Ultrawide-Bandgap Semiconductors for High-Frequency Devices

Cellular base stations for 5G/6G networks, as well as satellites and long-range radar for commercial, aerospace, and defense systems all demand power amplifiers (PAs) with high output power density and high efficiency. While silicon (Si), silicon germanium (SiGe), and gallium arsenide (GaAs) technologies are well established, it is now impossible to overlook gallium nitride (GaN) in applications where maximizing output power with minimal footprint is paramount. Backed by its deployment for solid-state lighting, power converters, and PAs, GaN technology has seen tremendous advancements in the last 20–30 years. The principal device for microwave and millimeter-wave (mm-wave) applications is the AlGaN/GaN high-electron-mobility transistor (HEMT), which is most commonly fabricated on either silicon carbide (SiC) or Si substrates. AlGaN/GaN HEMTs with an output power density of 40 W/mm <xref ref-type="bibr" rid="ref1" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[1]</xref> have been reported up to the X band, while highly scaled transistors with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub>/<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">MAX</sub> of >450 GHz have also been achieved <xref ref-type="bibr" rid="ref2" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[2]</xref>. In the last decade, a new wave of innovations has also emerged, such as the replacement of AlGaN barriers with scandium aluminum nitride (ScAlN) and the development of N-polar GaN technology with 8-W/mm power density at 94 GHz <xref ref-type="bibr" rid="ref3" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[3]</xref>.