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ultraviolet light

An invisible band of radiation at the upper end of the visible light spectrum. With wavelengths from 10 to 400 nm, ultraviolet (UV) starts at the end of visible light and ends at the beginning of X-rays. The primary source of ultraviolet light is the sun, and most of the UV that reaches earth is in the lower-frequency, longer-wavelength Ultraviolet "A" region (see below).

Although ultraviolet (UV) light is widely known as a disinfectant, it is also used to erase EPROM chips. After several minutes of exposure to UV light, the chip can be programmed again (see EPROM).


                      Wavelength in
 Ultraviolet Region   Nanometers (nm)

 Long Wave "A"   UVA    315-400
 Medium Wave "B" UVB    280-315
 Short Wave "C"  UVC    100-280

 Near            NUV    300-400
 Middle          MUV    200-300
 Far             FUV    122-200
 Vacuum          VUV    10-200
 Extreme         EUV    10-121

 Deep            DUV    Below 300




Ultraviolet in the Spectrum
The ultraviolet band comes after visible light and ends at the beginning of X-rays.






EPROM

(Erasable Programmable ROM) A rewritable memory chip that holds its content without power. EPROM chips are written on an external programming device before being placed on the circuit board. The chip requires an expensive ceramic chip package with a small quartz window that is covered with opaque, sticky tape. For reprogramming, the chip is extracted from the circuit board, the tape is removed, and it is placed under an intense ultraviolet (UV) light for approximately 20 minutes (see ultraviolet light).

Although still used, EPROMS evolved into EEPROMs and flash memory, both of which can be erased in place on the circuit board. See EPROM programmer, EEPROM and memory types.

A Floating Gate Holds the Charge
EPROMs use a transistor with a floating gate underneath a control gate. To program the bit, a high voltage is applied to the control gate. This causes electrons to tunnel through the insulating oxide layer into the floating gate, which impedes the subsequent operation of the control gate. The 0 or 1 is determined by whether the voltage on the control gate is blocked or not.

The floating gate can hold the charge for more than a decade or until the device is erased; however, EPROMs only support a few hundred erase-write cycles.




An EPROM Cell
An EPROM cell acts like a permanently open or closed transistor. Charging is accomplished by applying voltage to the control gate. When the "floating gate" is charged, it impedes the flow of electrons from the control gate to the silicon, and the 0 or 1 is determined by whether the voltage on the control gate is blocked or not.





The Glass Window
EPROM packages have a glass cover so that the chip can be exposed to ultraviolet (UV) light for erasure.






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