Performance enhancement of AlGaN/AlN/GaN high electron mobility transistors by thermally evaporated SiO passivation

A surface passivation technique has been developed for AlGaN/AlN/GaN high electron mobility transistors (HEMTs) by simple thermal evaporation of silicon monoxide (SiO) at room temperature. Detailed device characteristics were studied and compared with the most commonly used SiN_x passivation grown by plasma enhanced chemical vapor deposition at elevated temperatures. Both passivation techniques lead to a similar enhancement in the on-state drain current and transconductance as compared with the unpassivated HEMTs. However, we discovered that the gate leakage current in the SiO passivated devices was more than two orders of magnitude lower than the devices passivated by SiN_x. Furthermore, while the SiN_x passivated HEMTs exhibited a two orders of magnitude increase in off-state drain current, SiO passivation substantially reduced it, resulting in an overall improvement by a factor of 1429. The extent of the device surface damage caused by passivation was also investigated by characterizing other parameters. The subthreshold slope of the SiO passivated HEMTs was 95 mV dec⁻¹, nearly 5 times better than the SiN_x passivated devices. The extracted interface trap density was 1.16 × 10¹²cm⁻² eV⁻¹, about ten times lower than that in the SiN_x passivated HEMTs. Moreover, SiO passivation was found to enhance the gate Schottky barrier height by 60 meV whereas SiN_x passivation reduced it, which could partially explain the differences in the gate leakage current. Finally, SiO passivation enabled twice high breakdown voltage than SiN_x passivation. The relevant physical mechanisms were discussed.