Solid-state, non-volatile storage devices are an important part of modern computing technology. Unlike volatile storage like RAM that requires power to maintain stored data, non-volatile storage can retain data even when powered off. Solid-state refers to storage devices that have no moving mechanical parts, making them faster, more reliable, and less prone to damage compared to traditional hard disk drives. Some common examples of solid-state non-volatile storage devices include flash memory, ROM, and ferroelectric RAM (FRAM). These devices are used across many computing applications from personal devices to enterprise servers and data centers.
Flash Memory
Flash memory is one of the most widely used types of solid-state non-volatile storage today. There are two main types of flash memory:
NAND Flash
NAND flash got its name from its internal structure which resembles a NAND logic gate. It stores data in an array of memory cells made from floating-gate transistors. NAND flash is found in USB flash drives, memory cards, solid-state drives (SSDs), and other storage devices. Key advantages of NAND flash include:
- High storage density – NAND flash can store lots of data in a small physical footprint.
- Fast read/write speeds – NAND flash access times are orders of magnitude faster than mechanical hard drives.
- Shock resistance – With no moving parts, NAND flash storage can withstand bumps and vibrations.
- Low power consumption – NAND flash uses less power than hard disk drives.
Some disadvantages include slower write speeds compared to RAM, eventual degradation over time, and susceptibility to data corruption from radiation. Overall, NAND flash offers an excellent balance of speed, density, cost, and reliability for solid-state storage applications.
NOR Flash
The less dense NOR flash gets its name from its NOR gate-like internal structure. It is primarily used when random data access is needed since it supports full address and data buses for reading. This makes NOR flash slower at writing but faster at reading compared to NAND. NOR flash is commonly used for storing firmware such as boot code. It has higher endurance and longevity than NAND but lower storage density.
Read-Only Memory (ROM)
ROM is a non-volatile storage type which can only be programmed once. Once data is written, it cannot be modified. Some common types of ROM include:
Masked ROM
Masked ROM chips have their data hard-wired during manufacturing. The contents are fixed from the factory and cannot be altered. These are low cost and were used for early game cartridges and other permanent firmware storage.
Programmable ROM (PROM)
PROM chips can be programmed after manufacturing. But once written, their contents cannot be changed. PROMs had to be removed from the device in order to program them using specialized equipment.
Erasable Programmable ROM (EPROM)
EPROMs can be erased and reprogrammed repeatedly by exposing the chip to ultraviolet light. This clears its entire contents to all 1’s so it can be reprogrammed. EPROMs had a small transparent quartz window to allow UV light exposure for erasing.
Electrically Erasable Programmable ROM (EEPROM)
EEPROM improved upon previous ROM types by allowing electrical erasing and reprogramming without needing to remove the chip from the device. EEPROM allowed in-system rewriting of firmware and made reprogrammable non-volatile memory available for consumer devices.
Ferroelectric RAM (FRAM)
FRAM is a non-volatile RAM memory technology based on ferroelectric film capacitors. Unlike EEPROM which wears out after many rewrite cycles, FRAM can endure 1014 (100 trillion) writes. Key advantages of FRAM include:
- High write endurance – FRAM can handle many read/write cycles.
- Low power – FRAM consumes less power than comparable flash memory.
- High speed – Data can be written to FRAM faster than EEPROM.
- Shock resistant – Like all solid state memory, FRAM contains no moving parts.
These features make FRAM useful for storing frequently updated data like embedded systems configuration data, RFID tags, and energy metering. The downside is lower memory density compared to flash so it has not replaced flash for mass storage applications.
Magnetoresistive RAM (MRAM)
MRAM is an emerging non-volatile memory technology based on magnetic storage principles. It uses magnetic tunnel junctions to store data bits. MRAM has very fast read/write speeds, infinite endurance, and does not need power to retain data. Key advantages of MRAM:
- Very high read/write performance – MRAM has faster access times than both DRAM and flash memory.
- Virtually unlimited endurance – MRAM can be rewritten trillions of times without wear.
- Non-volatile – Data remains stored without power.
- High reliability and stability – MRAM is more resistant to radiation and temperature changes.
MRAM has the potential to become a so-called universal memory, combining the speed of RAM with the non-volatility of flash. But current densities and costs are prohibitive for mainstream applications. Improvements in MRAM manufacturing techniques could enable it to compete with existing memory technologies in the future.
Resistive RAM (RRAM)
Sometimes referred to as memristors, RRAM stores data by changing the resistance of certain materials inside memory cells. RRAM can enable high density solid-state storage along with good performance. Advantages of RRAM include:
- High density – RRAM has potential for greater storage density than NAND flash.
- Low power – RRAM consumes very little energy during operation.
- High speed – RRAM access times can potentially match or exceed DRAM speeds.
- Non-volatile – Data remains stored when powered off.
RRAM is still an emerging technology not yet at commercial production scale. But its potential for fast and dense solid-state storage within a simple structure makes it promising. Efforts are ongoing to resolve challenges like improving yield and endurance.
3D XPoint
3D XPoint is a new class of non-volatile memory developed by Intel and Micron. It uses a three-dimensional structure to greatly increase data density compared to flash memory. 3D XPoint fills the performance gap between DRAM and typical solid-state storage. Key features include:
- 1000x faster than NAND flash – 3D XPoint has very fast access times.
- 1000x more endurance than NAND flash – It can handle vastly more write cycles.
- 10x denser than conventional memory – Innovative stacked architecture enables high density.
- Non-volatile – Data persists without power.
The major disadvantage of 3D XPoint is higher cost per bit than NAND flash. But in applications where very high performance storage matters, 3D XPoint has compelling advantages over other non-volatile memory technologies.
Conclusion
Solid-state non-volatile memory plays an essential role across the computing landscape from portable consumer devices to data center servers. Non-volatile storage enables persisting data securely when power is removed while avoiding the limitations of mechanical hard drives. Leading solid-state technologies like NAND flash and 3D XPoint provide an excellent combination of speed, endurance, density, and price for memory intensive applications. Other innovative non-volatile memory types like RRAM, MRAM, and FRAM are emerging to potentially supplement or replace conventional solid-state storage with new capabilities. Continued improvements in solid-state memory will enable future computing systems to run faster while storing vastly more information.
