Editing EPROM (section)

==Operation==
[[File:Eprom.jpg|thumb|250px|left|An Intel 1702A EPROM, one of the earliest EPROM types (1971), 256 by 8 bit. The small quartz window admits UV light for erasure.]]
Development of the EPROM [[memory cell (computing)|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]].

In 1957, Frosch and Derick were able to manufacture the first [[silicon dioxide]] [[Field-effect transistor|field effect transistors]] at [[Bell Labs]], the first transistors in which drain and source were adjacent at the surface.<ref>{{Cite journal |last1=Frosch |first1=C. J. |last2=Derick |first2=L |date=1957 |title=Surface Protection and Selective Masking during Diffusion in Silicon |url=https://iopscience.iop.org/article/10.1149/1.2428650 |journal=Journal of the Electrochemical Society |language=en |volume=104 |issue=9 |pages=547 |doi=10.1149/1.2428650|url-access=subscription }}</ref> Following the invention of the [[MOSFET]] at Bell Labs, [[Frank Wanlass]] studied MOSFET structures in the early 1960s. In 1963, he noted the movement of charge through [[Gate oxide|oxide]] onto a [[Metal gate|gate]]. While he did not pursue it, this idea would later become the basis for EPROM technology.<ref>{{cite web |title=People |url=https://www.computerhistory.org/siliconengine/people/ |website=The Silicon Engine |publisher=[[Computer History Museum]] |access-date=17 August 2019}}</ref>

In 1967, [[Dawon Kahng]] and [[Simon Min Sze]] at Bell Labs proposed that the [[Floating gate MOSFET|floating gate of a MOSFET]] could be used for the cell of a reprogrammable [[Read-only memory|ROM]] (read-only memory).<ref name="computerhistory1971">{{cite web |title=1971: Reusable semiconductor ROM introduced |url=https://www.computerhistory.org/storageengine/reusable-semiconductor-rom-introduced/ |website=[[Computer History Museum]] |access-date=19 June 2019}}</ref> Building on this concept, [[Dov Frohman]] of [[Intel]] invented EPROM in 1971,<ref name="computerhistory1971"/> and was awarded {{US patent| 3660819}} in 1972. Frohman designed the Intel 1702, a 2048-bit EPROM, which was announced by Intel in 1971.<ref name="computerhistory1971"/>
 
Each storage location of an EPROM consists of a single [[field-effect transistor]]. Each field-effect transistor consists of a channel in the semiconductor body of the device. Source and drain contacts are made to regions at the end of the channel. An insulating layer of oxide is grown over the channel, then a conductive (silicon or aluminum) gate electrode is deposited, and a further thick layer of oxide is deposited over the gate electrode. The [[floating-gate]] electrode has no connections to other parts of the integrated circuit and is completely insulated by the surrounding layers of oxide. A control gate electrode is deposited and further oxide covers it.{{Sfn | Sah | 1991 | p = 639}}

To retrieve data from the EPROM, the address represented by the values at the address pins of the EPROM is decoded and used to connect one word (usually an 8-bit byte) of storage to the output buffer [[Amplifier|amplifiers]]. Each bit of the word is a 1 or 0, depending on the storage transistor being switched on or off, conducting or non-conducting.

[[File:Floating gate transistor.png|thumb|A cross-section of a floating-gate transistor]] <!-- not quite the picture I want -->
The switching state of the field-effect transistor is controlled by the [[voltage]] on the control gate of the transistor. Presence of a voltage on this gate creates a conductive channel in the transistor, switching it on. In effect, the stored charge on the floating gate allows the threshold voltage of the transistor to be programmed.

Storing data in the memory requires selecting a given address and applying a higher voltage to the transistors.  This creates an avalanche discharge of electrons, which have enough energy to pass through the insulating oxide layer and accumulate on the gate electrode. When the high voltage is removed, the electrons are trapped on the electrode.<ref>{{cite book|last=Oklobdzija|first=Vojin G.|title=Digital Design and Fabrication|year=2008|publisher=CRC Press|isbn=978-0-8493-8602-2|pages=5–17}}</ref> 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 (integrated circuit)|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 lamp|fluorescent lighting]] over several years.<ref>{{Citation | first = John E | last = Ayers | title = Digital integrated circuits: analysis and design | publisher = CRC Press | year = 2004 | isbn = 0-8493-1951-X | page = 591}}.</ref> 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.