We report a comparison of various gate dielectrics including SiO2, Si3N4, ZrO2 and Pb(Zr, Ti)O3
(PZT) on AlGaN/GaN heterojunction field-effect transistors, deposited by PECVD, MBE, and
sputtering respectively. In terms of I-V characteristics, maximum drain-source current could be
enhanced under positive gate voltage and the reverse leakage current level decreases by orders of
magnitude. In terms of DC measurements, very thin SiO2 layers can improve performance, which
may be due to the passivation effect to remove surface states. No significant difference exists
between control and the Si3N4 and ZrO2 samples. Slightly reduction in transconduction is observed
on the sample with PZT probably because of the much thicker layer was utilized. The thickness of
insulator layers examined from C-V measurements reveals a better crystal quality can be obtained
by PECVD deposition. While the RF S-parameters measurements shows the PZT gate dielectric
brings the highest cut-off frequency or the lowest gate capacitance confirmed also by C-V data,
which makes it a better candidate for microwave applications.
Epitaxial growth of ZrO2 has been achieved on MOCVD-grown GaN(0001) templates by
oxides molecular beam epitaxy using reactive H2O2 for oxygen and organometallic
source for Zr. Utilizing a low temperature buffer layer followed by high temperature
insitu annealing and high-temperature growth, monoclinic (100)-oriented ZrO2 thin films
were obtained. The full width at half maximum of ZrO2 (100) rocking curve was 0.4 arc
degree for 30-nm-thick films and the rms roughness for a 5&mgr;m by 5 &mgr;m AFM scan was 4 Å. The employment of epitaxial ZrO2 layer in the AlGaN/GaN heterojunction field effect
trasnsistor as a gate dielectric has resulted in the increase of the saturation-current density
and pinch-off voltage as well as in near symmetrical gate-drain I-V behavior.
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