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

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.