Erasable Programmable Read-Only Memory (EPROM)

The term "EPROM" is the acronym for "Erasable Programmable Read-only Memory". As its name implies, it is a semiconductor memory device that can be programmed with data which can only be read, but not altered, by the application circuit.  As such, programming an EPROM generally takes place prior to its attachment to the application circuit.  One of the most common applications for an EPROM is as a BIOS chip of a personal computer, which stores information about the computer's basic input/output system.

An EPROM is a non-volatile memory device, i.e., it can retain its stored data even if it is powered off.  Reprogramming an EPROM with new data is possible, but it has to undergo a special data erasure process that employs ultraviolet (uv) light before it can be done. There are some EPROMs though, known as one-time programmable (OTP) EPROMs, that are designed to be non-reprogrammable as a cheaper alternative for storing specific bug-free data that never require any change.

An EPROM, just like a Flash memory IC, is a "floating gate" device, i.e., it is a device that employs a 'floating gate' in each of its memory cells to store data.  A  "floating gate" is a structure embedded within the dielectric layer that isolates the silicon channel and the external gate of each memory cell transistor.  It is designed to store charge, the amount of which is used to represent whether the bit of data stored in the memory cell is a '1' or '0'.

The memory cells of a completely erased EPROM all contain a '1'. Programming a cell, which entails charging up the cell's floating gate to a certain level, gives it a new value of '0'. A memory cell sensor determines the amount of charge stored in the floating gate and compares it with a given threshold.  If the charge stored exceeds the threshold, then the stored data is considered a '0', otherwise it is a '1'.

Programming an EPROM requires higher voltages than just reading it, since injection of charge into the floating gate is needed.  Modern EPROMs just need a 5-V supply (Vcc) to be read, but require a second power supply (set to a higher voltage in the range of 12 to 25 volts during programming) to be programmed. This second supply voltage used during programming is often referred to as the "Vpp." In some EPROMS, programming also requires a higher voltage at Vcc aside from the Vpp voltage.

Just like other memory devices, a typical EPROM has data pins, address pins, and control pins (CE and OE).  The data pins are bi-directional, acting as inputs during programming and as outputs during reading. The CE or "chip enable" pin is used for activating or deactivating the entire EPROM chip itself, while the OE or "output enable" pin is used for activating or deactivating the EPROM's output pins only. 

Programming an EPROM basically requires: 1) setting the Vcc and Vpp to their 'program mode' levels; 2) applying the addresses and input data at the address and data pins; and 3) applying the required programming pulses in accordance with a programming 'algorithm'.  

Erasure of an EPROM requires the dissipation of the stored charge inside the floating gate, which is accomplished by exposing the cells to ultraviolet light with a wavelength of 2537 angstroms. This is why EPROMs have a glass window directly over the chip area - to allow ultraviolet radiation to reach the memory cells. This glass window must be covered after the chip has been programmed to protect the stored data from ambient light, which can indeed erase EPROM data if given enough time. A wide range of EPROM uv erasers are available in the market for erasing EPROMS conveniently.  

Primary Reference: "How EPROMs Work" by Dr Ah Clem Memory, http://xtronics.com