| dc.description.abstract | The AlGaN/GaN hetrostructure has been announced in 1991, and it has also been proven that the electron mobility of 2DEG could be indeed enhanced. Then the first GaN MESFET was delivered in 1993, and the first GaN HEMT was also fabricated in 1996, too. It is interesting that the developing speed of GaN HEMT was almost 10 times as the traditional AlGaAs/GaAs pHEMT. Except the successful experience of GaAs pHEMT, the excellent properties of GaN based materials are the key forces to push the research of GaN HEMT forward. The excellent properties of GaN HEMT such as high temperature stability, high breakdown voltage, high electron velocity, strong piezoelectric effect and high current density let the GaN HEMT be a good candidate for the applications of high speed and high power.
In comparison with the development of GaAs pHEMT, there are a lot of issues in the research of GaN HEMT. The main problems can be divided into two fields, epitaxy and process. Owing to the lattice mismatch between the GaN and substrate, the research of how to improve the quality of GaN buffer layer is always an important and fundamental issue of GaN HEMT epitaxy. For the process issue, the high ohmic contact resistance caused by the wide bandgap property, the sensitive surface state, and the serious thermal effect are still under intensive study.
In this dissertation, a high etching selectivity and low surface damage etching recipe for GaN/AlGaN hetrostructure was described in the chapter 2. After combining the design of using n+ GaN as the cap layer and the selective gate recess technology, a lower ohmic contact resistance and a fine breakdown characteristic can be expected. In the chapter 3, the delay time analysis was carried out to extract the distribution of delay times in the GaN HEMT and this discussion can help us understand which parts of delay times affect the device performance. In the chapter 4, the pulse I-V curves measurement system was setup to detect the status of device surface state. Then a new low-k material, BCB, was applied into the application of device passivation process. In comparison with traditional Si3N4 passivation layer, the BCB passivation layer provides a lower parasitic capacitance and a better rf performance. Additionally, the characterizations of pulse I-V and reliability test in BCB passivated GaN HEMT were also completed. In the chapter 5, the gamma gate engineering on GaN HEMT was successfully realized. Owing to the decrease of electric field strength, the gamma gate can enhance the device breakdown performance but the parasitic capacitance would not be obviously added. In the chapter 6, the heating effect in the power device was addressed and the various device layouts including various gate widths and various gate pitches were designed to discuss the relation between device layouts and heating effect. Then the thermal IR microscopy was used to detect the device surface temperature in various dc power consumptions and the thermal resistance could be obtained simultaneously. In the final chapter, we summarized the results obtained in this thesis. | en_US |