New DRAM can be layered on top of CPU cores

At the last IEDM (IEEE International ElectronDevices Meenting), there were two sessions devoted to a new type of DRAM cell that consists or 2 transistors and no capacitors (2TOC) that can be built in layers on top of a micro processor which doesn’t disturb the microprocessor silicon. I couldn’t access (behind paywalls) the actual research but one of the research groups was from Belgium (IMEC) and the other from the US (Notre Dame and R.I.T). This was written up in a couple of teaser articles in the tech press (see IEEE Spectrum tech talk article).

DRAM today is built using 1 transistor and 1 capacitor (1T1C). And it appears that capacitors and logic used for microprocessors aren’t very compatible. As such, most DRAM lives outside the CPU (or microprocessor cores) chip and is attached over a memory bus.

New 2T0C DRAM Bit Cell: Data is written by appliying current to the WBL and WWL and bit’s are read by seeing if acurrent can pass through the RWL RBL

Memory busses have gotten faster in order to allow faster access to DRAM but this to is starting to reach fundamental physical limits and DRAM memory sizes aren’t scaling like the used to.

Wouldn’t it be nice if there were a new type of DRAM that could be easlly built closer or even layered on top of a CPU chip, with faster direct access from/to CPU cores. through inter chip electronics.

Oxide based 2T0C DRAM

DRAM was designed from the start with 1T1C so that it could hold a charge. With a charge in place it could be read out quickly and refreshed periodically without much of a problem.

The researcher found that at certain sizes (and with proper dopants) small transistors can also hold a (small) charge without needing any capacitor.

By optimizing the chemistry used to produce those transistors they were able to make 2T0C transistors hold memory values. And given the fabrication ease of these new transistors, they can easily be built on top of CPU cores, at a low enough temperature so as not to disturb the CPU core logic.

But, given these characteristics the new 2T0C DRAMB can also be built up in layers. Just like 3D NAND and unlike current DRAM technologies.

Today 3D NAND is being built at over 64 layers, with Flash NAND roadmap’s showing double or quadruple that number of layers on the horizon. Researchers presenting at IMEC were able to fabricate an 8 layer 2T0C DRAM on top of a microprocessor and provide direct, lightening fast access to it.

The other thing about the new DRAM technology is that it doesn’t need to be refreshed as often. Current DRAM must be refreshed every 64 msec. This new 2T0C technology has a much longer retention time and currently only needs to be refreshed every 400s and much longer retention times are technically feasible.

Some examples of processing needing more memory:

  • AI/ML and the memory wall -Deep learning models are getting so big that memory size is starting to become a limiting factor in AI model effectiveness. And this is just with DRAM today. Optane and other SCM can start to address some of this problem but ithe problem doesn’t go away, AI DL models are just getting more complex I recently read an article where Google trained a trillion parameter language model.
  • In memory databases – SAP HANA is just one example but they are other startups as well as traditional database providers that are starting to use huge amounts of memory to process data at lightening fast speeds. Data only seems to grow not shrink.

Yes Optane and other SCM today can solve some of thise problems. But having a 3D scaleable DRAM memory, that can be built right on chip core, with longer hold times and faster direct access can be a game changer.

It’s unclear whether we will see all DRAM move to the new 2T0C format, but if it can scale well in the Z direction has better access times, and longer retention, it’s unclear why this wouldn’t displace all current 1T1C DRAM over time. However, given the $Bs of R&D spend on new and current DRAM 1T1C fabrication technology, it’s going to be a tough and long battle.

Now if the new 2T0C DRAM could only move from 1 bit per cell to multiple bits per cell, like SLC to MLC NAND, the battle would heat up considerably.

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