Silicon carbide (SiC) is a widely used semiconductor material known for its exceptional properties. It is commonly used in various applications due to its high thermal conductivity, wide bandgap, and excellent mechanical strength. However, there are different polytypes of SiC, including 4H SiC and 6H-SiC, which possess unique characteristics. In this article, we will explore the difference between 4H SiC and 6H-SiC, highlighting their crystal structures, properties, and applications.
Overview of Silicon Carbide
Silicon carbide is a compound composed of silicon and carbon atoms. It is a covalent material with a chemical formula SiC. Silicon carbide exists in various crystal structures, known as polytypes, with the most common ones being 3C, 4H, and 6H. These polytypes differ in their stacking sequences and arrangements of atoms, leading to variations in their physical and electrical properties.
Structure of Silicon Carbide
The crystal structure of silicon carbide determines its properties and performance. Both 4H SiC and 6H-SiC belong to the hexagonal crystal system. The difference lies in their stacking sequences. In 4H SiC, the layers are stacked in an ABCB sequence, while in 6H-SiC, the stacking sequence is ABABAB. This variation in stacking leads to differences in the symmetry, lattice constants, and electrical properties of these polytypes.
Types of Silicon Carbide
Silicon carbide is available in different types based on the number of layers in its crystal structure. Commonly used types include 3C, 4H, 6H, and 15R SiC. Among these, 4H SiC and 6H-SiC are widely studied and utilized for various semiconductor applications. Both types exhibit excellent material properties, but their specific characteristics set them apart.
Difference between 4H SiC and 6H-SiC
Crystal Structure
The crystal structure is the primary distinction between 4H SiC and 6H-SiC. As mentioned earlier, 4H SiC has an ABCB stacking sequence, resulting in a higher symmetry compared to 6H-SiC’s ABABAB stacking. This difference in symmetry affects the crystal growth process, resulting in variations in defect densities and crystal quality.
Physical Properties
In terms of physical properties, both 4H SiC and 6H-SiC exhibit similar characteristics. They possess high hardness, excellent thermal conductivity, and exceptional chemical resistance. However, due to the difference in crystal structure, 4H SiC has a higher thermal conductivity along the c-axis, while 6H-SiC shows higher thermal conductivity in the basal plane. This distinction makes each polytype suitable for specific applications requiring heat dissipation in different directions.
Electrical Properties
The electrical properties of 4H SiC and 6H-SiC also differ due to their crystal structures. 4H SiC has a higher electron mobility compared to 6H-SiC, making it ideal for high-frequency and high-power devices. On the other hand, 6H-SiC exhibits a lower concentration of deep-level defects, making it suitable for applications requiring high-quality substrates with low carrier recombination rates.
Applications
Both 4H SiC and 6H-SiC find applications in various fields. The unique properties of these polytypes make them ideal for different semiconductor devices. 4H SiC is commonly used in high-power electronic devices, such as MOSFETs, Schottky diodes, and bipolar junction transistors. It is also utilized in microwave applications, UV light-emitting diodes (LEDs), and radiation detectors. 6H-SiC, on the other hand, is preferred for applications that require high-quality substrates, including epitaxial growth and fabrication of electronic devices.
Comparison of 4H SiC and 6H-SiC
In summary, the main differences between 4H SiC and 6H-SiC are in their crystal structures, physical properties, and electrical properties. 4H SiC exhibits higher thermal conductivity along the c-axis, higher electron mobility, and is suitable for high-power applications. 6H-SiC, with its lower defect density and lower carrier recombination rates, is more appropriate for high-quality substrate applications. The choice between the two polytypes depends on the specific requirements of the semiconductor device and its intended application.
Conclusion
Silicon carbide, with its unique properties and crystal structures, offers a wide range of possibilities for semiconductor applications. Understanding the difference between 4H SiC and 6H-SiC is essential for choosing the appropriate polytype for specific device requirements. Both polytypes have their strengths and are suitable for different applications within the semiconductor industry. Whether it’s high-power electronics or high-quality substrates, silicon carbide continues to pave the way for technological advancements.
FAQs
Q1: Are 4H SiC and 6H-SiC the only polytypes of silicon carbide?
A: No, silicon carbide has several polytypes, but 4H SiC and 6H-SiC are the most commonly studied and utilized for semiconductor applications.
Q2: Can 4H SiC and 6H-SiC be used interchangeably in all applications?
A: No, the choice between 4H SiC and 6H-SiC depends on the specific requirements of the semiconductor device and its intended application. The differences in their crystal structures and properties make each polytype suitable for different applications.
Q3: Which polytype of silicon carbide has higher thermal conductivity?
A: The thermal conductivity of silicon carbide depends on the direction. 4H SiC has higher thermal conductivity along the c-axis, while 6H-SiC exhibits higher thermal conductivity in the basal plane.
Q4: What are some common applications of 4H SiC?
A: 4H SiC is commonly used in high-power electronic devices, such as MOSFETs, Schottky diodes, and bipolar junction transistors. It is also utilized in microwave applications, UV LEDs, and radiation detectors.
Q5: What are the advantages of using 6H-SiC as a substrate material?
A: 6H-SiC exhibits a lower defect density and lower carrier recombination rates, making it suitable for applications requiring high-quality substrates, epitaxial growth, and fabrication of electronic devices.
