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,
$\boldsymbol {f} _{\boldsymbol {T}}$
and
$\boldsymbol {f} _{\mathrm {\mathbf {max}}}$
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.