EPROM, unlike PROM and OTP can be erased. Early devices used strong ultraviolet light and would reset the entire chip back to its unprogrammed state.
The UV light enables charge on a floating gate to be dissipated. The EPROM was invented by Dov Frohman of Intel in 1971. An EPROM retains its data for about ten to twenty years and can be read an unlimited number of times.
A further refinement made it possible to erase them electrically and often called EEPROMs or E2PROM. Developed by George Perlegos at Intel (1978) it is composed of cells with two transistors. The floating gate is separated from the control gate by a thin oxide layer which enabled the device to be erased using a programming voltage instead of a UV source.
While the EEPROM can be reprogrammed, the number of times it can be altered is limited because the oxide insulating layer can be damaged by frequent rewriting.
Flash memory, invented at Toshiba in the mid-1980s, and commercialized in the early 1990s, is a form of EEPROM.