Today’s systems integrate processors, graphics, as well as memory and storage, often referred to as the memory/storage hierarchy. In the first tier of today’s hierarchy, SRAM is integrated into the processor for fast data access. DRAM, the next tier, is separate and used for main memory. Disk drives and NAND-based solid-state storage drives (SSDs) are used for storage.
DRAM and NAND are struggling to keep up with the bandwidth and/or power requirements in systems. DRAM is cheap, but it consumes power. DRAM is also volatile, meaning it loses data when the power is shut off in systems. NAND, meanwhile, is cheap and non-volatile—it retains data when the system is shut down. But NAND and disk drives are slow.
So for years, the industry has been searching for a “universal memory” that has the same attributes as DRAM and flash and could replace them. The contenders are MRAM, PCM and ReRAM.
Today, several next-generation memories, such as MRAM, phase-change memory (PCM) and ReRAM, are shipping to one degree or another. Some of the next new memories are extensions of these technologies. Others are based on entirely new technologies or involve architectural changes, such as near- or in-memory computing, which bring the processing tasks near or inside of memory.
Among the next new memory types are:
- FeFET or FeRAM: A next-generation ferroelectric memory.
- Nanotube RAM: In R&D for years, nanotube RAM is targeted to displace DRAM. Others are developing carbon nanotubes and next-generation memories on the same device.
- Phase-change memory: After shipping the first PCM devices, Intel is readying a new version. Others may enter the PCM market.
- ReRAM: Future versions are positioned for AI apps.
- Spin-orbit torque MRAM (SOT-MRAM): A next-generation MRAM targeted to replace SRAM.
There are additional efforts pushing in the vertical direction. For example, some are developing 3D SRAM, which stacks SRAM on logic as a potential replacement for planar SRAM.
Replacing SRAM
But in R&D, the industry is working on several new technologies, including a potential SRAM replacement. Generally, processors integrate a CPU, SRAM and a variety of other functions. SRAM stores instructions that are rapidly needed by the processor. This is called Level 1 cache memory. In operation, the processor will ask for instructions from L1 cache, but the CPU will sometimes miss them. So processors also integrate second- and third-level cache memory, called Level 2 and 3 cache.
SRAM-based L1 cache is fast. Latencies are less than a nanosecond. But SRAM also occupies too much space on the chip.
For years, the industry has been looking to replace SRAM. There have been several possible contenders over the years. One of those includes spin-transfer torque MRAM (STT-MRAM). STT-MRAM features the speed of SRAM and the non-volatility of flash with unlimited endurance.
DRAM contenders
Like SRAM, the industry for years has been trying to replace DRAM. In today’s compute architectures, data moves between a processor and DRAM. But at times this exchange causes latency and increased power consumption, which is sometimes called the memory wall.
DRAM has fallen behind in bandwidth requirements. Plus, DRAM scaling is slowing at today’s 1xnm node.
One new memory type is gaining steam, namely 3D XPoint. Introduced by Intel in 2015, 3D XPoint is based on a technology called PCM.Used in SSDs and DIMMs, PCM stores information in the amorphous and crystalline phases.
While PCM is ramping up, other technologies such as ferroelectric FETs (FeFETs) are still in R&D.
AI memory
Crossbar’s ReRAM technology also is targeted for machine learning. Machine learning involves a neural network. In neural networks, a system crunches data and identifies patterns. It matches certain patterns and learns which of those attributes are important.
Neuromorphic computing also uses a neural network. For this, advanced ReRAM is attempting to replicate the brain in silicon. The goal is to mimic the way that information is moving in the device using precisely-timed pulses, and there is much research underway in this area, particularly on the materials front.
