Silicon And Germanium (SiGe)

• The manufacturing process of silicon and germanium materials

1. Raw material and substrate selection

Silicon-germanium alloy layers are grown on a high quality silicon substrate using chemical vapor deposition (CVD) or molecular beam epitaxy (MBE) techniques.

The germanium content (X) can be precisely regulated (usually GE accounts for 10%-30%), forming a SI₁₋ₓGEₓ gradient or uniform structure.

2. Epitaxial growth technology

CVD method: the mainstream process, by heating the silicon substrate and introducing SIH₄ and GEH₄ mixed gas, the silicon germanium layer is decomposed and deposited at high temperature (500-800℃), which requires precise control of germanium doping concentration and epitaxial layer thickness.

MBE method: atomic layer growth in ultra-high vacuum environment, suitable for the preparation of ultra-thin heterojunction structures (such as SIGE/SI quantum well).

3. Doping and strain engineering

Doping boron (B) to form P-type silicon germanium, or phosphorus (P), arsenic (AS) to form N-type silicon germanium, the element adjusts the electrical conductivity and optimizes the carrier mobility to meet the requirements of high frequency devices.

Using the characteristic that the radius of germanium atom is larger than that of silicon, strain is introduced when growing SIGE layer on silicon substrate to improve electron mobility (strain silicon technology).

• Material properties

1. Electrical properties

High electron mobility: when the GE content is 30%, the mobility can reach 1500 CM²/ (V·S) (pure silicon is 1400 CM²/ (V·S)), suitable for high frequency transistors, millimeter wave communication (such as 5G band above 28 GHZ) and high speed data processing.

Low power consumption and low noise: SIGE devices consume 30% to 50% less power than traditional silicon devices at the same frequency, while having a lower noise factor, suitable for RF front-end modules

Variable band gap width: the band gap width decreases from 1.12 EV (pure silicon) to 0.66 EV (pure germanium) with the increase of GE content, expanding the range of light absorption.

Low saturation speed: high frequency response needs to be optimized through heterojunction design.

2. Mechanical and thermal properties

Strain characteristics: relieve the lattice mismatch between silicon and germanium and silicon substrate (about 4.2%), and avoid dislocation defects.

Thermal conductivity: between silicon (180W/ (M·K)) and germanium (59.9 W/ (M·K)), supports the heat dissipation requirements of high power density devices

3. Optical properties

It has enhanced light absorption in the mid-infrared (3-5ΜM) and is suitable for optical detectors and modulators.

The high absorption rate of near infrared light (1.3-1.55 ΜM) makes it widely used in optical fiber communication and lidar (LIDAR).

4. Technical requirements

Doping ratio of germanium: it should be controlled at 10%-30% to achieve the best performance balance and avoid defects caused by lattice mismatch.

Interface quality: the defect density of heterojunction interface should be less than 10⁴ CM⁻² to ensure device reliability.

• Industry applications

1. High frequency electronic devices

SIGE HBT (heterojunction bipolar transistor): used in power amplifiers (PA) for 5G base stations and radar to achieve high gain (>20 DB) and low noise (noise factor <1 DB).

RF CMOS Integration: Combining silicon and germanium with complementary metal oxide semiconductor (CMOS) to improve the performance of radio frequency integrated circuits (RFIC).

2. Photoelectric devices

GE/SIGE Photoelectric Detector: A high-speed response (> 10 GB/S) is achieved in optical fiber communication (1.3-1.55ΜM band) for the absorber layer of the photoelectric detector.

Optical communication: Silicon-germanium photodetectors are used in optical modules for data centers to improve data transmission rates (e.g., 400G/800G)

Silicon-based laser: an epitaxial layer for a laser that uses the SIGE/SI quantum well structure to break through the indirect band gap of silicon and emit light.

3. Stress silicon technology

High performance logic chips: Introduce SIGE stress layer in FINFET or nanosheet transistors to improve carrier mobility and switching speed (such as INTEL 14NM process).

4. Artificial intelligence and edge computing

AI Chip: The high frequency and low power consumption characteristics of SIGE are suitable for the signal processing requirements of AI accelerator, which is suitable for real-time decision-making systems of autonomous driving and intelligent terminals.

Sensors: Short wave infrared (SWIR) sensors use SIGE epitaxial layers to enhance the environmental perception accuracy of autonomous vehicles

• Technical index advantages

1. High frequency performance: In the millimeter wave band (24-100 GHZ), SIGE devices have better output power and efficiency than gallium arsenide (GAAS) with lower cost; the cutoff frequency (Fₜ) can reach more than 300 GHZ, better than pure silicon devices, meeting the requirements of 5G millimeter wave (24-100 GHZ).
2. Compatible with silicon process: compatible with existing silicon-based CMOS process, which can be directly integrated into the existing silicon-based production line to reduce manufacturing costs.
3. Stress engineering potential: the stress can be precisely controlled by the gradient design of germanium content to optimize device performance.
4. Optoelectronic integration: combining the low cost of silicon with the photoelectric properties of germanium to promote the development of silicon photonic chips.
5. Multi-scene adaptability: high temperature resistance (working temperature can reach more than 200℃) and radiation resistance, suitable for aerospace and industrial extreme environment.

• Future trends

Technology integration: Develop hybrid devices with gallium nitride (GAN) to break through the bottleneck of high frequency and high power applications.

Market expansion: The global silicon and germanium materials market is expected to exceed $5 billion by 2030, with the main growth coming from 5G, AI and autonomous driving.

Policy drive: China’s export control of germanium materials accelerates the technological upgrading of local industrial chain and promotes the domestic substitution of silicon and germanium

With high mobility, adjustable band gap width and compatibility with silicon process, silicon germanium has become the core material of high frequency electronic and optoelectronic devices. In the future, SIGE (such as GESN) or germanium/III-V compound heterojunctions with higher germanium content in optical interconnection and advanced logic chips will further expand the application boundary.