| dc.description.abstract | In recently years, the commercial outlook for GaN optoelectronic and electronic devices has grown considerably. GaN-based transistors, such as AlGaN/GaN high electron mobility transistors, are capable of delivering the high-power density under the harsh and high-temperature environments. However, it is difficult to fabricate working GaN-based HBTs due to the Schottky-like ohmic contacts on p-GaN, low base conductivity, and high leakage paths resulting from dislocations in materials and processing. Therefore, the research on an AlGaN/GaN HBT is one of the challenging research subjects. Different approaches have been utilized to obtain the n-p-n and p-n-p GaN-based HBTs such as direct mesa etch, base regrowth, emitter regrowth, and p-InGaN base. This dissertation is focused on efforts to develop fabrication technology for the GaN-based HBT and its related issues. The primary propose of this dissertation is to fabricate working AlGaN/GaN HBTs using the double mesa etching process.
In chapter 2, we present the design and growth of an AlGaN/GaN HBT. The proposed Al0.17Ga0.83N/GaN heterojunction bipolar transistor (HBT) was grown on c-face sapphire substrate by metalorganic chemical vapor deposition (MOCVD). Additionally, the fabricated Al0.17Ga0.83N/GaN n-p-n HBT with 75 x 75 um2 emitter area was demonstrated HBTs by direct mesa etching process. The details of the Al0.17Ga0.83N/GaN HBT fabrication are described in this chapter. Some of the important issues related to base layer and contact are discussed and discussed in detailed in the following chapters.
In chapter 3, the optimized etching process condition is studied for AlGaN/GaN HBTs. This chapter investigates the effect of Cl2/Ar dry etching on p-GaN. The root-mean-square (RMS) surface roughness is measured and depth display (Bearing analysis) is monitored. The current-voltage (I-V) characteristics of etched p-GaN with Ni (20 nm)/Au (20 nm) metallization are studied. Experimental results indicate that the etching rate does not increase significantly with the Cl2 flow rate at a constant power or chamber pressure. The Bearing ratio data exhibit a much stronger variation with etch conditions, the RMS displays the same trend but to a lesser extent. By the analysis of the RMS and the Bearing ratio, the optimal etching recipe is obtained and applied to the etching process of AlGaN/GaN HBTs.
In order to improve the base contacts after dry etching, base regrowth technique has been applied to fabricate GaN-based HBTs. In chapter 4, we use the regrown p-type AlInGaN and InGaN to decrease the base damage from the dry etching process. The p-type AlInGaN and InGaN contact layers are regrown on the etched p-GaN to study the Ni (20 nm)/Au (20 nm) contact current-voltage (I-V) characteristics. The thickness of the contact layer is 100 nm and regrown by metalorganic chemical vapor deposition. By using the regrown contact layer on etched p-GaN, Schottky barrier height (SBH) from the I-V characterization is reduced. The SBH of 0.65 eV from the contact to the etched p-GaN is reduced to 0.56 eV and 0.58 eV, respectively, after the AlInGaN and InGaN contact layers were formed. In addition to the I-V characterization of Ni/Au contacts, surface morphology and x-ray analysis are studied.
In chapter 5, the material properties of Al0.17Ga0.83N/GaN HBT presented in chapter 2 is investigated first. The epi-taxial layers are analyzed by x-ray diffraction pattern and secondary ion mass spectroscopy. The reciprocal space map verifies that the Al-content layer in emitter is a coherently strained structure. The threading dislocations are revealed by transmission electron microscopy (TEM) and etch pit density (EPD) measurement. The average value of the EPD is 3.38 x 108 cm-2. Additionally, the fabricated HBT with 75 x 75 um2 emitter area is demonstrated by direct mesa etching process. The measured gain exceeds 103 over the wide range of collector current in Gummel plots characteristic. In addition, the fabricated HBT with 75 x 75 um2 emitter area demonstrates a low offset voltage of 2.04 V and dc current gain of 1.22. The differential current gain is 1.32 in the common-emitter I-V characteristics. To analyze the device characteristics, a VBIC model for the intrinsic device and parasitic elements are used to implement an equivalent circuit model. From simulated results and fitting comparison, the high current gain in the Gummel plots is due to the extrinsic leakage current and poor base contact.
Finally, we summarize the results obtained in this dissertation and present some suggestions for further studies. The suggestions to fabricate GaN based HBT include the p-type InGaN base, the planar GaN bipolar transistors and the ZnO/InGaN HBTs using emitter regrowth. | en_US |