Power/Performance Bits: July 11

3D chip integrates computing, storage
Researchers at Stanford University and MIT developed a prototype 3D chip that integrates computation and data storage, based on carbon nanotubes and resistive RAM (RRAM) cells.

The researchers integrated over 1 million RRAM cells and 2 million carbon nanotube FETs, making what the team says is the most complex nanoelectronic system ever made with emerging nanotechnologies.

The RRAM and carbon nanotubes are built vertically over one another, making a dense 3D computer architecture with interleaving layers of logic and memory, with ultradense wires between these layers.

The carbon nanotube circuits and RRAM memory can be fabricated at much lower temperatures, below 200 C. “This means they can be built up in layers without harming the circuits beneath,” said Max Shulaker, an assistant professor of electrical engineering and computer science at MIT. The process is also CMOS compatible.

“The devices are better: Logic made from carbon nanotubes can be an order of magnitude more energy-efficient compared to today’s logic made from silicon, and similarly, RRAM can be denser, faster, and more energy-efficient compared to DRAM,” said H.-S. Philip Wong, a professor of electrical engineering and computer science at Stanford.

“The new 3D computer architecture provides dense and fine-grained integration of computating and data storage, drastically overcoming the bottleneck from moving data between chips,” according to Subhasish Mitra, a professor of electrical engineering and computer science at Stanford. “As a result, the chip is able to store massive amounts of data and perform on-chip processing to transform a data deluge into useful information.”

To demonstrate the potential of the technology, the researchers took advantage of the ability of carbon nanotubes to also act as sensors. On the top layer of the chip they placed over 1 million carbon nanotube-based sensors, which they used to detect and classify ambient gases.

Due to the layering of sensing, data storage, and computing, the chip was able to measure each of the sensors in parallel, and then write directly into its memory, generating huge bandwidth, Shulaker says.

“It leads to a fundamentally different perspective on computing architectures, enabling an intimate interweaving of memory and logic,” said Jan Rabaey, a professor of electrical engineering and computer science at UC Berkeley, who was not involved in the research. “These structures may be particularly suited for alternative learning-based computational paradigms such as brain-inspired systems and deep neural nets, and the approach presented by the authors is definitely a great first step in that direction.”

The team is working to improve the underlying nanotechnologies, while exploring the new architecture. The group will be working with Analog Devices to develop new versions of the system that take advantage of its ability to carry out sensing and data processing on the same chip.

Battery-free cellphone
Computer scientists and electrical engineers at the University of Washington built a  capable of making calls purely on a few microwatts of power harvested from either ambient radio signals or light.

The team eliminated a power-hungry step in most modern cellular transmissions — converting analog signals that convey sound into digital data. Instead, the battery-free cellphone takes advantage of tiny vibrations in a phone’s microphone or speaker that occur when a person is talking into a phone or listening to a call.

An antenna connected to those components converts that motion into changes in standard analog radio signal emitted by a cellular base station. This process essentially encodes speech patterns in reflected radio signals in a way that uses almost no power.

To transmit speech, the phone uses vibrations from the device’s microphone to encode speech patterns in the reflected signals. To receive speech, it converts encoded radio signals into sound vibrations that that are picked up by the phone’s speaker. In the prototype device, the user presses a button to switch between these two “transmitting” and “listening” modes.

Using off-the-shelf components on a printed circuit board, the team demonstrated that the prototype can perform basic phone functions — transmitting speech and data and receiving user input via buttons. Using Skype, researchers were able to receive incoming calls, dial out and place callers on hold with the battery-free phone.

The battery-free phone developed at the UW can sense speech, actuate the earphones, and switch between uplink and downlink communications, all in real time. It is powered by either ambient radio signals or light. (Source: Mark Stone/University of Washington)

The team designed a custom base station to transmit and receive the radio signals. But that technology conceivably could be integrated into standard cellular network infrastructure or Wi-Fi routers now commonly used to make calls.

“You could imagine in the future that all cell towers or Wi-Fi routers could come with our base station technology embedded in it,” said Vamsi Talla, a former UW electrical engineering doctoral student and Allen School research associate. “And if every house has a Wi-Fi router in it, you could get battery-free cellphone coverage everywhere.”

The battery-free phone does still require a small amount of energy to perform some operations. The prototype has a power budget of 3.5 microwatts.

The researchers demonstrated how to harvest this small amount of energy from two different sources. The battery-free phone prototype can operate on power gathered from ambient radio signals transmitted by a base station up to 31 feet away.

Using power harvested from ambient light with a tiny solar cell roughly the size of a grain of rice, the device was able to communicate with a base station that was 50 feet away.

Next, the research team plans to focus on improving the battery-free phone’s operating range and encrypting conversations to make them secure. The team is also working to stream video over a battery-free cellphone and add a visual display feature to the phone using low-power E-ink screens.

Self-powered smart window
Researchers at Princeton University developed a self-powered smart window they say promises to be inexpensive and easy to apply to existing windows. The smart window controls the transmission of visible light and infrared heat into the building, a solar cell uses near-UV light to power the system.

Because near-UV light is invisible to the human eye, the researchers set out to harness it for the electrical energy needed to activate the tinting technology.

“Using near-UV light to power these windows means that the solar cells can be transparent and occupy the same footprint of the window without competing for the same spectral range or imposing aesthetic and design constraints,” said Yueh-Lin (Lynn) Loo, director of the Andlinger Center for Energy and the Environment and a professor of chemical and biological engineering at Princeton. “Typical solar cells made of silicon are black because they absorb all visible light and some infrared heat – so those would be unsuitable for this application.”

Graduate student Nicholas Davy holds a sample of the special window glass. (Source: David Kelly Crow/Princeton)

The team used organic semiconductors — contorted hexabenzocoronene (cHBC) derivatives — for constructing the solar cells. The researchers chose the material because its chemical structure could be modified to absorb a narrow range of wavelengths — in this case, near-UV light. To construct the solar cell, the semiconductor molecules are deposited as thin films on glass with the same production methods used by organic light-emitting diode manufacturers. When the solar cell is operational, sunlight excites the cHBC semiconductors to produce electricity.

The smart window consists of electrochromic polymers, which control the tint. When near-UV light from the sun generates an electrical charge in the solar cell, the charge triggers a reaction in the electrochromic window, causing it to change from clear to dark blue. When darkened, the window can block more than 80% of light.

The team’s aim is to create a flexible version of the solar-powered smart window system that can be applied to existing windows via lamination.

“Someone in their house or apartment could take these wireless smart window laminates – which could have a sticky backing that is peeled off – and install them on the interior of their windows,” said Nicholas Davy, a doctoral student in the chemical and biological engineering department at Princeton. “Then you could control the sunlight passing into your home using an app on your phone, thereby instantly improving energy efficiency, comfort, and privacy.”

The researchers have started a company to commercialize the technology, and are exploring using the near-UV solar cells to power IoT sensors and other low-power consumer products.