Power/Performance Bits: May 22

Sensing without battery power
Engineers at the National University of Singapore developed an IoT-focused sensor chip that can continue operating when its battery runs out of energy. The chip, BATLESS, uses a power management technique that allows it to self-start and continue to function under dim light without any battery assistance.

The chip can operate in two different modes: minimum-energy and minimum-power. When the battery energy is available, the chip runs in minimum-energy mode to maximize the battery lifetime. However, when the battery is exhausted, the chip switches to the minimum-power mode and operates with a power consumption of about half a nanoWatt.

Power can be provided by a very small on-chip solar cell that is about half a square millimeter in area, or other forms of energy available from the environment, such as vibration or heat.

The team says the use of the two modes allows for a battery just a few millimeters in size, rather than a few centimeters. The device can uninterruptedly sense, process, capture and timestamp events of interest regardless of battery state, then wirelessly transmit the data to the cloud when the battery becomes available again.

Despite being in minimum-power mode when battery is not available, the reduced speed of the microchip is still adequate for IoT applications that need to sense parameters that vary slowly in time, including temperature, humidity, light, and pressure.

The BATLESS chip. (Source: National University of Singapore)

According to Massimo Alioto, an associate professor at NUS, “BATLESS is the first example of a new class of chips that are indifferent to battery charge availability. In minimum-power mode, it uses 1,000 to 100,000 times less power, compared to the best existing microcontrollers designed for fixed minimum-energy operation. At the same time, our 16-bit microcontroller can also operate 100,000 times faster than others that have been recently designed for fixed minimum-power operation. In short, the BATLESS microchip covers a very wide range of possible energy, power, and speed trade-offs, as allowed by the flexibility offered through the two different modes.”

The device is indifferent to battery availability and can self-start while being powered directly by the tiny on-chip solar cell, with no battery assistance. The team had demonstrated this at 50-lux indoor light intensity, which is equivalent to the dim light available at twilight, and corresponds to nanoWatts of power.

The team plans further work on battery indifferent systems that cover the entire signal chain from sensor to wireless communications, with the long-term goal of completely eliminating the need for batteries in such devices.

1D interconnects
Engineers at the University of California, Riverside, developed one dimensional interconnects made from zirconium tritelluride, or ZrTe₃, nanoribbons that can conduct a current density 50 times greater than conventional copper interconnect technology.

“Conventional metals are polycrystalline. They have grain boundaries and surface roughness, which scatter electrons,” said Alexander A. Balandin, a distinguished professor of electrical and computer engineering at UC Riverside. “Quasi-one-dimensional materials such as ZrTe3 consist of single-crystal atomic chains in one direction. They do not have grain boundaries and often have atomically smooth surfaces after exfoliation. We attributed the exceptionally high current density in ZrTe3 to the single-crystal nature of quasi-1D materials.”

In principle, such quasi-1D materials could be grown directly into nanowires with a cross-section that corresponds to an individual atomic thread, or chain. In the present study the level of the current sustained by the ZrTe₃ quantum wires was higher than reported for any metals or other 1D materials, the team reported. It almost reaches the current density in carbon nanotubes and graphene.

Microscopy image of an electronic device made with 1D ZrTe3 nanoribbons. The nanoribbon channel is indicated in green color. The metal contacts are shown in yellow color. Owing to the nanometer scale thickness the yellow metal contacts appear to be under the green channel while in reality they are on top. (Source: Balandin lab, UC Riverside)

Depending on how they are configured, the ZrTe3 nanoribbons could be made into either nanometer-scale local interconnects or device channels.

“The most exciting thing about the quasi-1D materials is that they can be truly synthesized into the channels or interconnects with the ultimately small cross-section of one atomic thread– approximately one nanometer by one nanometer,” Balandin said.

While the group’s experiments were conducted with nanoribbons that had been sliced from a pre-made sheet of material, industrial applications would need to grow nanoribbon directly on the wafer. This manufacturing process is already under development, and Balandin believes 1D nanomaterials hold possibilities for applications in future electronics.

Chromium memory
Researchers from the University of Washington, the University of Hong Kong, Carnegie Mellon University, the National Institute for Materials Science in Tsukuba, and Oak Ridge National Laboratory are working on a new atomically thin memory device using stacked sheets of 2-D magnetic insulator chromium tri-iodide (CrI₃) to control the flow of electrons based on the direction of their spins.

The researchers sandwiched two layers of CrI₃ between conducting sheets of graphene. They showed that, depending on how the spins are aligned between each of the CrI₃ sheets, the electrons can either flow unimpeded between the two graphene sheets or were largely blocked from flowing. These two different configurations could act as the bits to encode information.

“The functional units of this type of memory are magnetic tunnel junctions, or MTJ, which are magnetic ‘gates’ that can suppress or let through electrical current depending on how the spins align in the junction,” said Xinghan Cai, a UW postdoctoral researcher in physics. “Such a gate is central to realizing this type of small-scale data storage.”

In the experiment, the researchers sandwiched two atomic layers of CrI3 between graphene contacts and measured the electron flow through the CrI3. (Source: Tiancheng Song)

With up to four layers of CrI₃, the team discovered the potential for “multi-bit” information storage. In two layers of CrI₃, the spins between each layer are either aligned in the same direction or opposite directions, leading to two different rates that the electrons can flow through the magnetic gate. But with three and four layers, there are more combinations for spins between each layer, leading to multiple, distinct rates at which the electrons can flow through the magnetic material from one graphene sheet to the other.

The current device is infeasible for commercial use, as it requires modest magnetic fields and is only functional at low temperature, said Xiaodong Xu, a UW professor of physics and of materials science and engineering. “We hope that with developed electrical control of magnetism and some ingenuity, these tunnel junctions can operate with reduced or even without the need for a magnetic field at high temperature, which could be a game changer for new memory technology.”