| Summary: | 博士 === 國立交通大學 === 材料科學與工程學系 === 98 === AlGaN/GaN high electron mobility transistors (HEMTs) have been fabricated and characterized. To further improve the performance of the HEMTs, Ohmic and Schottky contacts have been extensively re-examined and new approaches are proposed. The effect of the device layout to the device characteristics is also discussed.
In this dissertation, the contact resistivity of Ohmic contacts of AlGaN/GaN HEMTs is improved by the insertion of a thin Si layer (3nm) between the semiconductor layer and traditional Ti/Al/Ni/Au metal stacks. Using this approach, low contact resistivity of 0.23 ohm-mm is realized, and the mechanism for this improvement is investigated by means of transmission electron microscope (TEM). As for improvement of the Ohmic contact surface morphology, WSiN cap layer is placed on top of the ohmic contact before annealing, afterward it is partially removed by low damage induced coupled plasma (ICP) etcher. Smooth surface with no degradation of the electrical performance are observed.
For Schottky contacts, we systematically investigated the characteristics of tungsten nitride with respect to the nitrogen composition, then diodes and GaN HEMTs are realized using the optimized W0.52N0.48 as Schottky contact metal. Reliability after electrical stress, thermal stability as well as excellent RF performance of 5W/mm at 2 GHz are measured with the WNx gate device.
In addition, the geometry and layout effecting the GaN HEMTs are also investigated. Both gate-drain and source-gate distances are varied and the gate behaviors are strongly dependent on these parameters. The information could provide a guideline when designing the device.
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