This post addresses the basics of resistive random access memory (ReRAM) structures, as well as the test hardware available to characterize them, using examples from Keithley Instruments' portfolio.
For many portable electronics, floating-gate NAND flash memory has long been the non-volatile memory (NVM) technology of choice. Flash cells are implemented on the foundation of MOSFET transistors, so they have standard source, gate, drain, and bulk/substrate connections. Fowler-Nordheim current tunneling through gate oxide and Hot Carrier Injection represent the two standard methods for storing and removing charge from the floating gate. These methods are degradation mechanisms of standard (non-NVM) MOSFET transistors, which are also responsible for the finite number of write/erase cycles (endurance) of flash memory.
Manufacturers of consumer products that incorporate memory devices are increasingly concerned that floating-gate flash memory won’t be able to continue providing higher storage capacities at the ever lower cost-per-bit requirements that drive the NVM market. Electrical characterization is key to making the transition from materials research to a commercial product. Resistive random-access-memory (ReRAM or simply RRAM) offers the potential to be an alternative to floating-gate flash technology and it’s now inching closer to commercial production. In fact, one manufacturer, Crossbar Inc., has already developed a high-density, high-speed, filament-based ReRAM memory structure.
A typical ReRAM cell has a switching material with different resistance characteristics sandwiched by two metallic electrodes. The switching effect of ReRAM is based on the motion of ions under the influence of an electric field or heat and also the switching material’s ability to store the state of the ion distribution. This in turn causes a measurable change in the device resistance.
ReRAM is faster and requires lower voltage than traditional flash memory. It is bit-alterable, making it suitable for use in both embedded and solid-state drive (SSD) applications. The ReRAM cell structure offers high area efficiency and scalability. It also offers the potential for 3D integration. ReRAM requires lower programming currents than phase-change memory (PC RAM) or magneto-resistive memory (MRAM), with comparable retention and endurance.
Table 1 summarizes the important test parameters for characterizing ReRAM devices.