The V/I characteristics of p Ge-n CdS heterojunctions exhibit a hysteresis or memory characteristic which is the result of two diode conduction states of resistances 10⁶ and 10Ω. The device switches from the high to the low state in less than 30 ns under reverse bias, and resetting to the high-resistance state occurs in less than 100 ns when a threshold forward bias current is exceeded. In either state, the device retains its state for periods greater than two weeks and appears to have potential application as a fast, nonvolatile memory element.