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

Al 2 O 3 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 Al 2 O 3 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 Al 2 O 3 passivation exhibited 77% higher output power using plasma-assisted ALD compared with thermal ALD.