| dc.description.abstract | In the recent years, the GaN-based electronic devices have attracted attention for high power microwave application, such as AlGaN/GaN high electron mobility transistors (HEMT). However, it is difficult to fabricate working GaN-based HBTs due to the plasma-induced damages on p-GaN, low base conductivity, and high leakage paths resulting from threading dislocations (TDs) in materials and processing. Therefore, this dissertation presents two technologies to fabricate the GaN-base heterojunction bipolar transistors (HBT).
In Chapter 2, we study the p-GaN after dry etching and then treat with different solutions, HCl and aqua regia. Next, different metal contacts on p-GaN were formed for Ohmic contact study. The details of the process flows are presented and related issues are discussed in this chapter. The transmission line method (TLM) measurement is used to characterize the electrical characteristics. The extracted Schottky barrier height (SBH) of Ni/Au, Pt/Au, and Cr/Au were obtained from the current-voltage characteristics to be 0.71, 0.75, and 0.88 eV, respectively. This indicates surface treatment with aqua regia prior to metal deposition can effectively removes the contamination and the Ni/Au metal contact on etched p-GaN shows better contact performance.
Additionally, we study the p-GaN after dry etching and then treat with Zinc- (Zn) diffusion in the furnace. Both TLM measurement and surface morphology including scanning electron microscope (SEM) and atomic force microscope (AFM) are presented. We also investigate the elements distribution on the samples by second ion mass spectroscopy (SIMS) measurement. The measured I-V characteristics of etched p-GaN show the higher specific contact resistance and sheet resistance by surface Zn-diffusion as diffusion time increases. The extracted SBH on etched p-GaN also increases with diffusion time.
In Chapter 3, the fabricated Al0.17Ga0.83N/GaN npn HBT with 110?110 ?m2 emitter area was demonstrated by direct mesa etching process. SIMS analysis for Mg, Si, Ga and Al elements confirms the location of the base-emitter junction. The details of the Al0.17Ga0.83N/GaN HBT fabrication are also described in this chapter. The DC measurement of the Al0.17Ga0.83N/GaN HBT includes the junction, Gummel plot and common-emitter I-V characteristics (CE-IV) are presented. Finally, the current gain of the transistor in collector-up Gummel plot is 4.3 with the VBE ＝0.9 V, while the CE-IV demonstrates the current gain of 126. Experimental results indicate that the differential current gain was obtained in the Gummel plot and CE-IV characteristics, which is because the leakage currents result in abnormal high gain form Gummel plot measurement.
In order to obtain good base contacts, some of the publications had demonstrated the base- or emitter-regrowth on fabricated GaN-based HBTs. In chapter 4, we used the ZnO film to be the regrown layer by sputter to decrease the base damage from the dry etching process. The SEM, Hall measurement, Photoluminescence (PL) and the X-ray diffraction (XRD) are used to characterize the n-ZnO films. The details of the ZnO/GaN HBT fabrication are also described in this chapter. Both Gummel plot and CE-IV characteristics are measured to verify the fabricated ZnO/GaN HBTs. The measured maximum current gain from Gummel plot is 168 at 300K, while the CE-IV demonstrates the current gain of 41. Moreover, the temperature dependence of DC characteristic is studied in this section.
In chapter 5, an npn AZO/GaN HBT by using emitter re-deposited is presented. The utilization of AZO is similar to the ZnO in the previous chapter. Except for the similar lattice constant with GaN, AZO shows a higher doping concentration and higher bandage than ZnO. The characteristics of carrier concentration, electrical resistivity and Hall mobility of AZO films were studied. The CE-IV characterization shows DC current gain of 1.2. In addition, improved device characteristics are observed after the P2S5/(NH4)2S treatment.
Finally, in chapter 6, we summarized the results obtained in this dissertation and presented some suggestions for further studies.
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