This semiconductor has two faces and many simultaneous functions

Gallium nitride semiconductors have been a boon to high-frequency and power electronics. They have also revolutionized energy-efficient LED lighting. But no semiconductor has been able to do both efficiently at the same time.

Cornell researchers, working with a team from the Polish Academy of Sciences, have developed the first double-sided, or “dualtronic,” chip that simultaneously combines its photonic and electronic functions, an innovation that could make functional devices smaller, more energy efficient, and lower manufacturing costs.

The team’s paper, “Using Both Sides of Polar Semiconductor Wafers for Functional Devices,” was published September 25 in NatureThe co-lead authors are doctoral students Len van Deurzen and Eungkyun Kim.

The project was led by Debdeep Jena, the David E. Burr Professor of Engineering in the School of Electrical and Computer Engineering and the Department of Materials Science and Engineering, and Huili Grace Xing, the William L. Quackenbush Professor of Electrical and Computer Engineering and of Materials Science and Engineering, both at Cornell Engineering.

Gallium nitride (GaN) is unique among wide-bandgap semiconductors because it exhibits significant electronic polarization along its crystal axis, giving each of its surfaces radically different physical and chemical properties. The gallium, or cation, side has proven useful for photonic devices such as LEDs and lasers, while the nitrogen, or anion, side can accommodate transistors.

The Jena-Xing lab has set out to fabricate a working device in which a high electron mobility transistor (HEMT) on one side drives light-emitting diodes (LEDs) on the other, a feat that has not been achieved in any other material.

“To our knowledge, no one has yet made devices that are active on both sides, not even for silicon,” van Deurzen said. “One reason is that using both sides of a silicon wafer doesn’t add any additional functionality because it’s cubic; the two sides are basically the same. But gallium nitride is a polar crystal, so one side has different physical and chemical properties than the other, which gives us an extra layer of flexibility in device design.”

The project was originally conceived at Cornell by Jena and former postdoctoral researcher Henryk Turski, co-senior author of the paper, along with Jena and Xing. Turski worked with a team at the Institute of High Pressure Physics of the Polish Academy of Sciences to grow transparent GaN substrates on a single-crystal wafer about 400 microns thick.

The HEMT and LED heterostructures were then grown in Poland by molecular beam epitaxy. Once epitaxy was complete, the chip was shipped to Cornell, where Kim built and processed the HEMT on the nitrogen pole face.

“The polar side of nitrogen is more chemically reactive, which means that during device processing, the electronic channel can be damaged quite easily,” Kim said. “One of the challenges of making polar transistors with nitrogen is making sure that all the plasma processes and chemical processing don’t damage the transistors. So it took a lot of process development to make and design this transistor.”

Next, van Deurzen built the LED on the metal pole face, using a thick positive photoresist coating to protect the previously treated N pole face. After each step, the researchers measured the characteristics of their respective devices and found that they had not changed.

“It’s actually a very feasible process,” van Deurzen said. “The devices don’t degrade. And that’s obviously important if you want to use this technology as a real technology.”

Since no one had yet made a double-sided semiconductor, the team had to invent a new method to test and measure it. They assembled a “raw” glass plate covered with a double-sided coating and connected one side of the wafer to it with wire, allowing them to probe both sides from above.

Since GaN substrates are transparent across the visible range, light was able to pass through. The HEMT device successfully drove a large LED, turning it on and off at frequencies in the kilohertz range, which is more than enough for a functional LED display.

Currently, LED displays have a separate transistor and independent manufacturing processes. An immediate application of the dualtronic chip is microLEDs: fewer components, smaller footprint, less energy and materials required, and faster manufacturing at lower cost.

“The iPhone is a good analogy,” Jena said. “It’s a phone, of course, but it’s a lot more than that. It’s a calculator, it’s a map, it lets you look at the Internet. So there’s an aspect of convergence. I would say our first demonstration of dualtronics in this paper is the convergence of two or three features, but it’s actually broader than that.”

“Now you won’t need different processors to perform different functions, and you’ll reduce the power and speed lost in the interconnections between them, which require more electronics and logic. Many of these features are reduced to a single wafer with this demonstration.”

Other applications include Complementary Metal-Oxide-Semiconductor (CMOS) devices with an induced-bias n-channel transistor (which uses electrons) on one side and a p-channel transistor (containing holes) on the other.

Moreover, because GaN substrates have a high piezoelectric coefficient, they can be used as bulk acoustic wave resonators to filter and amplify radio frequency signals in 5G and 6G communications. The semiconductors could also integrate lasers instead of LEDs for “LiFi” transmissions, i.e. light-based transmissions.

“We could extend this to enable the convergence of photonic, electronic and acoustic devices,” van Deurzen said. “We are essentially limited by our imagination in terms of what we can do, and unexplored features may emerge when we try them in the future.”