11.3 EPROM: Erasable Programmable ROM and UV Erasure

Explain how EPROM's floating-gate transistors store charge to hold data, and how UV-light erasure resets the entire chip for reprogramming.

EPROM (Erasable Programmable Read-Only Memory) solves PROM's biggest limitation — that a programming mistake permanently ruins the chip — by using a reprogrammable storage mechanism instead of destructive fuses. Each EPROM memory cell is built around a floating-gate MOSFET: a transistor with an extra, electrically isolated ('floating') gate buried inside its oxide layer. Programming an EPROM cell injects electrons onto this floating gate, where they remain trapped for years because the surrounding oxide is an excellent insulator; the presence or absence of this trapped charge shifts the transistor's threshold voltage enough to represent a stored 0 or 1. What makes EPROM distinct historically is its erasure method: exposing the die to strong ultraviolet light through a transparent quartz window on top of the chip package gives the trapped electrons enough energy to escape the floating gate, erasing the entire chip's contents at once and returning it to a blank, reprogrammable state.

Core concept: floating-gate charge storage

Unlike PROM's fuse links, an EPROM cell's floating gate is not physically destroyed by programming — it simply accumulates or releases trapped electrons. To program a cell, the device applies a higher-than-normal voltage that allows electrons to tunnel or be injected ('hot-electron injection') onto the floating gate. Because the floating gate is fully surrounded by insulating oxide with no direct electrical connection, those electrons have no path to leak away under normal operating conditions, which is exactly why the stored data is nonvolatile — it persists with power removed, sometimes for a decade or more.

Reading an EPROM cell works by sensing whether the trapped charge has raised the transistor's threshold voltage enough to prevent it from conducting at the normal read voltage. A charged floating gate (representing one stored logic state) makes the transistor behave as 'off' at the read voltage, while an uncharged gate (the other logic state) lets it conduct — the sense amplifier converts this conduct/don't-conduct difference into a clean digital 0 or 1 on the data bus.

• Programmability: individual cells can be written using elevated programming voltages • Erasure: strong UV light (typically for several minutes, through a quartz window) removes the trapped charge from every cell simultaneously • UV-based historical technology: the quartz window is the chip's most visually distinctive feature • Erasure is chip-wide, not selective — you cannot UV-erase a single byte

Step-by-step: program, use, erase, reprogram

Step 1 (erase, if reusing a part): place the EPROM under a UV eraser lamp for the manufacturer-specified time (commonly on the order of 15–20 minutes for older parts), which floods the floating gates with enough photon energy to let trapped electrons escape back through the oxide, returning every cell to its default (erased) logic state. Step 2 (program): use an EPROM programmer to apply elevated voltage pulses at each address whose bits must differ from the erased default, injecting electrons onto the corresponding floating gates. Step 3 (normal use): once programmed, the device is read exactly like a plain ROM — apply an address, the internal decoder selects a row, and the sensed conduct/don't-conduct pattern of that row's floating-gate transistors drives the data bus. Step 4 (later revision): if the stored program needs to change, the whole chip is UV-erased again and reprogrammed from scratch — there is no selective in-field, electrical-only erase in classic EPROM.

Why EPROM mattered, and its practical limits

EPROM was historically important because it let engineers reuse the same physical chips across many design iterations during development — burn a firmware image, test it, UV-erase, reprogram with the fixed version, and repeat — dramatically cutting the cost of debugging embedded firmware compared to using one-time PROMs for every revision.

The practical limits of EPROM are exactly what later technologies improved on: erasure requires physically removing the chip from the circuit board, placing it in a UV eraser box for many minutes, and it always erases the entire chip rather than a single byte or sector. It also requires that visible quartz window, which adds packaging cost and lets ambient UV/sunlight slowly and unintentionally erase data over years — a real reliability consideration for fielded EPROM-based products.

EPROM cell states

Exact logic-level convention for charged vs. uncharged varies by device family — the key concept is charged/uncharged, not a specific 0/1 assignment.

Floating-gate charge state → stored bit:
  Charged (electrons trapped on floating gate)   → one logic state (commonly "0")
  Uncharged (erased state, no trapped electrons) → other logic state (commonly "1")

Lifecycle:
  UV erase (chip-wide, ~minutes under UV lamp) → all cells return to erased/default state
  Program (electrical, per-cell)               → selected cells receive trapped charge

Code Examples

This illustrates EPROM's practical trade-off versus PROM: reuse is possible, but each revision costs a whole-chip UV erase cycle measured in minutes, plus physically removing and reinserting the chip — much slower than EEPROM/Flash's in-circuit electrical erase.

