Research Bits: March 14

Shift register-in-memory architecture; stacking LEDs; electrochemical transistor for bioelectronics.


Shift register-in-memory architecture

Researchers at the Singapore University of Technology and Design propose a new reconfigurable shift register-in-memory architecture for devices that can work both as a reconfigurable memory component and as a programmable shift register.

The device is based on phase-change alloys, which can switch reversibly between the glassy amorphous state and the ordered crystal state. It uses four material states: the amorphous state, fully crystalline state, partially crystallized state, and primed state, which represent different shift register/ memory modes.

“When operating as a memory, the device can be switched from the disordered glass state to crystalline state with 1.9 ns pulses, which is about one-third shorter than those of existing devices with germanium antimony telluride layers doped with nitrogen; and exhibit a resetting energy of 2 pJ. When operating as a shift register, the device can be switched between the serial-in–serial-out mode to serial-in–parallel-out mode, with a single cell, and exhibit many resistance levels, which has not been shown before,” said Desmond Loke, an assistant professor at SUTD.

The researchers say the shift register-in-memory architecture could be used to design a wide range of high-performance electronic systems and be applied to a variety of operation schemes and computations.

Stacking LEDs

Sharper, defect-free displays could be possible by stacking the red, green, and blue light-emitting diodes vertically instead of laying them side by side, according to research from MIT, Georgia Tech Europe, Sejong University, and other universities. Each stacked pixel measures about 4 microns wide, and the microLEDs can be packed to a density of 5,000 pixels per inch, which the researchers claim is the smallest pixel and highest density reported.

By altering the voltage applied to each of the pixel’s red, green, and blue membranes, they could produce various colors in a single pixel. “If you have a higher current to red, and weaker to blue, the pixel would appear pink, and so on,” said Jiho Shin, a postdoc at MIT. “We’re able to create all the mixed colors, and our display can cover close to the commercial color space that’s available.”

The team utilized a previously developed method to grow and peel away perfect, two-dimensional, single-crystalline material from wafers of silicon and other surfaces — an approach they call 2D material-based layer transfer, or 2DLT. This approach was used to grow ultrathin membranes of red, green, and blue LEDs. They then peeled the entire LED membranes away from their base wafers and stacked them together to make a layer cake of red, green, and blue membranes. They could then carve the cake into patterns of tiny, vertical pixels, each as small as 4 microns wide.

“In conventional displays, each R, G, and B pixel is arranged laterally, which limits how small you can create each pixel,” Shin said. “Because we are stacking all three pixels vertically, in theory we could reduce the pixel area by a third.”

As a demonstration, the team fabricated a vertical LED pixel, and showed that by altering the voltage applied to each of the pixel’s red, green, and blue membranes, they could produce various colors in a single pixel.

The team plans to improve the operation of the vertical pixels. So far, they have shown they can stimulate an individual structure to produce the full spectrum of colors. They will work toward making an array of many vertical micro-LED pixels.

“You need a system to control 25 million LEDs separately,” Shin said. “Here, we’ve only partially demonstrated that. The active matrix operation is something we’ll need to further develop.”

Electrochemical transistor for bioelectronics

A new type of transistor suited for use in bioelectronics has been developed by researchers at Northwestern University.

The electrochemical transistor is based on an electronic polymer and a vertical architecture. It conducts both electricity and ions and is stable in air. It is also compatible with blood and water and can amplify important signals, making it especially useful for biomedical sensing.

“This exciting new type of transistor allows us to speak the language of both biological systems, which often communicate via ionic signaling, and electronic systems, which communicate with electrons,” Jonathan Rivnay, professor of biomedical engineering at Northwestern. “The ability of the transistors to work very efficiently as ‘mixed conductors’ makes them attractive for bioelectronic diagnostics and therapies.”

The team said it could enable onsite signal processing in wearable devices, right at the biology-device interface. Potential applications include measuring heartbeat and levels of sodium and potassium in blood as well as eye motion for studying sleep disorders.

“All modern electronics use transistors, which rapidly turn current on and off,” said Tobin J. Marks, professor of catalytic chemistry, materials science, and chemical and biological engineering at Northwestern. “Here we use chemistry to enhance the switching. Our electrochemical transistor takes performance to a totally new level. You have all the properties of a conventional transistor but far higher transconductance (a measure of the amplification it can deliver), ultra-stable cycling of the switching properties, a small footprint that can enable high density integration, and easy, low-cost fabrication.”

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