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(EPROM) A type of storage device in which the data is determined by electrical charge stored in an isolated ("floating") MOS transistor gate. The isolation is good enough to retain the charge almost indefinitely (more than ten years) without an external power supply. The EPROM is programmed by "injecting" charge into the floating gate, using a technique based on the tunnel effect. This requires higher voltage than in normal operation (usually 12V - 25V). The floating gate can be discharged by applying ultraviolet light to the chip's surface through a quartz window in the package, erasing the memory contents and allowing the chip to be reprogrammed.