Repurposing memory for ultrafast switches

Novel switches that can effectively turn on and off electromagnetic signals of frequencies up to 260 GHz using a device known as a memristor have been developed by an international team^[1].

Switches are the most important part of an electrical circuit. Transistors — which are effectively miniaturized electrically operated switches — are crucial for radiofrequency communication applications such as smartphones, Wi-Fi, Bluetooth and satellite communications. While communication devices typically work at a few to tens of gigahertz (GHz, or billion cycles per second), future sixth generation (6G) systems will require hundreds of gigahertz. Transistors can struggle to provide effective switching between on and off conditions with such high-frequency signals.

The switches have been developed by KAUST scientists Sebastian Pazos, Mario Lanza and their colleagues, along with international collaborators from Ireland (Tyndall National Institute), Spain (Universidad Autónoma de Barcelona), and the United States (University of Texas at Austin).

“A memristor is a two-terminal electronic device that can change its electrical resistance when subjected to a tailored electrical stress,” says Pazos. “A memristor acts like a switch as it toggles between a high-resistance ‘off’ state that blocks the current flow and a low-resistance ‘on’ state that allows current to flow.”

Memristors are useful as radiofrequency switches due to their ultralow power consumption. Conventional RF switches employ semiconductor devices (transistors or diodes) that may require several tens of milliamperes to maintain their state, while memristors are nonvolatile: they “remember” their resistance when the control signal is switched off.

Lanza’s team built their memristor from a 2D layered material called hexagonal boron nitride between two gold electrodes. “Memristors made of single-layer 2D materials have previously been explored for use as RF switches, but they had limited endurance and the device-to-device and cycle-to-cycle variability was severe,” explains Pazos, the leading researcher of this study. “We thought of using multilayered hexagonal boron nitride, instead of monolayers, in an attempt to solve these issues.”

Key to the operation of their radiofrequency switches was the ability to create and control one or more conductive filaments through the multilayer hexagonal boron nitride by application of voltage/current pulses. In this way, their switches exhibited an “off” state with a resistance as low as 7 Ohms and blocked about 99% of the signal.

The losses when transmitting the radiofrequency signal in the “on” state can be as low as 5% at frequencies of 100 GHz. And importantly, while previous monolayer hexagonal boron nitride devices lasted for only 30 cycles between high- and low-resistivity states, the devices from Pazos et al. were sufficiently robust to last 2,000 cycles.

In addition, these switches were combined for the first time into circuits featuring multiple memristors, which improved their signal blocking capabilities well above 200 GHz. “In our group, we have already shown that hexagonal boron nitride memristors can be integrated with CMOS circuits, and we now want to take this further into the high frequency realm with devices such as configurable high frequency communication circuits and on-chip antenna arrays,” says Pazos.