Difference Between EPROM and EEPROM


EEPROM and EPROM are two types of memory storage elements developed in the 1970s. These are non-volatile erasable and reprogrammable memory types and are commonly used in hardware programming.

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 store 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 But they are superseded by new technologies such as EEPROM, which are cheaper, smaller and faster.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 EEPROM?

EEPROM stands for Electronically Erasable Programmable Read Only Memory, which was the most widely used memory cell type until Flash memory became available. EEPROM was developed by George Perlogos at Intel in 1978 based on the previously developed EPROM Technology. Intel 2816 is the first commercially launched EEPROM chip.

EEPROMs are also a large array of floating gate MOSFETs like EPROMs, but unlike the EPROMs, EEPROMs has a thinner layer of insulation between the gates. Therefore, the charges in the gates can be electronically changed. EEPROMs are both electronic programmable and erasable. They can be programmed, erased and then reprogrammed without removing from the circuit. But the circuit has to be designed to accommodate transmission of special programming signals.

Based on the data communication mode EEPROMs are categorized into Serial and Parallel interface types. In general, parallel bus chips have an 8-bit wide data bus which allows wider memory usage. In contrast, serial interface type has fewer pins; therefore, the operation has to be performed in a serial manner. Therefore, parallel EEPROM are faster and are used commonly compared to serial interface type EEPROMs.

EEPROM chips are widely used in computers and other electronic devices for storing small amounts of data that must be saved when the power is removed and needs to be retrieved during restarting. Information such as configuration details and calibration tables were stored in EEPROMs. EEPROMs were also used as BIOS chips. Now a variant of EEPROM, the FLASH ROM has taken over the market, due to its capacity, low cost and endurance.

What is the difference between EEPROM and EPROM?

• EPROMs have to be erased with exposure to UV light and EEPROMs can be erased electronically.

• EPROMs have a Quartz window in the package to expose the chip to UV light and EEPROMs are completely encased in an opaque plastic case.

• EPROM is the older technology.