Flip-Chip Packaging Design for Performance Enhancement of AlGaN/GaN High-Electron-Mobility Transistors

• Main Authors: Tsai, Szu-Ping, 蔡思屏
• Other Authors: 張翼
• Format: Others
• Language: en_US
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/33662m

博士 === 國立交通大學 === 材料科學與工程學系所 === 104 === GaN-based high-electron mobility transistors (HEMTs) are recognized as one of the promising candidates for applications in power electronics since they are capable of operating at high voltages and high temperatures and delivering high output currents. Hence, GaN-based HEMTs are of great potential to compete with the conventional metal-oxide-semiconductor (MOS) devices as the key components in power electronics applications. Although wire-bonding (WB) is still the main stream of chip-level packaging, flip-chip (FC) has drawn much attention especially in high-frequency applications owing to several advantages such as shorter interconnect length and smaller package size. In fact, FC packaging is also favorable for high-power applications. The compactness of FC packaging helps to increase the overall power density and the bumps can act as the paths that dissipate the heat generated by the chip. According to the theories, ID, max improvement of the AlGaN/GaN HEMTs could be made by increasing the tensile strain in the AlGaN/GaN layer. Moreover, thermo-mechanical stresses and strains will be induced after FC packaging. Combining these two characteristics, this dissertation investigates FC packaging design for performance enhancement of AlGaN/GaN high-power HEMTs. iv The content of this dissertation can be divided into two parts. The first part is about single chip FC package. The piezoelectric polarization of the AlGaN/GaN HEMTs is strongly related to its strain state in the active area. Thus, a comprehensive understanding on the strain behaviors inside the channel is important for electrical performance improvement. This part investigates the piezoelectric effect induced by FC bumps leading to the enhancement in device characteristics after packaging. The bump patterns were designed and intended to provide different levels of tensile strain due to the mismatch in the coefficient of thermal expansion (CTE) between the materials. FC packaging with the optimized bump pattern provided a maximum increase of 4.3 % in saturation current compared to the bare die; if compared with the conventional bump pattern, a 17 % improvement was achieved. The following part illustrates the potential of optimized FC structure with active-region bumps to modulate the strain state of AlGaN/GaN HEMT for enhancing piezoelectric effect. The equivalent thermo-mechanical strain in the channel has been observed from simulated results to be affected by the package dimension and the material selection to different degrees, which will be valuable information in future packaging design for device strain engineering. The second part is about multi-chip FC package of GaN HEMTs. This part demonstrates the usage of FC packaging to connect multiple AlGaN/GaN HEMTs in parallel for applications in power electronics. The electrical and thermal properties of both the bare die and the packaged devices were investigated via pulsed current–voltage (I–V) measurements. Compared to the bare die, triple output current, one-third on-resistance (Ron), less than one-fifth thermal resistance (Rth) with temperature insensibility were observed when three transistors were connected in parallel through FC packaging. Superior performance makes FC packaging a promising technology for high power GaN electronic applications.