Improving the performance of high-power electronics

The silicon that forms the foundation of most computer chips has fundamental limits to how much power it can manage, which constrains the speed and energy-efficiency of wireless communication systems.

A promising solution is to build future wireless electronics out of transistors made from gallium nitride, an advanced material that can handle the speed and energy required for demanding wireless applications like 6G and satellite communications. 

But even in the best transistors, a very large fraction of that energy becomes heat. As researchers pack more gallium nitride transistors into a smaller area on a silicon chip, localized hot spots degrade reliability and hamper performance.

Now, a team from MIT and elsewhere has broken through this bottleneck by embedding gallium nitride transistors into an ultrathin layer of diamond. The diamond acts as a heat spreader that normalizes the temperature and allows the transistors to approach peak performance without reducing reliability.

The researchers used this technique to manufacture a power amplifier for wireless communications, which outperformed every similar amplifier they found in the literature. 

While their fabrication technique is extremely precise and requires the integration of different material systems, it can be performed at the scale needed for commercial applications.

“No single material can do everything well in a wireless device, so these 3D heterogeneously integrated systems are here to stay. The key challenge left has been reliability and thermal management, and we might have now unlocked the final step we need to make these systems operate at scale and high volume,” says Pradyot Yadav, an electrical engineering and computer science (EECS) graduate student at MIT and lead author of a paper on this advance.

Yadav is joined on the paper by Tomás Palacios, the Clarence J. LeBel Professor of EECS, director of the Microsystems Technology Laboratories (MTL), and the MIT Institute for Soldier Nanotechnology; and Ruonan Han, a professor in EECS and a member of MTL and the Research Laboratory of Electronics; as well as others at Georgia Tech and Penn State University. The research was presented at the Radio Frequency Integrated Circuits Symposium, part of the IEEE International Microwave Symposium.

A multimaterial method

To build faster and more energy-efficient electronics, researchers are studying heterogeneously integrated systems in which multiple materials are stacked into a unified package to leverage the beneficial properties of each one. 

For instance, MIT researchers previously stacked gallium nitride (GaN) on top of silicon as well as on top of glass to create higher-performance chips.

But in a heterogeneously integrated chip, each material has a different operating temperature, which can degrade the reliability of an electronic device. 

“If we can incorporate a material that manages the heat so the GaN and silicon are at the same temperature, then the reliability of the entire 3D chip will improve. The best material for that is diamond,” Yadav explains.

The researchers use lab-grown, jewelry-grade diamond, the same type one would find in some engagement rings. Diamond has the highest thermal conductivity of any known material. 

Advances in the growth process have significantly reduced the cost of single-crystal diamond wafers, making their use in computer chips more feasible.

In prior work, scientists have grown ultrathin, single-crystal layers of diamond on top of GaN transistors to manage heat. 

But this growth process, which is not easy to scale up, introduces unwanted capacitances in the chip. These store energy flowing through the circuit, diverting it from the transistors and slowing down their operations. 

The MIT researchers developed a completely different approach that reduces these unwanted capacitive effects. They embedded extremely tiny GaN transistors, known as dielets, into an ultrathin interposer, or substrate, made of single-crystal diamond. This diamond layer spreads and manages the heat, so the GaN and silicon operate at the same temperature without the unwanted capacitances.

“By putting these GaN transistors into a diamond interposer, we are actually able to improve the performance of the device, as opposed to degrading it. We can get the best of both worlds,” Yadav says.

Meticulous manufacturing

The fabrication process begins with the use of a lightning-fast femtosecond laser to cut prepared gallium nitride dielets out of a wafer. 

The researchers use the laser to drill precisely sized cavities into the diamond substrate. They carefully place a die attach film, which is only 20 microns thick, at the bottom of the cavity and drop a dielet on top of the film. 

Once the dielet is in place, they apply heat and pressure to mold it with the film and diamond substrate.

“That interface is key. If you don’t have that thermal die attach film placed just right, then the heat flow through the diamond to the GaN transistor will not be good enough. So you really need to have a very smooth, clean surface,” Yadav says.

The researchers then stack additional dielectric and metal layers on top of the GaN and diamond to build a working circuit.

They used this technique to fabricate a power amplifier, which is one of the key building blocks of any wireless system. Power amplifiers convert small electrical signals into larger ones that can then be transmitted long distances.

The amplifier they developed achieved higher output power, efficiency, and gain than any similar device the researchers are aware of, including an amplifier they designed in prior work.

“The power amplifier is the beating heart of a wireless device front end. Its performance will dictate the entire performance of your communication system. Our amplifier is powerful enough to ensure that a signal can be propagated for miles,” Yadav says.

These results show how their technique could be well-suited for demanding applications, like high-power radars, space communications, and industrial drones. 

It could also be used to manage heat in systems that perform power conversions inside data centers, improving energy-efficiency. 

Yadav hopes other researchers will build on these advances as they develop more complex heterogeneously integrated systems, opening the door to new possibilities with next-generation electronics.

“When I started my PhD, we wondered if any of this was even doable. It seemed like science fiction. Now we’ve shown all these systems that have outperformed anything that exists on the market today. GaN and 3D heterogeneous systems are going to be at the forefront of so many future applications. It is rewarding to know that we contributed a little bit to that space,” he says.

This research was funded, in part, by the Department of War, the Air Force Office of Scientific Research, the MIT Institute for Soldier Nanotechnologies, and the Qualcomm Innovation Fellowships. Device fabrication and microscopy were conducted at MIT.nano and the Georgia Tech Institute for Matter and Systems.