The physics behind the excitement lies in a deceptively simple scaling law. In rectifying power electronics, the Baliga figure of merit, which captures how well a semiconductor performs as a switch, is proportional to the bandgap raised to the power of 5.5. That steep exponent means that moving from silicon’s 1.1 electronvolts to gallium nitride’s 3.4 electronvolts already transforms what is possible in converters, chargers and grid infrastructure. Moving to a colossal bandgap above 6 electronvolts, the regime the Cornell-led team now enters, promises another leap in critical electric field and therefore in the voltage a device can block per unit thickness. Thinner devices switch faster, dissipate less heat and shrink the passive components around them, which is why the pursuit of ever-wider bandgaps has become one of the most competitive frontiers in materials science.

Yet widening the bandgap has historically come at a punishing cost: the loss of dopability. The defining trick of semiconductors is that their conductivity can be tuned over many orders of magnitude by deliberately introducing impurity atoms, a capability that underpins every transistor and diode ever made. As bandgaps grow, dopants tend to become so deeply bound that they fail to release their carriers at room temperature, and materials that were once dismissed as insulators, including gallium nitride, silicon carbide, aluminium nitride, gallium oxide and germanium oxide, only became useful semiconductors after years of work reducing defect densities and finding the right dopants. Diamond and cubic boron nitride, despite their superb intrinsic properties, remain locked behind enormous synthesis barriers: high pressures, tiny crystals, prohibitive costs and difficulty scaling. The new result breaks that trend by achieving record bandgap and record conductivity on one of the cheapest substrates in the industry.

The material at the heart of the study is the corundum-structured alloy α-(Al_xGa_1−x)₂O₃, in which gallium atoms in α-gallium oxide are progressively replaced by aluminium to widen the gap. The films were grown by suboxide molecular-beam epitaxy, a technique in which the metal sources are supplied as suboxides rather than pure elements, allowing adsorption-controlled growth of stoichiometric oxide films. The team grew the layers on sapphire, which is itself crystalline aluminium oxide, meaning the substrate is chemically identical to the aluminium-rich end of the alloy system. Sapphire is abundant, inexpensive, available in exceptional crystalline quality and produced at massive scale for everything from watch faces to LED wafers, with crystals as large as hundreds of kilograms grown by the Kyropoulos method. Building colossal-bandgap electronics on such a substrate is a decisive economic advantage over diamond and cubic boron nitride, which require high-pressure synthesis and yield only small, costly samples.

The headline number is conductivity. In the colossal-bandgap regime above 6 electronvolts, the silicon-doped α-(Al_xGa_1−x)₂O₃ films exhibit room-temperature conductivities more than 100 million times higher than all previous reports for materials in this range. Previous efforts to dope the corundum-structure alloy had achieved conductive films only up to bandgaps of about 6.22 electronvolts, and the conductivity collapsed rapidly as aluminium content rose. The new work shows that silicon, acting as a donor on the cation sublattice, can still ionize and release electrons even when the bandgap exceeds 7 electronvolts, provided the growth conditions are tuned correctly. Temperature-dependent Hall measurements on the films revealed carrier freeze-out as samples were cooled, allowing the researchers to extract donor densities and donor activation energies and to confirm that the conduction observed at room temperature arises from thermally activated, deliberately introduced silicon donors rather than accidental defects.

Characterizing such films demanded an unusually broad arsenal of techniques. High-resolution scanning transmission electron microscopy revealed atomically sharp, flat interfaces between the sapphire substrate, the buffer layers and the doped channel, with misfit dislocations confined to the earliest interface. Spectroscopic ellipsometry, performed at multiple angles of incidence and azimuthal orientations to account for the anisotropy of the corundum crystal structure, was used to determine the bandgap of each layer. Secondary ion mass spectrometry quantified the silicon incorporation down to a noise floor of roughly 1.2 times 10 to the 19 per cubic centimetre, while atom probe tomography mapped the three-dimensional distribution of silicon, aluminium and gallium atoms, showing that the alloy mixing follows a random, binomial distribution with no clustering. X-ray diffraction confirmed the crystal phase and composition of the doped overlayers through sharp reflections corresponding to aluminium fractions near 0.52.

