Could Flexible Power Electronics Soon Become Universally Utilized?

Recent years have seen advancements in flexible devices for applications in wearables, large-scale electronics, and the Internet of Things. These applications involve energy devices such as batteries, sensors, and other semiconductor devices such as diodes and transistors. There is an increasing demand for new materials and fabrication techniques that allow for high-performance electronic components to be implemented directly on flexible substrates. Due to this increasing interest in new materials and processes, in addition to flexibility, research engineers have achieved stretchability, biodegradability, and biocompatibility.

Recent advancements in flexible electronics have paved the road for new applications such as flexible lighting and display technologies, wearables that monitor health and daily activities, implantable electronics for medical imaging and diagnostics, bioinspired soft robots, and energy harvesting devices. However, the flexible electronics developed until now are intended for usage in standalone solutions where high computation power is not required.

To achieve high computational capability, there is a need for implementing high-performance transistors on flexible substrates that allow for high device density. Apart from computational power, power electronic circuits use transistors to perform switching and control functions. Power electronics for wearable devices, for example, manage a few volts and a few milliamps, operating at frequencies ranging from a few hundred kHz to a few MHz. Therefore, high-performance transistors with high current handling capacity are desired for such applications. These devices, moreover, have to be highly efficient to minimize heat generation.

The ultrathin, flexible circuits capable of performing advanced computational functions and high power switching have been an engineering goal for many years. However, various technical difficulties have throttled back the miniaturization necessary for achieving such high performance. To overcome these difficulties, researchers at Stanford University have invented a manufacturing technique for implementing thin transistors of less than 100 nanometers in length on flexible substrates.

Currently, organic semiconductors and amorphous silicon are the base materials for flexible and stretchable electronic devices. But two-dimensional (2D) semiconductors are a promising candidate due to their excellent mechanical and electrical properties, even at the nanoscale. These 2D semiconductors, thus, allow for greater miniaturization, making them better candidates than the aforementioned conventional materials.

A 2D semiconductor is a type of natural semiconductor with the thickness on the atomic scale. With 3D semiconductors, the miniaturization of transistors is limited by the rate at which heat generated from static power is dissipated. In transistors with 2D semiconductors, leakage current is almost eliminated as all electrons are confined in atomically thin channels uniformly influenced by the gate voltage.

The challenge currently is that forming such atomically thin devices requires a heat-intensive process, where the materials would melt and decompose in the production process.

Eric Pop, a professor of electrical engineering at Stanford, and Alwin Daus, a postdoctoral scholar in Pop's lab, have developed a technique that involves forming an atomically thin film of 2D semiconductor molybdenum disulfide (MOS2) with nano-patterned gold electrodes. This step is performed on the conventional silicon substrate to get nanoscale dimensions with existing patterning techniques. After patterning on rigid silicon and allowing them to cool, researchers apply flexible material with a simple bath in deionized water. The entire device stack can then be fully transferred onto flexible polyimide.

Illustration of transfer process for 2D semiconductor with nano-patterned contacts (left) and photograph of flexible transparent substrate with transferred structures (right). Image Courtesy of Stanford University.

 

After a few additional fabrication steps, the results are flexible high-performance transistors. According to the researchers, the transistors can handle high electrical currents while operating at low voltages, consuming less power. Moreover, the gold metal contacts spread the heat generated by the transistors in operation, which can otherwise damage the flexible polyimide.

"In the end, the entire structure is just 5 microns thick, including the flexible polyimide," said Pop, who is the senior author of the paper. "That's about ten times thinner than a human hair."

"This downscaling has several benefits," said Daus, who is the first author of the paper. "You can fit more transistors in a given footprint, of course, but you can also have higher currents at lower voltage – high speed with less power consumption."

The researchers are now looking to refine these devices. They have built similar transistors with other 2D semiconductors: MoSe2 and WSe2, demonstrating the broad applicability of their technique. Moreover, they are planning to integrate radio circuitry within the devices to enable wireless communication.

"This is more than a promising production technique. We've achieved flexibility, density, high performance, and low power – all at the same time," Pop said. "This work will hopefully move the technology forward on several levels."

Flexible electronics presents a huge opportunity for scientific research and development to rapidly and considerably advance this area. Contributing to this development, Stanford University has an initiative on Flexible Electronics Translational Research or FlexTR. FlexTR is supported by the Beijing Institute of Collaborative Innovation (BICI) and directed by Professor Zhenan Bao, Department of Chemical Engineering at Stanford University.

 

About the Researchers

Alwin Daus is the first author of the paper, and Eric Pop is the senior author. Co-authors include postdoctoral scholars Sam Vaziri and Kevin Brenner, doctoral candidates Victoria Chen, Çağıl Köroğlu, Ryan Grady, Connor Bailey and Kirstin Schauble, and research scientist Hye Ryoung Lee.