What Is EPROM?

EPROM, or Erasable Programmable Read-Only Memory, is a type of non-volatile semiconductor memory that retains stored data without power and can be erased by exposure to ultraviolet light and reprogrammed with electrical pulses. Unlike earlier mask-programmed ROMs, which were fixed at the time of manufacture, and one-time-programmable PROMs, which allowed a single post-fabrication write cycle, EPROM introduced genuine reusability: a device could be erased, verified blank, and reprogrammed multiple times. This property made EPROM the dominant choice for storing firmware and microcode during the development and early production phases of microprocessor-based systems from the mid-1970s through the 1990s.

The key structural innovation in EPROM is the floating-gate metal-oxide-semiconductor field-effect transistor (MOSFET). Dawon Kahng and Simon Sze of Bell Labs proposed in 1967 that a conductive gate fully surrounded by insulating oxide could serve as the storage element in a reprogrammable memory cell. Dov Frohman at Intel translated this concept into a practical device, inventing the Intel 1702, a 2048-bit EPROM announced in 1971, as documented by the Computer History Museum's Silicon Engine record.

Programming and Erasure Mechanism

Data is written to an EPROM by applying a programming voltage, typically 12.5 to 25 volts depending on the generation, across the selected cell's control gate and drain. This high electric field induces hot-electron injection, forcing electrons through the thin gate oxide onto the floating gate. Once there, the trapped charge shifts the transistor's threshold voltage, changing the cell from a logic one to a logic zero state. The floating gate is electrically isolated on all sides, so charge leakage is negligible at operating temperatures; a properly programmed EPROM can retain data for 10 to 20 years. Erasure requires exposing the die through the device's characteristic transparent fused-quartz window to ultraviolet light at a wavelength near 253.7 nm, typically for 15 to 30 minutes. The UV photons excite the trapped electrons enough to overcome the oxide barrier and return the floating gate to a neutral state, restoring all cells to the unprogrammed condition. A full-UV erase returns the entire array to the blank state simultaneously; selective cell erasure is not possible.

Device Families and Evolution

EPROM families were identified by their storage capacity and programming voltage. The Intel 2716 (16 kbit) and 2732 (32 kbit) devices introduced in the late 1970s became widely used in embedded system designs. Successive generations from manufacturers including Intel, AMD, and Texas Instruments scaled capacity to 256 kbit (27256) and 1 Mbit (27010) by the late 1980s. TechTarget's EPROM definition notes that the quartz window, which distinguishes an EPROM from a windowed PROM and from its successor EEPROM, added to device cost and required that labels be affixed over the window during normal operation to prevent inadvertent UV erasure from sunlight or fluorescent lamps. ScienceDirect's overview of erasable programmable read-only memory documents how the introduction of electrically erasable PROM (EEPROM), which erases at the byte level using electrical signals, and subsequently flash memory, which extends this approach to large erase blocks, eventually displaced EPROM from most production applications.

Applications

EPROM has applications in a range of fields, including:

• Firmware storage for early microprocessors and microcontrollers during development
• ROM replacement in embedded systems requiring field updates
• BIOS and boot code in personal computers before the transition to flash-based firmware
• Programmable logic device configuration storage in prototyping environments
• Character generator and lookup-table functions in early graphics hardware