---
title: "UV-EPROM storing and erasing"
subject: "Embedded System"
chapter: 4
key: "semester_6/embedded_system:uv-eprom-store-erase"
source: https://www.bctengineer.com/q/semester_6/embedded_system/uv-eprom-store-erase
site: BCT Engineering
---

# UV-EPROM storing and erasing

Embedded System · Chapter 4: Memory

- Asked in: 2069 Bhadra, 6 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/embedded_system/uv-eprom-store-erase
- Whole chapter: https://www.bctengineer.com/md/semester_6/embedded_system/chapter-4

## Question

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

## 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.

## Answer

**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.

```text
 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).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Where is the charge stored in a UV-EPROM cell?
   A. On the floating gate
   B. In a capacitor
   C. In a fuse
   D. On the drain
   Answer: A. Each cell is a MOS transistor with an insulated floating gate that traps electrons. (Key point: Floating gate)

2. How does programming make a cell read 0?
   A. Electrons trapped on the floating gate raise the threshold voltage so the transistor stays off
   B. A fuse is blown
   C. UV light removes charge from the gate
   D. The control gate is grounded permanently
   Answer: A. Hot electrons cross the oxide and are trapped; the raised threshold keeps the transistor off, reading 0. (Key point: Programming mechanism)

3. What does a blank (erased) EPROM cell read?
   A. 0
   B. 1
   C. Undefined
   D. Alternates
   Answer: B. With no charge on the floating gate the transistor conducts normally and reads 1. (Key point: Blank state)

4. How is a UV-EPROM erased?
   A. Byte by byte electrically
   B. Exposing the whole chip to UV light through the quartz window
   C. Applying a reverse high voltage to the drain
   D. Blowing all fuses
   Answer: B. UV photons give trapped electrons energy to leave the floating gate; the whole chip is erased at once, taking about 15-20 minutes. (Key point: Erasing)

## Related in Embedded System

- [One Time Programmable ROM operation](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aotp-rom-storing) — Embedded System, chapter 4, asked 2×

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