Date

2020-12-28

Author

Çakmak, Hüseyin

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Gallium Nitride (GaN) based High Electron Mobility Transistor (HEMT) is the most powerful alternative for high-power and high-frequency applications due to its unique material properties, such as high breakdown field, high electron drift velocity, high operation temperatures, high radiation resistance. Low ohmic contact resistance (Rc) is critical to enhancing the device performance at high frequency and high output power. Various metal stacks with different metal types have been reported in literature. Annealing these metal stacks at high temperatures (above 800 °C) leads to a deterioration in the metal surface morphology and metal edge acuity which makes it difficult to carry out further operations to form the gate region. On top of increasing the production costs, high annealing temperatures may also result in thermal degradation of the epitaxial heterostructure. In this work, high temperature annealing of metal stacks was eleminated by the development of MOCVD regrown InGaN layers and ALD grown AZO non-alloyed ohmic contacts, which does not require high temperature annealing of metal pads. These contacts were specifically studied for Ka-band application of GaN-based HEMT devices. To our best knowledge, MOCVD regrown InGaN layers and ALD grown AZO films as non-alloyed ohmic contacts were implemented to GaN-based vi HEMT devices for Ka-band applications first time in literature. MOCVD regrown InGaN layers and ALD grown AZO films exhibited degenerate doping (1019cm-3) low resistivity (~10-3 .cm) crucial for ohmic contacts to GaN materials. Improved contacts resistance of non-alloyed regrown InGaN ohmic contacts down to 0.3 .mm compared to HEMT with alloyed ohmic contacts was accomplished. Alloyed ohmic contacts were also optimized with varying the barrier metal, metal thicknesses, and annealing conditions for comparison. 0.41 .mm contact resistance was achieved for the alloyed ohmic contact with Ni barrier and the corresponding HEMT device yielded 2.80 W/mm. An almost 7% improvement was observed in both drain-source current (Ids), transconductance (gm), and small-signal performance. Large-signal measurements showed that the output power of the HEMT with non- alloyed regrown InGaN ohmic contact was 3.07 W/mm which was 9% higher compared to HEMT with alloyed ohmic contacts. Although, initial trials did not yield expected results due to not optimized recess etching and post-plasma cleaning conditions, there is still room for improvement for the ALD of AZO ohmic contacts.

Subject Keywords

AlGaN HEMT, MOCVD, AZO, ALD, Ohmic Contacts, High Frequency

URI

https://hdl.handle.net/11511/89634

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Graduate School of Natural and Applied Sciences, Thesis