UV erasure is whole-chip and takes minutes; it cannot target a single byte.

Iteration 1:
  Blank EPROM  →  program with firmware v1  →  test on hardware  →  bug found

Fix and retest:
  Remove chip  →  place under UV eraser for ~15-20 minutes  →  entire chip erased
  Program with firmware v2 (bug fix)  →  test on hardware  →  passes

Result: same physical chip reused across iterations, at the cost of an
erase-and-reprogram cycle (many minutes) between each revision.

EPROM floating-gate cell (conceptual cross-section)

The floating gate is fully surrounded by insulating oxide, so trapped charge has no leakage path — that isolation is exactly why the stored data is nonvolatile.

Control Gate ───────────┐
                           │
   ┌───────────────────────▼───────────────────┐
   │            Oxide (insulator)               │
   │  ┌─────────────────────────────────────┐   │
   │  │        Floating Gate (isolated)      │   │ ← trapped electrons live here
   │  └─────────────────────────────────────┘   │
   │            Oxide (insulator)               │
   └───────────────────────┬───────────────────┘
   Source ──────[ channel ]──────── Drain

Important Rules

EPROM cells store data as trapped charge on an electrically isolated floating gate.

Programming is electrical and per-address; erasure is UV-light-based and always whole-chip.

The chip's quartz window is what allows UV light to reach the die for erasure.

Charge trapped on the floating gate persists without power — this is what makes EPROM nonvolatile.

Practical legacy

EPROM parts (often marked with part numbers like 27C-series) were a mainstay of 1980s–1990s embedded systems development, recognizable by their distinctive quartz erasure window. Although modern designs use EEPROM/Flash almost exclusively, EPROM is still an important conceptual stepping stone for understanding floating-gate storage, which is the same physical mechanism used inside every modern Flash memory chip.

Assuming EPROM can be erased electrically, one byte at a time, like EEPROM or Flash.

Classic EPROM erasure is UV-light-based and erases the entire chip at once; selective electrical erase is a capability of EEPROM and Flash, not EPROM.

Mixing up EPROM with EEPROM is one of the most common terminology errors in digital electronics courses — the extra 'E' in EEPROM specifically denotes electrical erasability.

[tricky] What physically enables EPROM to be erased and reprogrammed, unlike PROM? A. EPROM uses reversible fuse links that can be reconnected B. EPROM stores data as charge on an isolated floating gate, which UV light can discharge, whereas PROM destroys a physical fuse link that cannot be restored C. EPROM has no fixed address decoder D. EPROM is volatile, so it naturally resets between uses

B. EPROM stores data as charge on an isolated floating gate, which UV light can discharge, whereas PROM destroys a physical fuse link that cannot be restored — PROM's fuse-blowing is a one-way destructive change; EPROM instead traps charge on an insulated floating gate, and UV photons can give that trapped charge enough energy to escape, resetting the cell — a reversible, non-destructive mechanism that PROM's fuses lack.

[interview] Why can't EPROM erase a single memory location, only the whole chip? A. Because its address decoder only supports full-chip selection B. Because UV light is applied externally through the package window and illuminates the entire die at once, with no way to target one cell C. Because EPROM cells are wired in series D. Because the manufacturer disables partial erase in firmware

B. Because UV light is applied externally through the package window and illuminates the entire die at once, with no way to target one cell — UV erasure works by exposing the whole die to ultraviolet photons through the quartz window; there is no electrical addressing mechanism involved in erasure, so there is no way to confine the UV exposure to a single row or byte — every floating gate on the die is affected together.

Explain, using the floating-gate model, why EPROM data survives a power outage but a single UV erase session wipes the entire chip rather than a chosen region.

Data survives power loss because trapped charge on the floating gate has no electrical leakage path — the oxide fully insulates it, so it needs no power to persist. Erasure is global because UV light is applied through the package window to the entire die surface at once; the light energizes every floating gate it reaches simultaneously, and classic EPROM has no per-cell electrical mechanism to selectively discharge just one address — the erase mechanism (photons) simply cannot be aimed at a single memory cell.

Separate the question of 'why is it nonvolatile' (charge trapping) from 'why is erasure global' (how UV light reaches the die).