Silicon-germanium alloy SiGe
Si1-xGex, a silicon-germanium alloy in any ratio
SiGe is an alloy of silicon and germanium, written Si1-xGex, and it can be made in any ratio of the two (Wikipedia).
Evidence level
2 of 5
Lab-measured
- 0Predicted only. No one has made it, or no measurement exists.
- 1Calculated. Only calculated or predicted values from sources.
- 2Lab-measured. Measured properties with their conditions stated.
- 3Space-relevant tests. Measured in conditions relevant to space, such as vacuum or atomic oxygen.
- 4Ground-qualified. Qualified for an application on the ground.
- 5Flown or field-proven. Flight or field results exist.
Thermoelectric figure of merit
0.75 at 1000 K
Hot junction
1089 to 1255 K
Density in a weight model
4.31 g/cm3
Recipe, share of the weight
- Si 80%Silicon
- Ge 20%Germanium
Best for
High-temperature thermoelectric legs for radioisotope generators, and SiGe transistors for fast RF chips
Weakest at
Thermoelectric efficiency: NASA says the figure of merit of Si-Ge is much lower than that of lead telluride
Risk at a glance
- People Low. Low toxicity for germanium; no SiGe SDS
- If it fails Moderate. Grain growth in nanostructured SiGe
- Corrosion Low. NASA says SiGe is stable in air or vacuum at high temperature.
Quick facts: SiGe
| Property | Value | Basis |
|---|---|---|
| Density | 4.31 g/cm³ | 1 sourced value |
What is the density of Silicon-germanium alloy (SiGe)?
Silicon-germanium alloy (SiGe) has a density of 4.31 g/cm³. The source and test conditions are listed in the properties table on this page.
What is Silicon-germanium alloy (SiGe) used for?
Uses recorded in the sources include SiGe heterojunction bipolar transistors (HBTs) in RF and mixed-signal chips; SiGe strain layer in CMOS transistors; Radioisotope thermoelectric generators (MHW-RTG and GPHS-RTG; a power source, not a battery).
Why it exists
SiGe is the thermoelectric alloy used for deep-space radioisotope generators. NASA calls it a popular choice because it is high-temperature, mechanically robust and stable in air or vacuum. Alloying silicon with germanium scatters phonons and lowers lattice conductivity, which is the reason for the alloy. Its figure of merit is lower than that of lead telluride, but it tolerates hotter hot junctions. The same silicon-germanium material system is used in SiGe HBT transistors.
Where it's used
Radioisotope generators
MHW-RTG units with SiGe couples flew on Voyager 1 and 2, and GPHS-RTG units with the same SiGe technology flew on Galileo, Ulysses, Cassini and New Horizons (Wikipedia).
SiGe transistors
IHP presented a SiGe HBT with fT/fmax of 505 GHz/720 GHz at IEDM 2016. Wikipedia says the HBTs let CMOS logic be integrated into mixed-signal circuits.
Strain layers in CMOS
Wikipedia says SiGe is used as a strain-inducing layer for CMOS transistors.
Recipe
Si1-xGex, a silicon-germanium alloy in any ratio; the thermoelectric composition in the sources read is Si80Ge20
SiGe is an alloy of silicon and germanium in any molar ratio, written Si1-xGex (Wikipedia). The RTG thermoelectric material in the NASA report is Si1-xGex, doped with boron for p-type and with phosphorus for n-type. The 2023 paper's p-type composition is Si80Ge20B2. NASA's report describes powder processing with spark plasma sintering. Traditional samples were solidified and homogenized by zone-levelling (NASA report, citing Dismukes 1964).
| Ingredient | Share of the weight | What it does |
|---|---|---|
| Silicon Si | 80% | Host lattice of the alloy. Phonon scattering from the Si and Ge mass difference lowers lattice conductivity (NASA report, citing Abeles). |
| Germanium Ge | 20% | Alloying element. Its share is the Si80Ge20 composition of the 2023 paper; the alloy is made in any Si-Ge ratio (Wikipedia). |
Weight shares are as printed in the sources.
What it can do
Thermoelectric figure of merit
0.75 at 1000 K
measured
A p-type Si80Ge20B2 sample reached zT of 0.75 at 1000 K, and 0.8 with 4 wt% MnTe (Ivanova et al., 2023).
Hot junction
1089 to 1255 K
measured
NASA's 1968 study used SiGe hot junctions of 1089 K and 1255 K in its generator designs.
Density in a weight model
4.31 g/cm3
measured
NASA's 1968 weight model used 4.31 g/cm3 for Si-Ge elements, against a higher value for lead telluride. The value is a model input, not a measurement.
