storage (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.