Жадының EPROM түрі: өшірілетін және қайта бағдарланатын жад чипі

Early type of solid state computer memory
An EPROM (rarely EROM), or erasable programmable read only memory, is a type of programmable read only memory (PROM) chip that retains its data when its power supply is switched off. Computer memory that can retrieve stored data after a power supply has been turned off and back on is called non volatile. It is an array of floating gate transistors individually programmed by an electronic device that supplies higher voltages than those normally used in digital circuits. Once programmed, an EPROM can be erased by exposing it to strong ultraviolet (UV) light source (such as from a mercury vapor lamp). EPROMs are easily recognizable by the transparent fused quartz (or on later models resin) window on the top of the package, through which the silicon chip is visible, and which permits exposure to ultraviolet light during erasing.

Development of the EPROM memory cell started with investigation of faulty integrated circuits where the gate connections of transistors had broken. Stored charge on these isolated gates changes their threshold voltage. Following the invention of the MOSFET (metal–oxide–semiconductor field effect transistor) by Mohamed Atalla and Dawon Kahng at Bell Labs, presented in 1960, Frank Wanlass studied MOSFET structures in the early 1960s. In 1963, he noted the movement of charge through oxide onto a gate. While he did not pursue it, this idea would later become the basis for EPROM technology. In 1967, Dawon Kahng and Simon Min Sze at Bell Labs proposed that the floating gate of a MOSFET could be used for the cell of a reprogrammable ROM (read only memory). Building on this concept, Dov Frohman of Intel invented EPROM in 1971, Because of the high insulation value of the silicon oxide surrounding the gate, the stored charge cannot readily leak away and the data can be retained for decades. The programming process is not electrically reversible. To erase the data stored in the array of transistors, ultraviolet light is directed onto the die. Photons of the UV light cause ionization within the silicon oxide, which allows the stored charge on the floating gate to dissipate. Since the whole memory array is exposed, all the memory is erased at the same time. The process takes several minutes for UV lamps of convenient sizes; sunlight would erase a chip in weeks, and indoor fluorescent lighting over several years. Generally, the EPROMs must be removed from equipment to be erased, since it is not usually practical to build in a UV lamp to erase parts in circuit. Electrically Erasable Programmable Read Only Memory (EEPROM) was developed to provide an electrical erase function and has now mostly displaced ultraviolet erased parts.

For large volumes of parts (thousands of pieces or more), mask programmed ROMs are the lowest cost devices to produce. However, these require many weeks lead time to make, since the artwork or design in an IC mask layer or photomask must be altered to store data on the ROMs. Initially, it was thought that the EPROM would be too expensive for mass production use and that it would be confined to development only. It was soon found that small volume production was economical with EPROM parts, particularly when the advantage of rapid upgrades of firmware was considered. Some microcontrollers, from before the era of EEPROMs and flash memory, use an on chip EPROM to store their program. Such microcontrollers include some versions of the Intel 8048, the Freescale 68HC11, and the "C" versions of the PIC microcontroller. Like EPROM chips, such microcontrollers came in windowed (expensive) versions that were used for debugging and program development. The same chip came in (somewhat cheaper) opaque OTP packages for production. Leaving the die of such a chip exposed to light can also change behavior in unexpected ways when moving from a windowed part used for development to a non windowed part for production.