The SiC wafer application fields are mainly divided into the electronic power field, the radio frequency field, the photoelectric field, and other fields. Among them, the electronic power field and the radio frequency field are the most important applications, and the advantages of silicon carbide wafer usage are obvious. The article mainly introduces the reason of the SiC wafer application in radio frequency devices.
1. GaN HEMT Devices on the SiC Wafer Application in the 5G Base Station
Currently, a power amplifier (PA for short) used in base stations mainly adopts a silicon-based laterally diffused metal oxide semiconductor (LDMOS) technology. 5G base station AAU adopts Massive MIMO (massive multiple input multiple output) technology, resulting in increased equipment power.
LDMOS technology has limitations in high-frequency applications: the bandwidth of LDMOS power amplifiers will be greatly reduced as the frequency increases, LDMOS is only effective in the frequency range of 3.5GHz. Therefore, the performance of LDMOS in the 3.5GHz band has begun to decline significantly.
In addition, the AAU power of 5G base stations has been greatly increased, and the single-sector power has increased from about 50W in the 4G period to about 200W in the 5G period. The traditional LDMOS process is difficult to meet the performance requirements. The current PA market, including those used in base stations and mobile phones, the manufacturing process mainly includes traditional LDMOS, GaAs, GaN.
With the development of semiconductor material technology, gallium nitride (GaN) is becoming the main technical route for PA in the mid-high frequency bands. The advantages of GaN technology include energy efficiency improvement, wider bandwidth, greater power density, and smaller volume, making it a successful replacement for LDMOS.
GaAs has a microwave frequency and a working voltage of 5V to 7V, and has been widely used in PA for many years. The silicon-based LDMOS technology has a working voltage of 28V and has been used in the telecommunications field for many years, and it mainly plays a role in frequencies below 4GHz. But it is not widely used in broadband applications. By contrast, GaN has an operating voltage of 28V to 50V, with a higher power density and cut-off frequency, and can achieve a highly integrated solution in MIMO applications.
Massive-MIMO antennas require devices to be miniaturized. The size of devices made of GaN is 1/6 to 1/4 of the LDMOS size. Compared with LDMOS, GaN can increase the power by 4 to 6 times per unit area.
The application of high-frequency and high-power parts is the dominant field of the third-generation semiconductor GaN. GaN HMET devices on SiC substrate can be used.
2. Why Choose Silicon Carbide Substrate?
Each index of the substrate materials, such as the surface roughness, the coefficient of thermal expansion, the coefficient of thermal conductivity, and the degree of lattice matching with the epitaxial material, has a profound impact on producing the devices. The performance requirements and explanations to be investigated for qualified substrate materials are shown in the following figure:
|Substrate material performance requirements||Explanation|
|Good crystal structure characteristics||The epitaxial material and the substrate have the same or similar crystal structure; small lattice constant mismatch, good crystal performance, and low defect density|
|Good interface characteristics||Conducive to the nucleation of epitaxial materials and strong adhesion|
|Good chemical stability||It is not easy to decompose and corrode in the temperature and atmosphere of epitaxial growth.|
|Good thermal performance||The thermal conductivity is good and the thermal mismatch is small. The matching of the thermal expansion coefficient between the bottom and the epitaxial film is very important. If there is too much difference, the quality of the epitaxial film will decrease.|
|Good conductivity||Up and down structure can be made.|
|Good optical performance||The light emitted by the fabricated device is less absorbed by the substrate.|
|Good processability||The device is easy to process, including thinning, polishing and cutting, etc.|
|Low price||The development of industrialization requires that the cost should not be too high.|
|Large size||Strands require a diameter not less than 2 inches|
3. Comparison for sapphire, silicon and silicon carbide
Mismatch. For GaN lattice mismatch rate, sapphire is 13.9%, silicon is 16.9%, and silicon carbide is only 3.4%. The thermal mismatch rate of sapphire is 30.3%, that of Si is 53.5%, and only 15.9% for SiC single crystal. Therefore, in terms of crystal structure characteristics, the crystal structure of 4H-SiC and 6H-SiC and GaN are both wurtzite structures, with the lowest lattice mismatch rate and thermal mismatch rate. Thus, the application of SiC wafer is for growing high-quality GaN epitaxial layers.
Conductivity. Sapphire is insulating, and it cannot make vertical devices.
Thermal conductivity. The thermal conductivity of sapphire is only 0.3W·cm-1·K-1, and the thermal conductivity of silicon is 1.48W·cm-1·K-1, which is far lower than that of silicon carbide 3.4W·cm-1 · K-1.
Optical performance. Both sapphire and silicon carbide do not absorb visible light, the Si substrate absorbs light seriously, and the efficiency of the LED light output is low.
In summary, there are many advantages for growing gallium nitride on silicon carbide substrates. Due to the excellence of the silicon carbide properties, the SiC wafer application is widely.