XHEMTs on Ultrawide Bandgap Single-Crystal AlN Substrates

TL;DR

This paper introduces the AlN-based XHEMT, a novel high-electron-mobility transistor on single-crystal AlN substrates, achieving high RF performance and addressing key challenges in strain, polarization, and defect management.

Contribution

The paper presents the first demonstration of XHEMTs on AlN substrates, showcasing their potential for high-frequency applications with improved thermal and structural properties.

Findings

Achieved 5.92 W/mm output power at 10 GHz

Demonstrated 65% peak power-added efficiency

Overcame dislocation and thermal resistance issues

Abstract

AlN has the largest bandgap in the wurtzite III-nitride semiconductor family, making it an ideal barrier for a thin GaN channel to achieve strong carrier confinement in field-effect transistors, analogous to silicon-on-insulator technology. Unlike SiO$_2$/Si/SiO$_2$, AlN/GaN/AlN can be grown fully epitaxially, enabling high carrier mobilities suitable for high-frequency applications. However, developing these heterostructures and related devices has been hindered by challenges in strain management, polarization effects, defect control and charge trapping. Here, the AlN single-crystal high electron mobility transistor (XHEMT) is introduced, a new nitride transistor technology designed to address these issues. The XHEMT structure features a pseudomorphic GaN channel sandwiched between AlN layers, grown on single-crystal AlN substrates. First-generation XHEMTs demonstrate RF performance on par with the state-of-the-art GaN HEMTs, achieving 5.92 W/mm output power and 65% peak power-added efficiency at 10 GHz under 17 V drain bias. These devices overcome several limitations present in conventional GaN HEMTs, which are grown on lattice-mismatched foreign substrates that introduce undesirable dislocations and exacerbated thermal resistance. With the recent availability of 100-mm AlN substrates and AlN's high thermal conductivity (340 W/m$\cdot$K), XHEMTs show strong potential for next-generation RF electronics.