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| 32.3 Erasable Programmable Read Only Memory (EPROM) |
| An EPROM is erased by shining ultraviolet light on the cells through a transparent window in the package. The UV radiation renders the oxide slightly conductive by direct generation of electron-hole pair in the material. The erasure process is slow and can take from seconds to several minutes, depending on the intensity of the UV source. Programming takes several (5-10) microseconds/word. Another problem with the process is limited endurance , that is, the number of erase/program cycles is generally limited to maximum of 1000, mainly as a result of UV erase procedure. Reliability is also an issue. The device threshold might vary with repeated programming cycles. Most EPROM memories therefore contain on-chip circuitry to control the value of thresholds to within a specified range during programming. Finally, the injection always entails a large channel current, as high as 0.5mA at a control gate voltage of 12.5V. This causes high power dissipation during programming. |
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| The EPROM cell is extremely simple and dense, making it possible to fabricate large memories at a low cost. EPROMs were therefore attractive in applications that do not require regular programming. Due to cost and reliability issues, EPROMs have fallen out of favor and have been replaced by Flash Memories. |
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