In a routine evaluation of an electrically identical Schottky diode made by two manufacturers, it was discovered that the diode by one manufacturer was unexpectedly sensitive to gamma radiation. In brief, the breakdown voltage of the diode was reduced by more than 20 percent after a dose of 1 Ã 105 rads. Subsequent investigations of this anomaly showed that the behavior resulted from the fact that the diode metallization extended significantly beyond the guard ring over the oxide forming a parasitic MOS device. The parasitic device is large and is formed by a thick, "unhardened" oxide. The radiation effects properties of this device are therefore similar to those of an unhardened MOS device formed with a field oxide. It is the charge accumulation in this thick oxide which significantly alters the diode properties following exposure to gamma radiation. The problem is easily eliminated by reducing the metallization size so that it does not extend significantly over the thick oxide, thereby eliminating formation of the parasitic MOS device.