Theory played an equal role in interpreting the results. Collaborators at the University of Texas at Austin, including Viet-Anh Ha, Nick Pant and Feliciano Giustino, calculated the theoretical electron mobilities of α-(Al_xGa_1−x)₂O₃ as a function of aluminium content and carrier density using density functional theory and electron-phonon coupling methods. Their calculations, which included phonon scattering, ionized impurity scattering and alloy scattering, and which accounted for the directional dependence of transport parallel and perpendicular to the basal plane, provided benchmarks against which the measured mobilities could be judged. Because the alloy is highly anisotropic, with bandgap energies and dielectric properties that depend on crystallographic direction, such first-principles guidance is essential for designing devices that exploit the material’s best transport directions.

Crucially, the team did not stop at films. They fabricated a Schottky barrier diode, dubbed the AlphaDiode, and a field-effect transistor, the AlphaFET, both with colossal-bandgap channels. The transistor structure stacked an aluminium-rich buffer on sapphire, a silicon-doped α-(Al_0.51Ga_0.49)₂O₃ channel, and a heavily doped regrown contact layer to achieve low-resistance ohmic access to the channel. The demonstration of both a rectifying diode and a transistor proves that the material can be patterned, contacted and switched, the minimum requirements for a practical electronic platform. These first devices are proof of concept rather than production-ready components, but they establish that doping, regrowth and contact formation, the traditional stumbling blocks for ultra-wide-bandgap materials, can all be executed on this oxide system.

The broader significance is strategic. Power electronics built on silicon carbide and gallium nitride are already reshaping electric vehicles, data centres and renewable energy systems, and gallium oxide in its beta phase has attracted intense interest because it can be grown from its own melt. The corundum-structure alloy reported here extends the reachable bandgap beyond anything doped before, and it does so on sapphire wafers that are already manufactured at scale, which the authors argue facilitates rapid development and adoption of colossal-bandgap electronics. The team has filed a provisional patent covering the growth procedure for conductive silicon-doped α-(Al,Ga)₂O₃ and the device structures that use it. Challenges certainly remain, including the modest mobilities typical of oxide alloys, deep donor levels that limit carrier activation, and the need for p-type doping, which remains unsolved. But the central barrier, the belief that semiconductors above 6 electronvolts cannot be made conductive, has now been decisively broken, and with it the assumption that the future of power electronics must be built on ever more expensive and difficult crystals.

Subject of Research: Silicon-doped α-(AlxGa1−x)2O3 as a record 7-eV bandgap semiconductor for colossal-bandgap power electronics

Article Title: A 7-eV bandgap semiconductor based on silicon-doped α-(AlxGa1−x)2O3

Article References: Steele, J., Bhattacharya, D., Nomoto, K., Pieczulewski, N. A., Sorensen, P., Ha, V.-A., Pant, N., Shukla, I., Das, S., Kilic, U., Ramesh, M., Giustino, F., Mazumder, B., Senevirathna, M. K. I., Williams, M. D., Schubert, M., Muller, D. A., Xing, H. G., Jena, D., & Schlom, D. G. (2026). A 7-eV bandgap semiconductor based on silicon-doped α-(AlxGa1−x)2O3. Nature. https://doi.org/10.1038/s41586-026-11085-3

Image Credits: AI Generated

DOI: 10.1038/s41586-026-11085-3

Keywords: colossal-bandgap semiconductor, alpha-gallium oxide, aluminum gallium oxide alloy, silicon doping, suboxide molecular-beam epitaxy, sapphire substrate, power electronics, Baliga figure of merit, Schottky diode, field-effect transistor, wide-bandgap materials, molecular-beam epitaxy