A CDE Definition

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).

(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.