Impact of Passivation System on Device Performance and Proton Radiation Hardness in GaN-Based MIS-HEMTs
Authors: Chang Sung-Jae , Jung Hyun-Wook , Cho Kyu Jun , Kang Soo Cheol , NOH Youn Sub , Lee Sang-Heung , Kim Seong-il , Kim Hae Cheon , Ahn Hokyun , Lim Jong-Won 
Published: December 2020

GaN-based metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs) are intensively investigated for high power, high frequency [1] and aerospace application [2] due to its wide bandgap, high breakdown field, and high carrier density and mobility at AlGaN/GaN interface [3]. For the demonstration of high power radio-frequency and aerospace applicable devices, the passivation layer introduced on the AlGaN barrier plays significant roles for the enhancement of the frequency characteristics [4] and proton radiation hardness [2]. From this point of view, diverse passivation materials and systems have been researched in GaN-based MIS-HEMTs [2, 5, 6]. However, the detailed impact of passivation layer on the device properties and the immunity to the proton irradiation is not clear in many cases, so far.
In this work, we have prepared the SiN/Al2O3 bi-layer passivation system to study the impact of mechanical stress introduced by the passivation layer deposition on the device performance (Sample A: SiN = 20 nm, Sample B: SiN/Al2O3 = 5/15 nm, Sample C: SiN/Al2O3 = 5/35 nm). The correlation between the induced mechanical stress on the GaN channel and carrier transport properties in terms of carrier concentration and mobility at hetero-interface in GaN-based MIS-HEMTs was examined. The Al2O3 layer deposited by the atomic layer deposition (ALD) provided the tensile stress which compensated the compressive stress generated by the SiN layer deposited by the plasma enhanced chemical vapor deposition (PEDVD). When the compressive stress reduces through the deposition of Al2O3 on the SiN layer, the carrier concentration and mobility at hetero-interface were increased. With decreasing compressive stress, the piezoelectric field enlarges that enhances polarization charges [7] and carrier concentration. The carrier mobility was also improved with diminishing compressive stress since the mean free path related to the lattice scattering is decreased, enhancing transconductance as well. As a result, GaN-based MIS-HEMTs with slight tensile stress in Sample C exhibits the best device performance.
The proton radiation hardness according to the passivation system was studied for various passivation systems. When we compared the device characteristics before and after the proton irradiation with the fluences of 1013 and 1014 cm-2 at an energy of 5 MeV, the device properties such as threshold voltage, drain current, and transconductance were changed. After the proton radiation, the threshold voltage was negatively shifted that is identical characteristics to the metal-oxide-semiconductor device [8], rather than GaN-based HEMTs [9] as relatively large amount of holes was trapped inside of the passivation system due to the lower hole mobility than electron. The alternation of the device characteristics depended on the passivation system. The ALD-deposited Al2O3 showed superior quality compared to the PECVD-deposited SiN that was revealed by the pulse-mode stress measurements. Therefore, the hole trap sites generated by the proton irradiation and located interior of the passivation system are less when the Al2O3 layer was employed as a passivation layer, showing smaller threshold voltage shift. Moreover, the carrier mobility deterioration caused by the Coulomb scattering between the trapped holes inside the passivation system and carriers at hetero-interface was improved with the Al2O3 passivation layer. The drain current and transconductance degradation were improved by applying Al2O3 passivation layer.
Our systematic measurement results highlight that the optimized SiN/Al2O3 bi-layer passivation system is promising technique for the enhancement of the device performance and improvement of proton radiation hardness in GaN-based MIS-HEMTs.
Acknowledgement
This work was supported by the National Research Council of Science & Technology (NST) grant by the Korean government (MSIP) (No. CRC-19-02-ETRI).
References
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Figure 1