Drawing No. EH–MT–013 // Engineering Materials
Germanium Properties
Reviewed September 2026
Germanium is a dense, brittle elemental semiconductor with lower thermal conductivity and lower melting temperature than silicon.
Representative properties
The stated basis, temperature and direction are part of the value. Switch units without changing the physical reference.
What changes these properties?
Engineering variability
- Doping and temperature affect thermal/electronic properties.
- Fracture strength is not a stable bulk constant for polished wafers or optical components.
- Thermal conductivity falls strongly with temperature and defect concentration.
- The [100] elastic values should not be used blindly for other crystal orientations.
Selection note
Used in infrared optics, radiation detectors, semiconductor devices and specialized substrates where its electronic/optical properties justify cost and brittleness.
Where a property is shown as qualitative rather than numeric, that is intentional: the selected source or the material physics does not justify a single comparable scalar.
Germanium in practice
The first semiconductor, displaced from electronics by silicon, and now specified almost entirely for its optical behaviour.
Where it is specified
Infrared optics is the dominant use: germanium is transparent from about 2 to 14 µm and is the standard window and lens material for thermal imaging cameras, FLIR systems and CO&sub2; laser optics.
Also used in fibre-optic core doping (germanium dioxide raises refractive index), high-efficiency multi-junction solar cells for satellites where it serves as both substrate and bottom junction, SiGe heterojunction transistors for RF front ends, and gamma-ray spectrometry detectors — high-purity germanium gives the best available energy resolution.
Chosen over what
Against silicon in electronics: higher carrier mobility, but a narrow 0.66 eV bandgap gives high leakage current that rises steeply with temperature, and germanium dioxide is water-soluble and useless as a passivation layer. Those two facts ended germanium's dominance in the 1960s.
Against zinc selenide for infrared optics: germanium has higher refractive index (about 4.0) and better thermal conductivity, but is opaque in the visible, so ZnSe is preferred where visible alignment is needed. Germanium's very high index requires antireflection coating — an uncoated surface reflects over 35%.
Against chalcogenide glass: germanium has better transmission and thermal conductivity; the glasses are cheaper and mouldable into aspheres.
How it fails
Thermal runaway in optics. Germanium's absorption rises sharply with temperature above roughly 60–80 °C, and since absorbed energy raises temperature further, a high-power CO&sub2; laser window can enter runaway and be destroyed. This is a real operational limit, not a theoretical one.
Brittleness and cleavage — it is a brittle diamond-cubic crystal like silicon and fractures readily under point loading or thermal shock.
High leakage at temperature in any electronic use, which is why germanium detectors for gamma spectrometry must be cooled to liquid nitrogen temperature.
Cost and supply. Germanium is a by-product of zinc refining and coal fly ash with a small, concentrated supply chain, and has been subject to export controls — a procurement risk as much as a technical one.
Fabrication notes
Single-point diamond turning works well on germanium, which is one of its advantages: aspheric and diffractive surfaces can be cut directly rather than ground and polished, and this is routine for thermal imaging lenses.
Antireflection coating is effectively mandatory given the refractive index; diamond-like carbon coatings are common on exterior surfaces for durability.
Crystal growth is by Czochralski for optical and electronic grades; detector-grade material requires zone refining to impurity levels around one part in 1012, among the purest materials routinely manufactured.
Handling requires care with toxicity of some germanium compounds, though the bulk metal is not considered hazardous.
Specifications and standards
ASTM F389 for germanium resistivity; MIL-G-174 for optical germanium; ISO 10110 for optical element drawings; ASTM E1926 and IEEE 325 for germanium radiation detector characterisation.
References
Sze and Ng, Physics of Semiconductor Devices; Umicore and II-VI/Coherent optical germanium technical data; Knoll, Radiation Detection and Measurement for HPGe detectors.
How to interpret this material
Stiffness
Young’s modulus describes elastic stiffness. For crystals and composites, orientation can make one scalar modulus only a directional reference.
Strength / failure
Ductile metals can use yield and tensile strength. Brittle ceramics/semiconductors and anisotropic composites require fracture, flexural or directional strength descriptions instead.
Thermal response
Conductivity, heat capacity and expansion are temperature dependent. The last column is a melting or transition descriptor only where that physical concept is appropriate.
Technical sources
The database favors manufacturer, industry, government or academic technical references and labels secondary condition-specific datasets when a primary source does not expose a complete property set.