Difference Between PROM and EPROM


In electronics and computing, memory elements are essential to store data and retrieve them afterwards. In earliest stages, magnetic tapes were used as memory and with the semiconductor revolution memory elements were also developed based on semiconductors. EPROM and EEPROM are non-volatile semiconductor memory types.

If a memory element cannot retain data after disconnecting from the power, it is known as a volatile memory element. PROMs and EPROMs were pioneering technologies in nonvolatile memory cells (i.e. they are able to retain data after disconnecting from power) which led to the development of modern solid state memory devices.         

What is PROM?

PROM stands for Programmable Read Only Memory, a type of non-volatile memory   created by Weng Tsing Chow in 1959 on the request of US Air Force as an alternative for the memory of Atlas E and F ICBM models onboard (airborne) digital computer. They are also known as One-Time Programmable Non-Volatile Memory (OTP NVM) and Field Programmable Read Only Memory (FPROM).  Currently these are widely used in microcontrollers, mobile phones, Radio Frequency Identification cards (RFIDs), High Definition Media Interfaces (HDMI), and video game controllers.

Data written on a PROM is permanent and cannot be changed; therefore, they are commonly used as static memory such as firmware of devices. Early computer BIOS chips were also PROM chips. Prior to programming, the chip has only bits with a value one “1”. In the programming process, only required bits are converted into zero “0” by blowing each fuse bits. Once the chip is programmed the process is irreversible; therefore, these values are unchangeable and permanent.

Based on the manufacturing technology, data can be programmed at wafer, final test, or system integration levels. These are programmed using a PROM programmer which blows the fuses of each bit by applying a relatively large voltage to program the chip (usually 6V for 2nm thick layer).  PROM cells are different from ROMs; they can be programmed even after manufacturing, whereas ROMs can only be programmed at manufacturing.

What is EPROM?

EPROM stands for Erasable Programmable Read Only Memory, also a category of non-volatile memory devices which can be programmed and also erased. EPROM was developed by Dov Frohman at Intel in 1971 based on the investigation into faulty integrated circuits where gate connections of the transistors had broken.

An EPROM memory cell is a large collection of floating gate Field Effect Transistors. Data (each bit) is written onto individual Field Effect Transistors inside the chip using a programmer which creates source drain contacts inside. Based on cell address a particular FET stores data and voltages much higher than the normal digital circuit operating voltages are used in this operation. When the voltage is removed, the electrons are trapped in the electrodes. Due to its very low conductivity the silicon dioxide (SiO2) insulation layer between the gates preserves the charge for long periods, hence retaining the memory for ten to twenty years.   

An EPROM chip is erased by exposure to strong UV source such as a Mercury vapor lamp. Erasure can be done using a UV light with a wavelength shorter than 300nm and exposing for 20 -30 minutes at close range (<3cm). For this, EPROM package is built with a fused quartz window that exposes the silicon chip to the light. Therefore, an EPROM is easily identifiable from this characteristic fused quartz window. Erasure can be done using X-rays too.

EPROMs are basically used as static memory stores in large circuits. They were widely used as the BIOS chips in computer motherboards, but they are superseded by new technologies such as EEPROM, which are cheaper, smaller and faster.

What is the difference between PROM and EPROM?

• PROM is the older technology while both PROM and EPROM are nonvolatile memory devices.

• PROMs can be programmed only once while EPROMs are reusable and can be programmed multiple times.

• The process in the programming of PROMS is irreversible; hence the memory is permanent. In EPROMs memory can be erased by exposure to UV light.

• EPROMs have a fused quartz window in the packaging to allow this. PROMs are enclosed in complete plastic packaging; therefore UV has no effect on PROMs

• In PROMs data is written/ programmed onto the chip by blowing the fuses at each bit using much higher voltages than the average voltages used in digital circuits. EPROMS also use high voltage, but not enough to alter the semiconductor layer permanently.