Differences Between Semiconductor Substrate And Epitaxy

1. Substrate

1. Definition and function

·Physical support: The substrate is the carrier of the semiconductor device, usually a circular or square single crystal wafer (such as a silicon wafer).

·Crystal template: Provides a template for atomic arrangement for epitaxial layer growth to ensure that the epitaxial layer is consistent with the substrate crystal structure (homoepitaxial) or matches (heteroepitaxial).

·Electrical basis: Part of the substrate directly participates in the conduction of the device (such as silicon-based power devices), or acts as an insulator to isolate the circuit (such as a sapphire substrate).

2. Comparison of mainstream substrate materials

3. Core considerations for substrate selection

· Lattice matching: reduce epitaxial layer defects (e.g. GaN/sapphire lattice mismatch reaches 13%, requiring a buffer layer).

· Thermal expansion coefficient matching: avoid stress cracking caused by temperature changes.

· Cost and process compatibility: For example, silicon substrates dominate the mainstream due to mature processes.

2. Epitaxial Layer

1. Definition and Purpose

Epitaxial growth: Deposition of single crystal thin film on the substrate surface by chemical or physical methods, with the atomic arrangement strictly aligned with the substrate.

Core Functions:

• Improve material purity (the substrate may contain impurities).
• Build heterogeneous structures (such as GaAs/AlGaAs quantum wells).
• Isolate substrate defects (such as micropipe defects on SiC substrates).

2. Classification of Epitaxial Technology

3. Key parameters of epitaxial layer design

• Thickness: from a few nanometers (quantum well) to tens of microns (epilayer of power devices).
• Doping: Precisely control the carrier concentration by doping impurities such as phosphorus (N-type) and boron (P-type).
• Interface quality: Lattice mismatch needs to be alleviated by buffer layer (such as GaN/AlN) or strained superlattice.

4. Challenges and solutions of heteroepitaxial growth

• Lattice mismatch:
• Gradient buffer layer: Gradually change the composition from substrate to epitaxial layer (such as AlGaN gradient layer).
• Low-temperature nucleation layer: Grow thin layers at low temperature to reduce stress (such as low-temperature AlN nucleation layer of GaN).
• Thermal mismatch: Select a combination of materials with similar thermal expansion coefficients, or use a flexible interface design.

3. Synergistic application cases of substrate and epitaxy

Case 1: GaN-based LED

Substrate: sapphire (low cost, insulation).

Epitaxial structure:

• Buffer layer (AlN or low-temperature GaN) → Reduce lattice mismatch defects.
• N-type GaN layer → Provide electrons.
• InGaN/GaN multi-quantum well → Light-emitting layer.
• P-type GaN layer → Provide holes.

Result: Defect density is as low as 10⁸ cm⁻², and luminous efficiency is significantly improved.

Case 2: SiC power MOSFET

Substrate: 4H-SiC single crystal (withstand voltage up to 10 kV).

Epitaxial layer:

• N-type SiC drift layer (thickness 10-100 μm) → withstand high voltage.
• P-type SiC base region → control channel formation.

Advantages: 90% lower on-resistance than silicon devices, 5 times faster switching speed.

Case 3: Silicon-based GaN RF devices

Substrate: High-resistance silicon (low cost, easy to integrate).

Epitaxial layer:

• AlN nucleation layer → alleviates the lattice mismatch between Si and GaN (16%).
• GaN buffer layer → captures defects and prevents them from extending to the active layer.
• AlGaN/GaN heterojunction → forms a high electron mobility channel (HEMT).

Application: 5G base station power amplifier, with a frequency of more than 28 GHz.