高功率氮化鎵元件之封裝與熱分析研究

Abstract

氮化鋁鎵/氮化鎵高電子遷移率電晶體(AlGaN/GaN HEMT)具有寬能隙（約3.5 eV）、高崩潰電壓、高臨界崩潰電場以及高電子飽和漂移速率、高峰值電子速率、高電子飽和速率等優點，因此適用於功率電子方面及高頻通訊方面的應用。 本研究致力於氮化鎵高功率元件的封裝與散熱分析，由於功率元件在電性操作下必定會有功率的損失，尤其是閘極開關瞬間的切換耗損，而其損失的能量大部份會轉化為熱能的形式，經熱傳導由元件傳到封裝外殼，再以自然熱對流傳熱至外部環境。氮化鎵電晶體為橫向傳導功率元件，其熱源分佈與矽場效功率元件不同，本研究以5 mm閘極寬元件為基礎，藉由Silvaco TCAD電熱模擬，找出元件熱點位置，並列出多項封裝方式，以Ansys Icepak熱模擬分析來探討元件工作時的溫度分佈。本研究首先以V型凹槽銅座及多層陶瓷基板設計以提升氮化鎵元件的散熱能力，由紅外線熱像儀來實際量測元件主動區在操作下的溫度分佈趨勢，並利用拉曼光譜實驗的縱向溫度量測確認元件內部接面熱點產生的位置，再作橫向區域溫度量測，將測量結果與模擬作比較與驗證。 接著將先前的研究方法套用至80 mm閘極寬之氮化鎵元件來達到提升元件之輸出功率的目的，以TO-257串疊封裝將常開型元件轉換為常閉型元件，並且為提升封裝元件之功率，另將氮化鎵元件並聯，結合氮化鎵電晶體元件的高速開關速度特性及低壓場效電晶體元件穩定的閘極開關形成常閉型(normally-off)開關的串疊電路，可直接取代現行MOSFET功率元件，並利用此結構可保護氮化鎵電晶體元件避免其損壞與增加元件之穩定性，使氮化鎵電晶體元件利用更加廣泛。

Work on wide band gap materials and devices has been going on for many years. One of the important wide band gap materials showing great promise for the future for both switching and power applications is Gallium Nitride (GaN). Heteroepitaxial GaN devices on Si is restricted to lateral device structures. The most common lateral device structure is the HEMT (High Electron Mobility Transistor). Wide band gap and high critical field give GaN an advantage when high power is a key desirable device feature. However, the relatively poor thermal conductivity of GaN makes heat management for GaN devices a challenge for system designers to contend with. This study presents the packaging development of high power AlGaN/GaN HEMTs on Si substrate. By foremost carrying out electro-thermal simulation with Silvaco TCAD and related thermal measurements with infrared thermography and Raman spectroscopy for basic 5 mm GaN HEMTs, the location of hot spot in operating device can be obtained. Based on the outcome, further packaged GaN HEMT is analyzed. The packaging structure is designed on the device periphery surface for enhancing Si substrate thermal dissipation. The effects of structure design and fabrication processes on the device performance were studied. In addition, this study investigates the thermal performance of packaged normally-on multi-finger AlGaN/GaN HEMTs that are cascaded with a low-voltage MOSFET and a SiC Schottky barrier diode (SBD). The analytical results are confirmed by comparing them with the infrared thermographic measurements and numerical results obtained from simulation using Ansys Icepak. To increase the output power, the GaN HEMTs are connected in parallel and the related researches are also displayed. Finally, the packaged GaN devices are applied in power conversion systems.