What are the differences between silicon-based gallium nitride epitaxial wafers and sapphire epitaxi
1. Core foundation: Differences in substrate material properties
| Characteristic parameters | Silicon substrate (Si) | Sapphire substrate (Al₂O₃) | GaN epitaxial layer (reference) | Core Impact |
|---|---|---|---|---|
| Crystal structure | face-centered cubic (diamond type) | Hexagonal wurtzite | Hexagonal wurtzite | Sapphire is closer in structure to GaN, while silicon and GaN have very different structures, which can easily lead to defects in the epitaxial layer. |
| Lattice constant (a-axis) | 5.43 Å | 4.76 Å | 4.51 Å | The lattice mismatch rate between sapphire and GaN is ≈5.5%, and the mismatch rate between silicon and GaN is ≈20%. The greater the mismatch, the more epitaxial defects there are. |
| Thermal expansion coefficient (25℃) | 2.6 × 10⁻⁶ /℃ | 7.5 × 10⁻⁶ /℃ | 5.6 × 10⁻⁶ /°C (c-axis) | The thermal mismatch rate between silicon and GaN is ≈54%, and that between sapphire is ≈34%. Thermal mismatch can easily lead to cracking or residual stress in epitaxial wafers. |
| Thermal conductivity | ~150 W/(m·K) | ~40 W/(m·K) | ~200 W/(m·K) | Silicon has a much better heat dissipation capability than sapphire and is more suitable for high-power devices. |
| Electrical properties | Semiconductor (conductive, supports n/p type doping) | Insulators (resistivity > 10¹⁴ Ω・cm) | Semiconductors (wide bandgap, mainly n-type) | Silicon substrate can be used as a "conductive substrate", while sapphire can only be used as an "insulating substrate", which directly affects the device structure design. |
| Commercial size | 6-12 inches (mature silicon wafer industry chain) | 2-8 inches (mainstream 4-6 inches) | - | Silicon substrates can reuse large-scale semiconductor production capacity, with significant cost advantages; large-scale sapphire preparation is difficult and costly. |
| Mechanical hardness | Mohs hardness 6.5 | Mohs hardness 9 (second only to diamond) | Mohs hardness 5.5 | Sapphire processing is difficult, while silicon wafer cutting/polishing is more mature. |
2. Differences in epitaxial growth processes
1. GaN epitaxy on sapphire
Simple buffer layer design: Only a 10-20nm low-temperature GaN buffer layer is needed to alleviate the lattice/thermal mismatch with sapphire and reduce dislocation defects.
Stable growth temperature: Sapphire is resistant to high temperatures (melting point 2050℃), and higher growth temperatures (1050-1100℃) can be used, which promotes the densification of GaN crystals and higher quality epitaxial layers (dislocation density can be reduced to below 10⁸ cm⁻²).
No conductive treatment required: Sapphire insulation, the epitaxial layer is directly used for insulating substrate devices after growth, without the need for additional insulation process.
2. GaN-on-silicon epitaxy
Complex buffer layer design：Need to useAlGaN graded buffer layer(Al composition from high to low) orSuperlattice buffer layer(AlGaN/GaN alternating growth), through "step-by-step stress release" to reduce the huge mismatch between silicon and GaN, otherwise the epitaxial wafer will be prone to cracking.
Stress control is difficult: During the growth process, temperature, pressure and gas flow need to be precisely controlled to avoid warping or cracking of the epitaxial layer due to thermal mismatch (the thicker the silicon wafer, the more difficult it is to control stress).
Compatible with silicon process：Existing silicon-based semiconductor production lines (such as photolithography and etching) can be used, but the "heterogeneous integration" compatibility of GaN and silicon (such as interface reaction and doping diffusion) needs to be resolved.
3. Key Performance Comparison
| Performance Dimension | GaN-on-silicon epitaxial wafers | GaN epitaxial wafers on sapphire |
|---|---|---|
| Epitaxial layer quality (dislocation density) | High (10⁹-10¹⁰ cm⁻², depends on buffer layer optimization) | Lower (10⁸-10⁹ cm⁻², more complete crystals) |
| heat dissipation capacity | Excellent (silicon substrate directly conducts heat, suitable for high power) | Poor (sapphire has poor thermal conductivity and requires additional heat dissipation design) |
| Conductive properties | Supports vertical conduction (substrate can be doped to n-type) | Only supports lateral conduction (substrate insulation) |
| Device integration | High (compatible with silicon-based CMOS process, enabling heterogeneous integration) | Low (sapphire is not compatible with silicon processes, making integration difficult) |
| cost | Low (large-size silicon wafer industry chain is mature and mass production costs are low) | High (sapphire material and processing costs are high, and size is limited) |
4. Differences in Application Scenario
1. GaN epitaxial wafers on sapphire: the "absolute workhorse" of optoelectronic devices
LED field: More than 90% of blue-green LEDs (such as lighting and display backlights) use sapphire-based GaN epitaxial wafers. The crystal integrity directly determines the internal quantum efficiency (luminous brightness) of the LED.
UV optoelectronic devices: Deep ultraviolet LEDs (sterilization, water purification), ultraviolet lasers (lidar, lithography), the wide bandgap characteristics of sapphire can prevent the substrate from absorbing ultraviolet light.
Other optoelectronic devices：Photodetectors, solar cells (substrates for high-efficiency III-V stacked cells).
2. GaN-on-Silicon Epitaxial Wafers: The Future Trend for Power Electronics and RF Devices
Power electronics field: Fast charging power supply modules (such as mobile phone PD fast charging), new energy vehicle OBC (on-board charger), inverter (photovoltaic power station), silicon-based GaN's low on-resistance and high switching speed can reduce energy consumption by more than 30%.
RF device field: 5G base station RF power amplifiers (PAs) and satellite communication devices, the high integration of silicon-based components can reduce device size and lower system costs.
Other fields: Millimeter-wave radar (autonomous driving), aerospace high-reliability power modules.
5. Summary: Core Differences and Selection Logic
| Dimensions | GaN-on-silicon epitaxial wafers | GaN epitaxial wafers on sapphire |
|---|---|---|
| Core Advantages | Low cost, good heat dissipation, compatible with silicon process, large size | High crystal quality, good insulation, suitable for optoelectronic devices |
| Core Shortcomings | High epitaxial defects and difficult stress control | High cost, poor heat dissipation, and limited size |
| Core Applications | Power electronics (fast charging, in-vehicle), radio frequency (5G) | Optoelectronic devices (LEDs, UV devices) |
| Technology maturity | Rapidly developing (commercialization in 5-10 years) | Highly mature (commercially available for 20+ years) |
Extension: Future Development Direction
Sapphire base: Develop towards larger size (8 inches) and thinner substrate (laser lift-off technology, improved heat dissipation) to consolidate the advantages of optoelectronic devices.
GaN on silicon: Focus on breakthroughs in "defect density reduction" and "12-inch large-size epitaxial" technologies, aiming to replace some silicon-based power devices (such as MOSFET) and seize trillion-level markets such as new energy and 5G.
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