Fastest-ever flexible diode provides 'last missing piece' needed to realize bendable phones

flexible diode
(Left) Photograph of the flexible IGZO Schottky diodes. (Right) The highest frequency achieved by the flexible diodes is 6.3 GHz, which can accommodate all transmission frequencies of wireless communication. Credit: Zhang, et al. ©2015 Macmillan Publishers Limited

(—While there are hints that Samsung and LG are developing flexible phones that can fold, roll up, and even be stretched into larger screens, there are still some obstacles to overcome before such bendable phones become a reality. Arguably the largest obstacle is the need for a high-speed flexible diode, which is what detects and modulates the cell phone's signal. The diode must operate at high speeds in order to match the transmission frequencies used by wireless cellular communication, Bluetooth, Wi-Fi, and GPS signals (which range from 935 MHz to 5 GHz).

In a new paper published in Nature Communications, a team of researchers led by Professor Aimin Song at the University of Manchester in the UK with collaborators at Shandong University in China have developed a flexible Schottky that achieves a of 6.3 GHz, which to the best of their knowledge makes the device the fastest of its kind to date. It is also the first flexible diode to reach what is widely considered the "benchmark speed" of 2.45 GHz, which covers the principal frequency bands used in most current wireless communications, with the exception of 4G and the newest 5G Wi-Fi channels.

"A flexible GHz diode is the last missing jigsaw piece needed to achieve flexible mobile phones and rapidly growing wearable electronics," Song told "Thin-film IGZO transistors have already been demonstrated that operate at frequencies beyond 100 MHz, which is enough to construct chips for data processing in at least entry-level smart phones. Now having a fast-enough diode for front-end signal receiving and modulation shall bring the flexible mobile phones that were envisaged many years ago much closer to the market."

In the new study, the researchers fabricated the diode using the flexible semiconductor film indium-gallium-zinc-oxide (IGZO). They demonstrated the high-speed operation using low-temperature processing on , which has not been achieved before now. The previous best IGZO Schottky diodes could achieve speeds of up to 3 GHz, but could not be made on flexible plastic substrates because they used high-temperature processing, and so could only be fabricated on glass substrates. The previous best flexible Schottky diodes, which were made of crystalline silicon microparticles, obtained speeds of up to 1.6 GHz but still required complex and expensive processing.

One of the keys to achieving the high-speed flexible diode was controlling the IGZO thickness to optimize performance. As the researchers explained, it is very surprising that this could be done at all, as it has been thought that a thick IGZO layer is essential for high-speed operation due to having a low capacitance. Here the researchers found that, in fact, the series resistance increases as IGZO thickness increases, which plays a critical role in determining the diode speed. While the researchers initially expected that the thickest IGZO layer would have the best performance, they found that it actually exhibits the worst performance. By investigating the factors underlying this surprising finding—which include the relationships between resistance, reactance, and capacitance—the researchers could optimize the diode's performance by using an 80-nm-thick IGZO layer.

With a better understanding of the IGZO semiconductor material's electronic properties, the researchers were able to create a promising component of next-generation phone technology. They expect that post-treatment processes could further improve the diode's performance—both its speed and its output voltage, which is necessary to maintain a strong signal.

"Our future research will focus on integration of the fast diodes and transistors for a range of demo circuits, or even simple systems, in order to demonstrate the amazing potential of flexible thin-film electronics," Song said.

Explore further

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More information: Jiawei Zhang, et al. "Flexible indium-gallium-zinc-oxide Schottky diode operating beyond 2.45 GHz." Nature Communications. DOI: 10.1038/ncomms8561
Journal information: Nature Communications

© 2015

Citation: Fastest-ever flexible diode provides 'last missing piece' needed to realize bendable phones (2015, July 15) retrieved 18 August 2019 from
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Jul 15, 2015
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Jul 15, 2015
The electronics needed for the display are the only ones that need to be flexible, the rest can be rigid since they are small enough.

You forget that the small rigid chips are extremely fragile and need a rigid packaging to protect and interface them with wires. A simple example on a larger scale is jiggling a wire that is soldered onto a PCB. The wire is flexible, but the connection with the board is not and will eventually fail. Same thing applies to the bonding wires of the microchip.

The rigid packaging in the flexible phone will simply shift the problem onwards. Stress failures will develop at the rigid-flexible interface because the packaging doesn't deform with the rest of the structure.

Additionally, an antenna design that does not lose performance when deformed needs to be designed.

That depends entirely on how much the antenna has to deform. Does it need to crumple up, or just bend a little?

Jul 15, 2015
Is this a solution looking for a problem? Why do we need flexible phones?

Jul 16, 2015
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Jul 17, 2015
that the small rigid chips are extremely fragile
Well, this is just the point. These chips are microprocessors: they will always have larger size, than the detecting Schottky diode at the input.

The diode is a part of an entire microchip that does the recieving, filtering and pre-processing of the signal. Ultimately, you'd want it all to be a single SoC.

Is this a solution looking for a problem? Why do we need flexible phones?

So your next iPhone wouldn't break when you put it in your pocket and sit down.

On other electronics as well; part of the reason for the failed graphics chips in Apple's plastic macbooks was because the unibody designs just wasn't rigid enough, so the circuit boards inside bent and the chips on them didn't.

Jul 18, 2015
I'd prefer real solutions to real world problems not this circular self impressing monkey talk shit.

Does this make the world a better place or is just more 'shiny crow charmed/wowed by worthless baubles?

The sheer triviality of this crap, is staggering. Billions spent on jerking off the monkey parts of the avatar.

Jul 18, 2015
. A normal metal connection would eventually fail. I am thinking an interconnect technology based on liquid metals might just do the trick.

You also have to think about how to connect the substrate to the chip packaging without a chance of separation later on, because if the substrate starts to peel off, it will form cavities where the liquid metal can leak.

The problem is a difficult one, because stretching or bending the substrate will put pressures on the liquid metal and squirt or suck it along any cracks that form at the interface between the substrate and the chip packaging, and the interface between the substrate and the embedded wires.

The liquid may travel up the wire cavity and squirt out of the device entirely, because adhesion is easily defeated by peeling forces.

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