Significance Statement
Recently, GaN-based Schottky Barrier Diodes (SBDs) grown on silicon substrate have attracted significant attention for power electronics applications. The SBDs play a critical role in many power conversion system. To achieve a small form factor of the system, an increased operating speed is inevitable, while it is also a great challenging to maintain the conversion efficiency. GaN has a superior material such as the large bandgap (Eg = 3.4 eV), high electron saturation velocity (vsat ∼ 2.5 × 107 cm/s), high carrier density (n ∼ 1 × 1013 cm−2), and excellent breakdown field (Ec ~ 3.3 MV/cm), which is an excellent candidate for the SBDs to operate at high speed with low loss. In addition, growth of a high-quality GaN epitaxial layer on large silicon substrates has shown substantial progress, which allows the Ga-on-Si devices to be realized on large-scale silicon substrates (8 inch and above), and makes it possible to hove low cost and high performance GaN-based Schottky Barrier Diodes.
One issue remains challenging for the GaN Schottky Barrier Diodes is the relatively high turn-on voltage VON due to the large bandgap of the material, which can increase the conduction loss and lower the efficiency. Different approaches have been proposed to reduce VON of AlGaN/GaN Schottky Barrier Diodes. Among them, using the anode recess technology is the most straightforward and effective way to reduce VON while maintaining high reverse breakdown capability. The recessed structure allows the current flows from the etched sidewall with an increased tunneling probability, resulting in a reduced VON. By employing the recess technology in the GaN Schottky Barrier Diodes, it is important to understand and control of the surface condition of etching, which is closely related to the channel/buffer leakage current and premature breakdown.
In this work, we found a low-damage process using ICP-RIE to obtain optimized semiconductor/anode interface. The experimental results indicate that the pressure is critical during the recess process for low surface roughness. At increased pressures, the mean energy of ion bombardment becomes lower due to the increased collisions. Consequently, the reduced etching rate leads to clearly improved surface roughness. The measured results demonstrate the VON of GaN Schottky Barrier Diode is successfully reduced to ~ 0.7 V from a typical device of 1.2 V without anode recess. The improved surface roughness leads to an obvious leakage current suppression. Also, an excellent reverse breakdown voltage up to 2070 V for the SBD with LAC = 21 mm (anode-cathode distance) can be achieved, which is an evidence of the high quality surface condition after recess process. The Baliga’s Figure-of-Merit (B-FOM) is often used to evaluate the power devices in terms of power handling capability and conduction loss. The achieved B-FOM in this work is up to 1127 MW/cm2, which is among the highest compared with the published GaN-on-Si Schottky barrier diodes up to date.
Journal Reference
IEEE Electron Device Letters (Volume:37 , Issue: 1 ), 2015.
Chuan-Wei Tsou; Kai-Pin Wei ; Yi-Wei Lian ; Shawn S. H. Hsu.
Electr. Eng. Dept., Nat. Tsing Hua Univ., Hsinchu, Taiwan
Abstract
In this letter, we demonstrate high-performance AlGaN/GaN Schottky barrier diodes (SBDs) on Si substrate with a recessed-anode structure for reduced turn-on voltage VON. The impact of the surface roughness after the recessed-anode formation on device characteristics is investigated. An improved surface condition can reduce the leakage current and enhance the breakdown voltage simultaneously. A low turn-on voltage of only 0.73 V can be obtained with a 50-nm recess depth. In addition, the different lengths of Schottky extension acting like a field plate are investigated. A high reverse breakdown voltage of 2070 V and a low specific ON-resistance of 3.8 mΩ · cm2 yield an excellent Baliga’s figure of merit of 1127 MW/cm2, which can be attributed to the low surface roughness of only 0.6 nm and also a proper Schottky extension of 2 μm to alleviate the peak electric field intensity in the SBDs.
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