How EPROM is erased and reprogrammed?

EPROM (Erasable Programmable Read-Only Memory) is erased and reprogrammed using ultraviolet (UV) light through a process designed to discharge the stored electrical charges from its memory cells.

How EPROM is Erased

• Each memory cell in an EPROM contains a floating-gate transistor that stores data as an electrical charge on the gate.
• To erase the stored information, the chip is exposed to UV light (typically 253.7 nm wavelength) through a quartz window located on the top of the chip package.
• The UV photons ionize the silicon oxide layer, allowing the trapped electrons to leak away from the floating gate into the silicon substrate, effectively removing the stored charge.
• The entire chip is erased at once, resetting all bits to the "1" (high) state.
• The erasure typically takes 15–20 minutes under a high-intensity mercury-vapor UV lamp, depending on factors like lamp power, distance, and chip cleanliness.

How EPROM is Reprogrammed

• After erasure, data is written using a programmer device that applies higher-than-normal voltages (typically around 12–25 volts) to selected transistors.
• This process injects electrons onto the floating gate through avalanche breakdown, trapping them and switching the cell to the "0" (low) state.
• Each memory location is selected and written sequentially according to the input data pattern.