Evaluation of Thermal Versus Plasma-Assisted ALD Al2O3 as Passivation for InAlN/AlN/GaN HEMTs

Al₂O₃ films deposited by thermal and plasma-assisted atomic layer deposition (ALD) were evaluated as passivation layers for InAlN/AlN/GaN HEMTs. As a reference, a comparison was made with the more conventional plasma enhanced chemical vapor deposition deposited SiN_x passivation. The difference in sheet charge density, threshold voltage, f_T and f_max was moderate for the three samples. The gate leakage current differed by several orders of magnitude, in favor of Al₂O₃ passivation, regardless of the deposition method. Severe current slump was measured for the HEMT passivated by thermal ALD, whereas near-dispersion free operation was observed for the HEMT passivated by plasma-assisted ALD. This had a direct impact on the microwave output power. Large-signal measurements at 3 GHz revealed that HEMTs with Al₂O₃ passivation exhibited 77% higher output power using plasma-assisted ALD compared with thermal ALD.