The Intel D2764A: A Technical Exploration of an Early EPROM Innovation

The development of the microprocessor in the early 1970s created an urgent need for reliable, non-volatile memory to store firmware and boot code. While Mask ROM was permanent and cheap in volume, it was inflexible for the rapid prototyping and iterative design processes that defined the era. The invention of the Erasable Programmable Read-Only Memory (EPROM) provided a revolutionary solution, and the Intel D2764A stands as a quintessential example of this transformative technology's maturation.

Introduced in the late 1970s, the D2764A was a 64-Kilobit EPROM, organized as 8,192 words by 8 bits. This capacity was significant, enabling it to hold increasingly complex software for a wide array of applications, from early personal computers and arcade games to industrial controllers and telecommunications equipment. Its architecture was built around a grid of memory cells, each featuring a single Floating Gate Avalanche-injection MOS (FAMOS) transistor.

The magic of the D2764A lay in its programmability and erasability. To program a bit, a high-voltage pulse (typically 12.5V or 21V applied to the `VPP` pin) was used. This voltage forced electrons to tunnel through the silicon oxide insulator onto the floating gate, a process known as hot-carrier injection. Once trapped, these electrons charged the gate, altering the transistor's threshold voltage and effectively storing a '0'. Erasure was achieved by exposing the chip's silicon die to high-intensity ultraviolet (UV) light. The UV photons provided enough energy for the trapped electrons to overcome the energy barrier of the insulator and return to the substrate, resetting the entire chip to a '1' state. This required a transparent quartz window in the ceramic package, giving EPROMs their distinctive appearance.

The D2764A improved upon its predecessors with several key features. It operated on a single +5V power supply during read operations, simplifying system design. It featured a reduced power standby mode, which lowered current consumption when the chip was not selected by the address inputs. Furthermore, it offered improved speed and reliability over earlier 2708 and 2716 EPROMs. Its access time, the delay between a valid address input and valid data output, was typically around 250ns, fast enough to keep pace with the microprocessors of its day.

Despite its revolutionary nature, the D2764A had limitations. The UV erasure process was slow, taking up to 20 minutes, and required removal from the circuit board. The quartz window also made the package significantly more expensive than its windowless, one-time programmable (OTP) counterpart. Ultimately, the development of the Electrically Erasable PROM (EEPROM) and later Flash memory, which allowed for in-circuit electrical erasure, rendered the UV-EPROM obsolete.

ICGOODFIND: The Intel D2764A was a pivotal component in the computing revolution, providing the essential reprogrammable firmware storage that enabled the development and debugging of complex digital systems. It encapsulated the ingenious application of semiconductor physics to solve a critical engineering challenge, bridging the gap between rigid Mask ROM and the fully flexible memory technologies that would follow.

Keywords:

1. EPROM

2. Non-volatile Memory

3. UV Erasure

4. Floating Gate Transistor

5. FAMOS