What Is Germanium?

Germanium is a semiconductor element with atomic number 32 and atomic mass 72.63, positioned in Group 14 of the periodic table between silicon and tin. It appears as a lustrous gray-white metalloid, brittle at room temperature, with a melting point of 938 degrees Celsius and a density of 5.32 g/cm³. Germanium's band gap of 0.67 electron volts at room temperature is narrower than silicon's 1.12 eV, which gives it higher intrinsic carrier density and superior electron and hole mobility but limits high-temperature device operation. Electron mobility in germanium (approximately 3,900 cm²/V·s) and hole mobility (approximately 1,900 cm²/V·s) are several times larger than the corresponding silicon values.

The element was discovered in 1886 by Clemens Winkler, who isolated it from the mineral argyrodite. Winkler's discovery confirmed a prediction Dmitri Mendeleev had made in 1871 as a gap in his periodic table, which he called "eka-silicon." Germanium's historical importance to electronics is immense: the first point-contact transistor, demonstrated by John Bardeen and Walter Brattain at Bell Labs on December 16, 1947, used a germanium substrate. That device opened the transistor era. As documented in E.E. Haller's history of germanium at Lawrence Berkeley National Laboratory, silicon subsequently displaced germanium in mainstream integrated circuits during the 1960s because of silicon's larger band gap, lower cost, and the existence of a stable native oxide amenable to planar processing.

Epitaxial Growth and Semiconductor Thin Films

High-quality germanium for device applications is grown as single-crystal wafers using the Czochralski method or by epitaxial deposition onto silicon substrates. Epitaxial growth techniques, including molecular beam epitaxy (MBE) and chemical vapor deposition (CVD), allow precisely controlled films of germanium or germanium alloys to be deposited one atomic layer at a time. Because germanium and silicon have a 4.2 percent lattice mismatch, epitaxially grown germanium films on silicon are initially strained, and the strain can be engineered to modify carrier transport properties. Strain-induced enhancement of electron mobility in germanium channels has made strained Ge an attractive option for high-performance p-channel field-effect transistors (pFETs) in advanced complementary metal-oxide-semiconductor (CMOS) nodes beyond 10 nm. The ScienceDirect overview of silicon-germanium alloys details the range of epitaxial architectures used to integrate germanium into silicon-based manufacturing processes. Buffer layer techniques, including graded SiGe virtual substrates, are used to accommodate the lattice mismatch and achieve low threading dislocation densities in the final germanium film.

Silicon-Germanium Integration

The ability to alloy germanium with silicon produces silicon-germanium (SiGe) alloys with bandgap and lattice parameters that vary continuously between the two elemental end points. This tunability underpins the silicon-germanium heterojunction bipolar transistor (HBT), first reported in the 1980s, which achieves cutoff frequencies exceeding 500 GHz in advanced processes. SiGe BiCMOS technology, which integrates SiGe HBTs alongside CMOS logic on the same substrate, is the dominant platform for radio-frequency integrated circuits in mobile phones, GPS receivers, and wireless LAN chipsets. Germanium is also regaining prominence in photonics: germanium-on-silicon photodetectors and electro-optic modulators operate in the near-infrared wavelengths used by optical fiber data links, enabling CMOS-compatible silicon photonics transceivers. The ScienceDirect overview of Si-Ge alloys summarizes the breadth of device architectures enabled by controlled germanium content in silicon-based processes.

Applications

Germanium has applications in a wide range of disciplines, including:

• High-speed heterojunction bipolar transistors for RF and millimeter-wave circuits
• Silicon-germanium photonic detectors and modulators for optical interconnects
• Infrared optics and thermal imaging lenses
• High-efficiency multijunction solar cells in space photovoltaics
• Gamma-ray and neutron detectors for nuclear and medical imaging
• Thermoelectric devices for waste heat recovery