Silicon carbide (SiC) is widely used in medium and high voltage power semiconductor device fabrication due to its inherent wide band gap and high thermal conductivity material properties. Today, Schottky diodes, MOSFETs and JFETs are the most popular SiC power devices on the market. In particular, SiC Schottky diodes have been used successfully in power applications for almost 20 years. The earliest SiC Schottky diodes used a pure Schottky barrier diode (SBD) structure. Later, this evolved into a structure called a Junction Barrier Schottky (JBS) with low reverse leakage current. The newest structure, called the Merged PN Schottky (MPS), shows a vastly improved inrush current handling capability.
Vaillant Semiconductor released 650V SiC MPS diodes based on 100mm SiC wafers in 2014 and 650V SiC MPS diodes based on 150mm high-quality SiC wafers in 2017. Earlier this year, Vaillant introduced 1200V SiC MPS diodes and AEC-Q101 automotive-qualified 650V SiC MPS diodes based on proven 150mm wafer technology.
Vaillant has more than 50 years of history and rich experience in the design of power semiconductor devices. The design process includes setting design goals based on customer needs, equipment and process simulation using EDA tools, mask design and process design, foundry manufacturing, assembly, and reliability testing. After multiple rounds of trial, optimization, life testing and application testing, a qualified product with optimized design is released.
With carefully designed P+ islands and a unique ohmic contact process, a high-efficiency inrush current conduction path without significant impact on the effective Schottky area is achieved. This gives WeEn SiC MPS diodes excellent inrush current handling capability without loss of nominal current conduction capability.
Figure 1: Schematic cross-section and current distribution of WeEn SiC MPS diode
A SiC wafer suitable for producing power devices consists of two layers: a thick substrate layer and a thin epitaxial layer grown on it. Thick substrates provide large SiC wafers with the mechanical stability required during semiconductor processing, handling and shipping. However, the electrical function of the substrate is minimal. The diode function that blocks high reverse voltages is covered by an epitaxial layer, and only in forward operation does the substrate function as a current conduction path. Unfortunately, this current conduction path acts as a series resistance. Commercially available SiC substrates do not offer high doping concentrations, so series resistance is very noticeable, especially for 650V SiC devices. This can lead to undesired power losses.
Figure 2: Comparison of forward IV characteristics between Vaillant NXPSC04650 4A, 650V MPS diodes and JBS diodes from other companies @25℃
Silicon carbide is a very hard material that presents several challenges for mechanical treatments such as grinding: crack prevention, surface roughness and thickness uniformity – just to name a few. Nonetheless, leading manufacturing processes and excellent quality control enable Wien to offer SiC products with substrate thicknesses that are only 1/3 the thickness of the substrates compared to standard products on the market. The thin die provides WeEn SiC diodes with better current conduction capability and lower thermal resistance.
Strict production management and quality control are the basic elements to ensure stable product performance. In order to provide customers with the most reliable SiC diode products, Wien has established comprehensive quality and reliability control systems and procedures. All SiC products must pass 100% Static Parametric Test, 100% Inrush Current Handling Test (I FSM ) and 100% Avalanche Capability Test (UIS). Products meet JEDEC standards or more stringent reliability testing requirements; for example, the HTRB test cycle has been extended from 1000 hours to 3000 hours.
Due to its excellent material properties, silicon carbide Schottky diodes perform far better than silicon PN junction diodes. Combining advanced chip design capabilities and mature manufacturing processes, high-quality SiC Schottky diodes are now manufactured by Vaillant.