| Summary: | 博士 === 國立清華大學 === 電子工程研究所 === 105 === AlGaN/GaN single heterojunction bipolar transistors (SHBTs) without using regrown emitter junction are demonstrated. Secondary ion mass spectroscopy analysis shows that a severe co-diffusion of Al and Si exists in AlGaN/GaN heterostructures grown at 780°C by plasma-assisted molecular beam epitaxy. The altered composition and doping profiles greatly degrade the common-emitter current gain of AlGaN/GaN HBTs to ≤ 0.8. A GaN spacer layer is inserted at the emitter-base junction to alleviate this problem. In an AlGaN/GaN HBT structure inserted with a 20 nm unintentionally doped GaN spacer layer, a current gain β about 2 is achieved. The current gain is improved about 2.5 times larger than the structure without the spacer layer. The light-emitting phenomenon is also demonstrated and investigated in this article.
Using high resolution X-ray photoelectron spectroscopy (XPS), the surface binding energy and composition ratio of Ga and N are determined. The XPS peak shift in dry etching processed p-type GaN represents the existence of an n-type thin layer on the surface originated from nitrogen vacancies. This extra surface layer makes the formation of a low contact resistance difficult. On the contrary, a lower contact resistance is obtained when an n-type metal contact (Ti/Ai/Ti/Au) stacks is used on the etched p-type GaN surface, which confirms that the source of surface damage is coming from surface nitrogen vacancies. In order to overcome the surface damage problem in the base region, a digital etching technique is developed. The mole fraction ratio of N/Ga measured by high resolution XPS decreases from 1 to 0.706 after the conventional base-mesa process. Applying the digital etching technique to the base surface processing, the N/Ga ratio increases from 0.706 to 0.877. Besides, the base contact resistance can be reduced about 23% than the sample treated with inductively coupled plasma etching.
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