Operation EPROM

an intel 1702a eprom, 1 of earliest eprom types (1971), 256 8 bit. small quartz window admits uv light erasure.

development of eprom memory cell started investigation of faulty integrated circuits gate connections of transistors had broken. stored charge on these isolated gates changed properties. eprom invented dov frohman of intel in 1971, awarded patent 3660819 in 1968.

each storage location of eprom consists of single field-effect transistor. each field-effect transistor consists of channel in semiconductor body of device. source , drain contacts made regions @ end of channel. insulating layer of oxide grown on channel, conductive (silicon or aluminum) gate electrode deposited, , further thick layer of oxide deposited on gate electrode. floating-gate electrode has no connections other parts of integrated circuit , insulated surrounding layers of oxide. control gate electrode deposited , further oxide covers it.

to retrieve data eprom, address represented values @ address pins of eprom decoded , used connect 1 word (usually 8-bit byte) of storage output buffer amplifiers. each bit of word 1 or 0, depending on storage transistor being switched on or off, conducting or non-conducting.

a cross-section of floating-gate transistor

the switching state of field-effect transistor controlled voltage on control gate of transistor. presence of voltage on gate creates conductive channel in transistor, switching on. in effect, stored charge on floating gate allows threshold voltage of transistor programmed.

storing data in memory requires selecting given address , applying higher voltage transistors. creates avalanche discharge of electrons, have enough energy pass through insulating oxide layer , accumulate on gate electrode. when high voltage removed, electrons trapped on electrode. because of high insulation value of silicon oxide surrounding gate, stored charge cannot readily leak away , data can retained decades.

the programming process not electrically reversible. erase data stored in array of transistors, ultraviolet light directed onto die. photons of uv light cause ionization within silicon oxide, allow stored charge on floating gate dissipate. since whole memory array exposed, memory erased @ same time. process takes several minutes uv lamps of convenient sizes; sunlight erase chip in weeks, , indoor fluorescent lighting on several years. generally, eproms must removed equipment erased, since not practical build in uv lamp erase parts in-circuit. electrically erasable programmable read-only memory (eeprom) developed provide electrical erase function , has displaced ultraviolet-erased parts.