Germanium (Ge) is, alongside silicon, one of the classic elemental semiconductors.

It was once the material behind the very first transistor, opening the door to the semiconductor era — and in recent years, it has been showing signs of a comeback as a material once again drawing attention.

・Chemical symbol: Ge

・Atomic number: 32

・Crystal structure: Diamond-type structure (same as silicon)

・Band gap: 1.1 eV (narrower than silicon’s)

・Electron mobility: 3,900 cm²/V·s (about 2.7 times that of silicon)

・Hole mobility: 1,900 cm²/V·s (about 4 times that of silicon)

Because both electrons and holes can move quickly through it, germanium is well suited to high-speed devices and high-sensitivity detection.

・In 1947, the world’s first transistor was built from germanium at Bell Labs.

・During the 1950s, it was used in radios, amplifiers, and early computers.

・However, its instability at high temperatures — due to increasing leakage current — led silicon to take over as the dominant material from the 1960s onward.

Even so, germanium retained its core strengths: fast response and low noise.

・Its narrow band gap allows electrons to become excited with relatively little energy.

・This makes it well suited to high-speed switching and high-sensitivity sensors.

・On the other hand, rising temperatures cause an excess of carriers to be generated, leading to unstable operation.

These properties make germanium best suited to low-temperature, high-precision applications.

1.Optical communications Germanium transmits infrared (IR) light very well. It is combined with materials such as InGaAs and silicon to build photodetectors for fiber-optic communications.

2.Infrared detection and thermal imaging Because it can absorb long-wave infrared light in the 8–14μm range, it is used in thermal cameras and nighttime surveillance equipment.

3.High-performance CMOS technology (Si-Ge based) Forming a germanium layer on a silicon substrate improves electron mobility, benefiting communication ICs, RF devices, and high-speed CPUs.

4.Solar cells (high-efficiency types) Germanium is used as a substrate for multi-junction cells (such as GaInP/GaAs/Ge stacks), which are used in satellites and space-based solar cells.

Combining silicon and germanium allows their respective strengths to be leveraged together.

Characteristics:

・High-speed operation (from germanium) combined with stability (from silicon)

・Improved carrier mobility through controlled strain engineering

・Compatibility with existing silicon manufacturing processes

Applications:

・High-frequency communication ICs (RF CMOS)

・Next-generation logic devices (FinFET, GAAFET)

・Sensor elements (infrared, pressure)

As a result, Si-Ge is being re-evaluated as a partner material for strengthening silicon rather than replacing it.

・Increased leakage current at high temperatures

・Germanium substrates are expensive, and large-diameter wafers remain scarce

・Its oxide layer is of lower quality than SiO₂

・Achieving compatibility with existing process flows remains difficult

That said, these challenges are gradually being overcome through techniques such as epitaxial growth on silicon and Germanium-on-Insulator technology.

・GeSn (germanium-tin) alloys: Converts germanium into a direct-gap material, opening the door to optical devices.

・Germanium quantum dots: Applied in infrared detection and quantum information devices.

・Optoelectronic integrated chips: Integration with silicon photonics.

Germanium is drawing renewed attention as a material capable of combining high speed with strong optical response.

・A low-loss, high-sensitivity device material well suited to the coming era of 6G/7G communications.

・Potential applications in optical transmission and optical computing within AI chips.

・High-performance detection elements for extreme environments, including aerospace, defense, and medical applications.

Rather than reclaiming its former role as the star material, germanium is emerging as a bridge to the next generation of technology.

・Germanium is an elemental semiconductor excelling in speed, sensitivity, and infrared response.

・Though once the leading material, it was largely displaced by silicon’s superior stability.

・It is now drawing renewed attention through Si-Ge alloys and optical applications.

・As a material strong in speed and light, germanium remains indispensable to future technologies.

1.In what ways does germanium outperform silicon?

2.Where does germanium fall short?

3.What germanium applications are drawing attention today?

 

 

Column Supervisor: Koji Kakumoto (Otis Group Co., Ltd.)

After studying abroad and working in planning and development at a trading company, he joined Otis Group Co., Ltd. in 2011. While primarily working in the Corporate Planning Department, he has also served concurrently in manufacturing and technical divisions, and since 2018 has served as Representative Director, working to drive business growth and strengthen the organization.

 

This article is a general technical explanation intended for educational purposes and does not refer to any specific company, product, or technology.