Since the 1990s, researchers have doped gallium nitride (GaN) by adding magnesium impurities to create holes, but the process is highly inefficient. For every hundred magnesium atoms introduced into the crystal, only three or four holes might appear at room temperature, and even fewer at low temperatures.
Rather than using impurities, Ph.D. student Reet Chaudhuri stacked a thin GaN crystal layer – called a quantum well – atop an AlN crystal. The difference in their crystal structures was found to generate a high density of mobile holes. Compared with magnesium-doping, the researchers discovered that the resulting 2D hole gas makes the GaN structures almost 10 times more conductive.
“In 1992, researchers discovered that when aluminum nitride is deposited on top of gallium nitride, you get free electrons at the interface, the so-called two-dimensional electron gas (2DEG). Having electrons conduct inside GaN makes what we call n-type electronic devices,” said Chaudhuri, the paper’s lead author. “The polarization theory that explains why we get mobile electrons in this structure also predicts that we should expect holes when the structure is flipped. But to date, there had not been any report of holes in an undoped type-III-nitride semiconductor structure. And that’s what we have found in this work.”
Using the new material structure developed by Chaudhuri and fellow student Samuel James Bader, some of the most efficient p-type GaN transistors have recently been demonstrated in a joint project with Intel. As the team is now able to produce such semiconductor structures, it plans to combine them with already available n-type transistors to build more complex circuits. This may open up new possibilities in high power conversion, 5G wireless and energy efficient electronics, including chargers.
“It’s very difficult to simultaneously achieve n-type and p-type in a wide bandgap semiconductor. Right now, silicon carbide is the only other one that has both besides GaN. But the mobile electrons in silicon carbide are more sluggish than those in GaN,” said co-senior author Huili Grace Xing, the William L. Quackenbush Professor in electrical and computer engineering and in materials science and engineering. “Using these complementary operations enabled by both n-type and p-type devices, much more energy efficient architecture can be built.”
Another advantage of the 2D hole gas is that its conductivity improves as the temperature is lowered, meaning that researchers will now be able to study fundamental GaN properties in ways that haven’t been previously possible. Equally important is its ability to retain energy that would otherwise be lost in less efficient power systems.
R. Chaudhuri, et al., A Polarization-Induced 2D Hole Gas in Undoped Gallium Nitride Quantum Wells, Science 27. Sep 2019, Vol. 365, Issue 6460, pp. 1454-1457 DOI: 10.1126/science.aau8623