"Erasure, however, has to be accomplished by non-electrical methods, since the gate electrode is not accessible electrically. Shining ultraviolet light on any part of an unpackaged device causes a photocurrent to flow from the floating gate back to the silicon substrate, thereby discharging the gate to its initial, uncharged condition. This method of erasure allows complete testing and correction of a complex memory array before the package is finally sealed. Once the package is sealed, information can still be erased by exposing it to X radiation in excess of 5*104 rads, a dose which is easily attained with commercial X-ray generators." (5*104 rad = 500 J/kg)[5]
"In other words, to erase your EPROM, you would first have to X-ray it and then put it in an oven at about 600 degrees Celsius (to anneal semiconductor alterations caused by the x-rays). The effects of this process on the reliability of the part would have required extensive testing so they decided on the window instead. (any temperature between 450 - 1410 °C should work).
EPROMs had a limited but large number of erase cycles; the silicon dioxide around the gates would accumulate damage from each cycle, making the chip unreliable after several thousand cycles. EPROM programming is slow compared to other forms of memory. Because higher-density parts have little exposed oxide between the layers of interconnects and gate, ultraviolet erasing becomes less practical for very large memories. Even dust inside the package can prevent some cells from being erased.
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