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P-GaN gate HEMTs have record threshold voltage

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Shandong University team reports an novel enhancement-mode p-GaN gate HEMTs with the highest threshold voltage a 7.1V

Researchers at Shandong University in China have reported an enhancement-mode P-GaN/AlGaN/GaN metal-insulator-semiconductor HEMT (MIS-HEMTs) by combining thermal oxidation treatment of P-GaN with atomic layer deposition (OTALD) prior to gate metal deposition.

This increased threshold voltage significantly to 7.1 V, with improved gate breakdown voltage and off-state breakdown voltage of 26.9 V and 1980 V respectively.

Low activation rate of Mg dopants in P-GaN typically results in the threshold voltage less than 2 V without additional processing, limiting the application of P-GaN/AlGaN/GaN HEMTs in high gate voltage drive circuits. In response, the researchers applied an integration of oxygen annealing treatment (OT) of P-GaN and atomic layer deposition (ALD) technology.

Device fabrication began with the P-GaN cap layer etch and mesa isolation. The ohmic contacts were then formed via magnetron sputtering and annealing in N2 atmosphere. After depositing SiO2 passivation layer, the opening of the gate region was patterned by inductively coupled plasma (ICP) etching.

Fabrication of the MIS structure on the opening of the P-GaN gate region by OTALD technology started with the thermal oxidation treatment. Followed by a 5 nm Al2O3 dielectric layer deposition by plasma enhanced atomic layer deposition (PEALD) process. Finally, gate metal stack was deposited by magnetron sputtering to form the gate metal field plate. The untreated fresh P-GaN gate HEMTs were fabricated as references.

Atomic force microscopy (AFM) measurement shows decreased surface roughness after oxygen annealing treatment process, indicating that this process produces a flat oxide layer on the P-GaN surface and effectively removes other contaminants. Cross-sectional HRTEM presents a dense and smooth oxide layer is formed. The presence of this oxide layer provides a gradually-varied transition between Al2O3 and P-GaN, leading to a significantly improved interface quality between these two materials.