(Invited) Wide Bandgap Semiconductor Homo- and Hetero-P-N Junctions
Authors: Fu Kai 
Published: August 2024

Wide bandgap (WBG) semiconductors, such as gallium nitride (GaN), gallium oxide (Ga2O3), aluminum nitride (AlN), and diamond, hold immense promise for power electronics applications by drastically reducing power loss, enhancing switching frequency, and reducing system volume. However, effective doping (p-type or n-type) remains a substantial challenge for these materials, given that the p-n junction is a fundamental building block for device design. Selective-area doping, crucial for high-performance electronic devices and integrated circuits, poses another significant obstacle.

We report recent progress on selective-area doping for GaN homo-p-n junctions. Although GaN shows substantial development in both p-type and n-type doping, achieving selective doping, especially p-type, remains challenging through techniques such as ion implantation, diffusion, or in-situ doping. Our investigation delves into the challenges of p-GaN regrowth using metal-organic chemical vapor deposition (MOCVD). Interface contaminants post etching and regrowth, such as silicon (Si), carbon (C), and oxygen (O), have been systematically investigated. An optimized interface treatment has been identified to effectively reduce surface charge and, consequently, reverse leakage current to the level comparable to the as-grown p-n junction. Additionally, a novel hydrogen plasma treatment has been proposed to realize selective doping for GaN, successfully applied to p-GaN gated high electron mobility transistors (HEMTs) to improve the dynamic performance by avoiding the etching damage and edge termination for vertical GaN p-n diodes to realize high breakdown voltage.

β-Ga2O3 has emerged as a standout in power electronics applications, surpassing Si (3000×), SiC (10×), and GaN (4×) in Baliga’s figure of merit. Its great potential for mass production, facilitated by the availability of high-quality and large-size wafers, significantly enhances its attractiveness. However, challenges, including the absence of p-type doping due to deep acceptor states and a flat valence band, as well as exceptionally low thermal conductivity, limit its performance at high power and temperatures. Overcoming these challenges necessitates hetero-integration with semiconductors possessing p-type characteristics and high thermal conductivity. P-type diamond stands out due to its exceptional thermal conductivity, high critical breakdown field, and well-established p-type characteristics. Moreover, since the n-type doping of diamond also poses challenges, fusing the p-type diamond and n-type Ga2O3 to create a diamond/Ga2O3 p-n-heterojunction can effectively overcome doping bottlenecks for both materials. Our successful construction of a diamond/β-Ga2O3 hetero-p-n junction through the mechanical integration of bulk p-type diamond and bulk n-type Ga2O3 presents robust electrical performance up to 125 ℃, with hysteresis lower than 0.7 V @ 1 μA. Impressively, the ideality factor of the p-n junction is remarkably low at 1.28, and the rectification ratio exceeds 108. This approach has also been applied to other hetero-p-n junctions, such as the diamond/GaN structure, demonstrating consistent robust performance. These findings underscore the significant potential of the mechanical integration approach, offering a promising avenue to simplify the fabrication process and facilitate the widespread application of WBG heterojunctions.