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CeRAM gains ARM's attention

Posted: 20 Feb 2014 ?? ?Print Version ?Bookmark and Share

Keywords:nonvolatile memory? semiconductor? correlated electron random access memory? CeRAM? metal-to-insulator transition?

We have recently witnessed many types of memory devices and materials advance in the quest to provide a nonvolatile memory that will scale lithographically and eventually outperform and replace flash memory (today's silicon-based NV memory workhorse) and unify the rest of the semiconductor memory device spectrum.

New to the scene is correlated electron random access memory (CeRAM). Proponents say it offers a number of attractive characteristics: thin film, fast bulk switching, no need for forming, stability over a wide temperature range, low-power and low-voltage operation, and scalability. All these would appear to make it a worthy candidate as a next-generation memory. CeRAM is the product of work by a team at the University of Colorado, Colorado Springs, under the direction of Dr. Carlos Paz de Araujo. They are responsible for the material research and development of this new type of memory, and they have verified its principles of operation and feasibility.

In 1986, Paz de Araujo and Dr. Larry D. McMillan founded the privately held Symetrix Corp. to conduct advanced research and development in the global semiconductor chip industry. Symetrix recently announced that CeRAM has clearly attracted the attention of ARM and a number of other important companies. On Feb. 4, Symetrix announced:

ARM is evaluating CeRAM technology as part of its strategy in embedded nonvolatile memory offerings and their discussions with Symetrix started over three months ago. Symetrix will provide its technology and the results from Symetrix programs ongoing at the University of Texas (Dallas) and the University of Colorado (Colorado Springs) to chip foundries engaged by ARM. Other chip companies are also working with Symetrix under similar terms.

Figure 1: An I-V curve showing electrical characteristics for CeRAM memory.

What is CeRAM?
CeRAM is based on a transition metal oxide, in this case nickel oxide (NiO). The premise is that, by cleaning up NiO through a suitable doping technique, it is possible to obtain electrically conducting NiO that can make very rapid, reversible, nonvolatile bulk transitions between its electrically insulating and conducting states. In the past, these transitions were possible only at a high pressure and temperature, but they now can be achieved at room temperature with low switching voltages and currents. Key to the operation is a reversible metal-to-insulator transition (MIT) that has its roots in the work of Sir Nevill Mott and John Hubbard.

Interpreting the mechanism responsible for the observed electrical memory characteristics requires casting off many of the fundamentals that underpin the silicon-based solid state electronics industry. In the name of the new memory, "correlated" is the single word that describes the difference between this new type of solid state electronics and conventional single-crystal silicon-based electronics. In the latter, electrons are considered uncorrelated. (In simple terms, the difference is about the interaction of electrons with one another, which requires casting off the reliance on carrier transport considerations based on structural periodicity.) Proponents of the new memory say that, though an understanding of conventional electronics relies on a rigid density of states, meaning the act of doping does not affect the density of states of the solvent (i.e., silicon in today's electronics), for the CeRAM, the density of states is not rigid. Instead, its manipulation is key to the two resistance states that are the basis of this new approach.

One unique feature of CeRAM operation is its single-site oxidation and reduction (meaning the loss and gain of an electron). For the active material of the CeRAM, the oxidation and reduction occurs at the same nickel-ion site by means of quantum tunnelling effects. From that point forward, the explanation of what is happening gets extremely complex and relies on effects that will not be familiar ground to those used to dealing with single-crystal electronics.

CeRAM: A two-terminal view
It will be essential for potential users of the CeRAM memory to understand the electrical characteristics, specifically current as a function of voltage. See the I-V curve illustrated in figure 1.

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