Impact of Parasitic Conductive Interfaces on the DC and RF Performance of GaN-on-GaN HEMTs

A. Bassal^a, S. Chevtchenko^a, F. Schnieder^a, T. Flisgen^a,b, O. Bengtsson^a, F. Brunner^a, and O. Hilt^a

Published in:

phys. stat. sol. (a), vol. 222, no. 22, pp. 2500406, doi:10.1002/pssa.202500406 (2025).

Abstract:

Radio frequency (RF) AlGaN/GaN high-electron-mobility transistors (HEMTs) on semi-insulating GaN substrates showed an unexpectedly low output power of ≤1.7 W mm^-1 and a power added efficiency (PAE) of ≤15% for V_ds = 20 V at 20 GHz, despite their good DC current densities (>1 A mm^-1) and minor trapping effects in pulsed characterization, comparable to GaN-on-SiC HEMTs. With theoretical calculations of P_out and PAE, and the detailed analysis of transistors and structures, a 1500 Ω/□ parasitic conductive interface (PCI) between the GaN substrate and the epitaxial layers was identified as a root cause. GaN substrates suffer from environmental Si contamination forming such a PCI. It was first detected by eddy current measurements showing that the insulating GaN:Fe buffer and GaN:Mn substrate did not effectively suppress the PCI. Nevertheless, it was effectively isolated by the GaN:Fe buffer to maintain a good DC performance for GaN-on-GaN HEMTs and allowing for a record g_m-max of 535 mS mm^-1. The S-parameters confirmed this and indicated that the PCI causes a feed-back effect, reducing the k-point and f_max. Furthermore, loading effects were revealed on the ports of the HEMTs due to the PCI’s resistive nature, and coplanar waveguide measurements showed 5 dB mm^-1 of propagation loss and severe changes in the characteristic impedance.

^a Wide-Bandgap Electronics Department Ferdinand-Braun-Institut (FBH), Gustav-Kirchhoff-Straße 4, 12489 Berlin, Germany
^b Fachgebiet Theoretische Elektrotechnik, Brandenburgische Technische Universitat Cottbus - Senftenberg, Siemens-Halske-Ring 14, 03046 Cottbus, Germany

Keywords:

GaN-on-GaN HEMTs, GaN substrate, loading effect, parasitic conductive interface

© 2025 The Author(s). physica status solidi (a) applications and materials science published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
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