Wide-band-gap semiconductors are materials that can withstand strong electric fields and switch between conducting and insulating states. The wider this gap, the higher the voltages a device can withstand, but the harder it is to control conductivity. Researchers from Cornell University led by Darrell Schlom found a way to retain control over conductivity at a record-wide band gap by adding silicon to thin films of aluminum gallium oxide.
In a semiconductor, electrons occupy two energy ranges: in the lower "valence band" they are bound to atoms and cannot carry current, while in the upper "conduction band" they move freely. The band gap is the energy an electron needs to move from one band to the other. The larger it is, the stronger the electric field the material can withstand. Already-known wide-band-gap semiconductors such as gallium nitride and silicon carbide are used in fast phone chargers and in electric vehicles.
Aluminum gallium oxide is promising because its band gap can be smoothly varied from 5.4 to 8.6 electronvolts (eV), the widest range among known semiconductors. However, as the aluminum fraction rises, it becomes increasingly difficult to supply the material with free electrons. Until now, films with a high aluminum content conducted current orders of magnitude worse than gallium-rich films, making them unsuitable for electronics.
Schlom's team grew films on a sapphire substrate by molecular beam epitaxy, layer-by-layer growth of material with precision down to a single atomic layer. During growth, small amounts of silicon were added to the films, whose atoms donate extra electrons to the material. The scientists then measured the band gap and electrical properties of the films and also tested their operation in two key components, a diode and a transistor.
The silicon-doped films conducted current at a band gap above 7 eV, surpassing the previous record held by cubic boron nitride. In the range above 6 eV, which the authors call the "colossal band gap," conductivity at room temperature turned out to be more than 100 million times higher than in any previously published reports. Both the diode and the transistor operated successfully with these materials.
The approach could make the production of next-generation semiconductors far more practical. Compared with other ultra-wide-band-gap materials such as diamond and cubic boron nitride, sapphire is common, cheap and already produced in large volumes with high quality. Schlom's team hopes their films will open the way to a new generation of power electronics that will waste far less energy as heat when converting and controlling electricity.