مسائل مربوط به مواد و پردازش در الکترونیک قدرت GaN عمودی Materials and processing issues in vertical GaN power electronics

1. Introduction Power electronics is the application of electronic switching devices for efficient conversion and the control of electrical power [1]. For the past 60 years, silicon-based power devices have been the dominant player used in power circuits to accomplish these tasks. The design of new device structures such as thyristors and insulated gate bipolar transistors (IGBT) and the optimization of the fabrication processes have enabled a significant improvement in device performance. However, Si-based power devices are now reaching their theoretical limit. Innovation in power devices is needed to further improve the performance of power electronics and bring us closer to realize a sustainable society. Wide-bandgap (WBG) semiconductors such as silicon carbide (SiC) and gallium nitride (GaN) are promising material candidates for the next generation power devices. Due to their high critical electric field, WBG semiconductors can enable the fabrication of power devices with lower capacitance and resistance for a given breakdown voltage than their Si counterparts. Table 1 shows the comparison of material parameters for GaN with Si and SiC. The Baliga’s figure of merit (BFOM) (ϵSμ E ) n C 3 [2,3], which describes the fundamental relationship between on-resistance and breakdown voltage (BV), shows that GaN promises the best performance among the current candidates for power electronics. GaN-based devices can meet the increasing performance demands of the evolving power systems, operating at higher power densities and temperatures than existing Si power devices. Lateral power transistors and diodes based on AlGaN/GaN heterostructures have already demonstrated excellent electrical characteristics [4–9], and enhancement-mode GaN products are commercially available since June 2009 [10]. However, the power handling of GaN transistors with a lateral configuration is typically limited to a few kW. For higher power ratings, the chip size of lateral GaN transistors increases significantly, resulting in inefficient utilization of the material, difficult current extraction, and poor reliability. The vertical buffer breakdown also limits the operating voltage of the device below 1 kV due to the difficulty of growing thicker (Al)GaN layers on a silicon substrate [11]. Additionally, lateral GaN transistors and diodes suffer from electron trapping at the surface due to the presence of high surface electric field under off-state operation [12,13]. This current collapse phenomenon can severely degrade the device performance and affect its long-term stability and reliability, especially for very high-voltage operations [14,15]. Power devices based on vertical GaN are promising candidates to overcome the challenges described above for the lateral GaN transistors. The breakdown voltage in vertical GaN devices can be increased by increasing the thickness of the drift region, while keeping the device footprint constant. The maximum electric field in vertical GaN devices is far away from the surface, which minimizes trapping effects and reduces dynamic on-resistance. Moreover, the vertical current extraction in vertical GaN transistors allows for the delivery of much higher power density than in lateral GaN devices.