Solid state storage has become an increasingly popular option for computer data storage in recent years. Unlike traditional hard disk drives that rely on spinning platters and moving read/write heads, solid state drives have no moving parts. This makes them faster, more durable, smaller, lighter, and less power-hungry than HDDs. There are three main types of solid state storage: flash memory, MRAM, and PCM. Each has its own advantages and best use cases.
Flash Memory
Flash memory is by far the most common type of solid state storage in use today. There are two main types of flash memory: NOR flash and NAND flash. NOR flash is faster for reading data but slower for erasing and writing data. This makes it a good fit for storing firmware and configuration data that doesn’t need to be updated often. NAND flash is slower for reads but much faster for writes and erases. This makes it ideal for storage devices like SSDs that need to constantly write new data.
Inside a NAND flash memory chip, data is stored in an array of floating gate transistors called memory cells. Each cell can store one or more bits of data based on the number of voltage levels it can retain. Single-level cell (SLC) NAND stores 1 bit per cell, multi-level cell (MLC) stores 2 bits per cell, triple-level cell (TLC) stores 3 bits per cell, and quad-level cell (QLC) stores 4 bits per cell. SLC is the fastest and most durable, while QLC is the slowest and least durable but allows for highest storage density and lowest cost per GB.
Flash memory has no moving parts and uses the principles of electron tunneling and hot electron injection to write data and trap-based mechanisms to erase data. It offers excellent random I/O performance, reading and writing data directly in small blocks. However, flash memory has limitations around endurance and requires advanced controller technology like error correction and wear leveling to account for things like program/erase cycling fatigue that causes storage cells to wear out over time.
Pros of Flash Memory:
- Very fast read and write speeds
- Low latency
- Good random I/O performance
- High reliability and durability
- Small physical size
- Lightweight and low power consumption
- Resistant to shock, vibration, temperature extremes
- No moving parts
Cons of Flash Memory:
- More expensive per GB than hard disk drives
- Wear out over time with program/erase cycling (endurance limitations)
- Requires advanced controller technology
- Lower storage densities than HDDs
- Not well-suited for sequential data writes
Due to its excellent performance characteristics, flash memory is commonly used for consumer devices like USB flash drives, memory cards, and solid state drives (SSDs). It’s also widely used in enterprise storage systems and embedded systems where durability, performance, power usage, form factor, and shock resistance are important.
Magnetoresistive RAM (MRAM)
Magnetoresistive RAM, or MRAM, stores data bits using magnetic charges instead of electric charges used in flash memory. Each MRAM cell contains two ferromagnetic plates separated by a thin insulating layer. One plate has a fixed magnetic polarity, while the polarity of the other plate can be flipped to store a 0 or 1. To read the bit value, an electrical current is passed through the cell. The resistance encountered depends on whether the free layer’s magnetic polarity is aligned or opposed to the fixed layer’s, allowing the bit value to be determined.
MRAM writes data faster and uses less power than existing memories like flash or DRAM. It offers non-volatile storage since the magnetic charges remain when power is removed from the device. Unlike flash memory, MRAM can endure practically unlimited read/write cycles. And it provides better random access capabilities than flash.
Pros of MRAM:
- Very high read/write speeds
- Unlimited endurance
- Lower power consumption
- Non-volatile storage
- Better random access than flash
- Smaller physical size than DRAM
Cons of MRAM:
- Lower storage density than Flash or DRAM
- High manufacturing cost per bit
- Currently low production capacities
- Prone to write errors and data loss
While MRAM has been researched since the 1990s, it is still a relatively new technology currently transitioning into mass production. It’s widely considered one of the most promising upcoming memory technologies, offering a “universal memory” that combines the speed of SRAM, the density of DRAM, and the non-volatility of flash memory. MRAM has potential for uses like cache memory and storage class memory in enterprise servers where speed, endurance, and persistence matter.
Phase Change Memory (PCM)
Phase change memory, also sometimes referred to as PCM, PCME, PRAM or PCRAM, relies on changing the physical state of a chalcogenide glass material to store data. Each PCM storage cell contains a material like Germanium-Antimony-Tellurium (GST) sandwiched between two electrodes. The GST can be switched between an amorphous (disordered) state with high electrical resistance representing a 1 bit, and a crystalline (ordered) state with low resistance representing a 0 bit, by heating and cooling it appropriately.
To write a 0 bit, a strong electrical current pulse rapidly heats the GST into an amorphous state. To write a 1 bit, weaker pulses heat it above its crystallization point but cool it slowly, allowing crystals to form. Fast transistor-driven pulses can quickly switch the bit values by inducing the physical state change. Reads are performed by applying a small voltage and assessing resistance. Unlike MRAM, PCM reads have lower latency and require less power.
Pros of PCM:
- Faster writes than flash memory
- High read speeds
- Good endurance
- Non-volatile storage
- High-density storage capability
- Low power consumption
Cons of PCM:
- Slower writes than DRAM and SRAM
- Limited temperature tolerance
- Prone to resistance drift over time
- Requires error correction
- Only moderate write endurance
PCM is sometimes considered a “Storage Class Memory” with characteristics spanning traditional memory and storage. Intel and Micron sell PCM technology marketed as 3D XPoint used in products like Optane SSDs. PCM currently fills a niche role between DRAM and NAND flash, useful for things like persistent memory and caches. As manufacturing processes mature, it holds promise for more widespread memory and storage applications in the future.
Comparison of Flash Memory, MRAM, and PCM
| Technology | Flash Memory | MRAM | PCM |
|---|---|---|---|
| Main Composition | Floating gate transistors | Magnetic tunnel junctions | Phase change material (chalcogenide glass) |
| Storage Mechanism | Trapping electric charge | Changing magnetic polarity | Changing physical state of material |
| Memory Type | Non-volatile | Non-volatile | Non-volatile |
| Read Latency | 10-50 microseconds | 2-5 nanoseconds | 10-20 nanoseconds |
| Write Speed | Fairly slow | Very fast | Faster than flash but slower than DRAM |
| Endurance | About 10k-100k cycles | Unlimited reads/writes | About 10M cycles |
| Storage Density | High | Low | Potentially high |
| Main Uses | USB drives, SSDs, memory cards | Enterprise storage, cache | 3D XPoint, persistent memory |
In summary, flash memory, MRAM, and PCM all offer significant advantages over traditional hard disk drives in areas like speed, durability, power efficiency, size, and weight. They represent major evolutions in data storage technology that will continue gaining market share in both consumer and enterprise storage applications going forward. Each type has relative strengths and weaknesses that make them suitable for different use cases. Continued improvements in cost, capacity, and capabilities are likely to expand solid state storage adoption substantially in the coming decade.
Conclusion
The three main types of solid state storage technologies are flash memory, magnetoresistive RAM, and phase change memory. Flash memory stores data in arrays of floating gate transistors and is the most common solid state storage used today, offering excellent performance for consumer and enterprise storage. MRAM uses magnetic charges to represent bits, allowing very fast writes and virtually unlimited endurance. It holds promise to become a universal memory. PCM takes advantage of physical state changes in chalcogenide glass material to enable high-speed non-volatile storage. Each technology has its strengths and best suited applications.
Flash excels at random I/O, PCM offers a balance of speed and density, and MRAM touts high endurance. All three provide durable, high-performance storage without the mechanical limitations of hard disk drives. As solid state storage technologies mature and overcome challenges around capacity, cost, and reliability, they are likely to continue gaining market share across computing systems from mobile to the cloud.
