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UV-EPROM storing and erasing

Short answer

A blank UV-EPROM has all cells reading 1. Programming applies high voltage to inject electrons onto the floating gate, turning the cell off and reading 0. Erasing exposes the chip to UV light through a quartz window, which discharges all floating gates simultaneously, resetting every cell to 1.

Explain the operations of storing and erasing the data in UV-EPROM.

UV-EPROM (Erasable Programmable ROM) stores each bit in a MOS transistor with an extra floating gate, an insulated gate between the control gate and the channel. The chip has a quartz window that lets UV light reach the cells.

Storing (programming)

  • A blank EPROM has no charge on the floating gates, so all cells read 1 (the transistor conducts normally).
  • An EPROM programmer applies a high voltage (Vpp, about 12-21 V) to the control gate and drain of the selected cell.
  • Hot electrons gain enough energy to cross the thin oxide and become trapped on the floating gate.
  • The trapped negative charge raises the threshold voltage, so the transistor stays off when read: the cell reads 0. Cells that are not selected keep 1.
  • The insulating oxide holds the charge for about 10 years without power.
 control gate  ----------
 floating gate ----------   <- electrons trapped = 0
 source |  channel  | drain

Erasing

  • The chip is removed from the circuit and exposed to ultraviolet light (wavelength 253.7 nm) through the quartz window for about 15-20 minutes.
  • The UV photons give the trapped electrons enough energy to leave the floating gate through the oxide (photocurrent), so the gate is discharged.
  • All cells return to 1. The whole chip is erased at once (individual bytes cannot be erased), and it can then be reprogrammed. The number of erase cycles is limited (typically about 100 to 1000).

✓ Answer checked by Claude Opus 5.5 (high effort)

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