes the junction to remain in its high current‐state after the optical exciation is removed. The junction may be switched back to the low‐current state electrically. The basic junction characteristics have been measured, and a simple model for the switching phenomenon has been developed.