Introduction to EPROM and EEPROM

Executive Summary: EPROM vs. EEPROM in 2026

The core difference lies in the erasure method and granularity: EPROM (Erasable Programmable Read-Only Memory) requires strong ultraviolet (UV) light to erase data and must be erased entirely. EEPROM (Electrically Erasable Programmable Read-Only Memory) uses electric signals for erasure and allows for byte-level modification without removing the chip from the circuit.

Modern Context: While Flash memory has largely superseded both for mass storage, EEPROM remains vital in 2026 for storing configuration parameters in embedded systems (IoT), whereas EPROM is now a legacy technology found primarily in vintage computing and industrial restoration.

Table of Contents

Ⅰ What is EPROM?

EPROM (Erasable Programmable Read-Only Memory) is a type of non-volatile memory chip that retains data without power but requires ultraviolet (UV) light for erasure. Unlike modern Flash memory, an EPROM chip is easily identifiable by the transparent quartz glass window on its package. Through this window, the silicon die is visible, allowing UV radiation to reset the memory cells. Once programmed, this window is typically covered with an opaque sticker to prevent accidental data corruption from ambient sunlight.

Historically serving as a bridge between permanent ROM and volatile RAM, EPROM chips solved the "write-once" limitation of older PROM technology. To program an EPROM, a dedicated device (programmer) applies a high voltage (VPP = 12V~24V) to inject electrons into the floating gate.

• Identification: Model numbers often start with "27" (e.g., the 27C020 is a 2M-bit EPROM).

• Default State: A blank (erased) EPROM reads as all "1s" (high level).

• Erasure: Requires 15-20 minutes of exposure to strong UV light.

1. Key Features of EPROM

EPROM technology relies on physical external hardware for its lifecycle management. A "programmer" device generates the specific high-voltage pulses required to trap electrons in the memory cells. The system writes data row-by-row, a process significantly slower than modern 2026 standards.

Despite its obsolescence in consumer tech, EPROM offers robust data retention:

• Longevity: A programmed EPROM can retain data for decades (often cited as 10-20+ years) without power.

• Read Cycles: Unlimited read endurance, making it ideal for firmware storage in legacy industrial machines.

• Legacy Use: Before Flash memory became ubiquitous, EPROM was the standard for BIOS chips on PC motherboards.

2. Working Principle (Hot Electron Injection)

EPROM operates using a Floating Gate Transistor architecture. This structure, a precursor to modern Flash, uses a double-layer gate design to trap electrical charge.

Figure 1: Double-layer grid structure of EPROM

The Switching Mechanism:

• State "1": No electrons in the floating gate. The control gate attracts electrons in the substrate, opening the channel (conductive).

• State "0": Electrons are injected into the floating gate. This negative charge blocks the control gate's field, closing the channel (non-conductive).

Figure 2: EPROM writing process (Hot Electron Injection)

Writing Data (Avalanche Injection): When a high voltage is applied to the drain, electrons gain immense kinetic energy ("hot electrons"). These energetic electrons punch through the thin SiO2 insulating barrier and become trapped in the floating gate. They remain there for years until UV light provides enough photon energy to allow them to escape, effectively erasing the chip.

Ⅱ What is EEPROM?

EEPROM (Electrically Erasable Programmable Read-Only Memory) is a user-modifiable ROM that can be erased and reprogrammed using electrical signals, eliminating the need for UV light. Unlike EPROM, EEPROM allows for byte-level modification, meaning individual bytes of data can be changed without erasing the entire chip. This capability makes it indispensable in modern 2026 electronics for storing calibration data, configuration settings, and user preferences.

1. Evolution from ROM to EEPROM

The memory hierarchy evolved to solve the "update problem" in electronics:

1. Mask ROM: Data burned at the factory. Cannot be changed. Low cost for millions of units, but zero flexibility.

2. PROM (Programmable ROM): Blank chips that users could write once. If an error occurred, the chip was scrapped.

3. EPROM: Erasable via UV light. Reusable, but required physical removal from the circuit board to erase.

4. EEPROM: Erasable in-circuit via electricity. The standard for parameter storage in modern embedded systems.

2. Working Principle (Fowler-Nordheim Tunneling)

EEPROM utilizes the quantum mechanical principle known as Fowler-Nordheim Tunneling to move electrons. This allows operation at lower voltages compared to EPROM and enables on-board programming.

Modern motherboards and microcontrollers use this capability to update firmware or BIOS settings safely. By applying a specific programming voltage (often generated internally by the chip's charge pump from a standard 3.3V or 5V supply), the system can push or pull electrons through the oxide barrier.

Figure 3: Electron tunneling in EEPROM

The Tunneling Effect: The Silicon Dioxide (SiO2) layer surrounding the floating gate in EEPROM is engineered to be extremely thin. When a high electric field is applied, electrons can "tunnel" through this barrier without damaging the physical structure, allowing for substantially higher write cycles (typically 100,000 to 1 million cycles) compared to EPROM.

Figure 4: Erasure process (removing electrons)

Ⅲ Key Differences: EPROM vs. EEPROM

In 2026, understanding the distinction is critical for engineers working on legacy system maintenance (EPROM) versus modern embedded IoT design (EEPROM).

1. Performance & Erasure Comparison

The fundamental difference lies in the erasure mechanism and granularity:

2. Application Scenarios in 2026

EEPROM Use Cases:
   While heavy storage is handled by Flash (NAND/NOR), EEPROM dominates the "small data" sector. It is used in smart meters, automotive sensors, and IoT devices to store network keys, calibration variables, and user settings (e.g., preserving volume levels when a TV is unplugged). It communicates via simple protocols like I2C or SPI.

EPROM Use Cases:
   EPROM is effectively obsolete for new designs. However, it remains relevant for:

• Restoration: Repairing vintage computing hardware (Apple II, Commodore 64).

• Industrial Legacy: Maintaining 1980s/90s era CNC machines and avionics that were certified with specific EPROM parts.

• Education: Teaching the physics of semiconductors and memory states.

Related Resources:

What is a Memory Controller?

Introduction to Flash Memory (NAND/NOR)