How it compares
The NASA 1968 study says lead telluride has a higher figure of merit than Si-Ge, but a much higher density than Si-Ge (4.31 g/cm3), and it chose Si-Ge for generators with hotter hot junctions (1089 to 1255 K against 811 K for lead telluride). Against germanium on its own, the sources read here give no direct comparison of thermoelectric behaviour.
The same measurements for each material, so you can compare them. Each row names the key condition under the value; the full conditions and methods are in the numbered notes under Technical details. Other materials are shown beside it; each was measured under its own conditions, so open its page before comparing closely. Compare every grade in the family.
| Measurement | SiGe | Si | Ge | As | B |
|---|---|---|---|---|---|
| Physical and electrical | |||||
| Density assumed for Si-Ge elements in a generator weight modelValue stated in a NASA Lewis analysis | 4.31 g/cm³5 | 2.328 g/cm³ | 5.327 g/cm³ | 5727 kg/m3 | 2.35 g/cm³ |
| Temperature behavior | |||||
| Hot-junction temperature, SiGe stage of a 1968 generator study | 1089 to 1255 K4 | no data | no data | no data | no data |
| Hot-junction temperature, MHW-RTG SiGe couplesValue printed on a reference page (Wikipedia) | 1273 K | no data | no data | no data | no data |
| Cold-junction temperature, MHW-RTG SiGe couplesValue printed on a reference page (Wikipedia) | 573 K | no data | no data | no data | no data |
| Other measurements | |||||
| Thermoelectric figure of merit, p-type Si80Ge20B2Value printed in the paper abstract | 0.75 - | no data | no data | no data | no data |
| Thermoelectric figure of merit, Si80Ge20B2 with 4 wt% MnTeValue printed in the paper abstract | 0.8 - | no data | no data | no data | no data |
| Thermoelectric figure of merit, p-type SiGe (up to) | 0.7 -1 | no data | no data | no data | no data |
| Thermoelectric figure of merit, n-type SiGe (up to) | 1 -2 | no data | no data | no data | no data |
| Seebeck coefficient, p-type Si80Ge20B2 (first value in abstract) | 252 uV/K3 | no data | no data | no data | no data |
| Seebeck coefficient, p-type Si80Ge20B2 with 4 wt% MnTe (second value in abstract)Value printed in the paper abstract | 197 uV/K | no data | no data | no data | no data |
Why choose it, and why not
Good for
- High-temperature thermoelectric legs. NASA and Wikipedia report SiGe couples in RTGs with hot junctions of 1089 to 1273 K.
- Fast RF transistors. IHP reports a SiGe HBT with fT/fmax of 505 GHz/720 GHz.
Watch out for
- Figure of merit: zT is about 0.75 at 1000 K for the p-type sample read, and NASA says it is much lower than lead telluride.
- Germanium supply: about 20 percent of a Si80Ge20 alloy is germanium, and USGS reports high US import reliance.
- Grain growth at high temperature can erase the gain from nanostructuring.
SiGe is well established as a high-temperature thermoelectric alloy for deep-space radioisotope generators, and as the base of SiGe transistors for fast RF chips. Ten values are sourced here, covering zT, Seebeck coefficient, junction temperatures and one design density. Thermal conductivity, melting point and density at a stated composition were not found as printed values in the pages read, and no SiGe-specific hazard data were read.
Technical details
For scientists and researchers. Everything above comes from the record below: every number with its method, its conditions and its source.
Notes
The method, full conditions and any caveat for the numbered rows above.
| # | Measurement | Method | Conditions | Caveat |
|---|---|---|---|---|
| 1 | Thermoelectric figure of merit, p-type SiGe (up to) | Value printed on a reference page (Wikipedia); zT is dimensionless and no unit is printed | temperature 1000 °Cbound up to, as printed | — |
| 2 | Thermoelectric figure of merit, n-type SiGe (up to) | Value printed on a reference page (Wikipedia); zT is dimensionless and no unit is printed | temperature 1000 °Cbound up to, as printed | — |
| 3 | Seebeck coefficient, p-type Si80Ge20B2 (first value in abstract) | Value printed in the paper abstract; the abstract reads it as a fall from 252 to 197 when MnTe is added | composition Si80Ge20B2, pristine, p-typetemperature 1000 K | — |
| 4 | Hot-junction temperature, SiGe stage of a 1968 generator study | Values stated in a NASA Lewis parametric analysis (the study gives 811 K for lead telluride) | basis design hot-junction temperatures in a parametric study, not measured material limits | Design temperatures for a generator model. The property name hot_junction_temperature is not in the canonical list (see gaps). |
| 5 | Density assumed for Si-Ge elements in a generator weight model | — | basis input to a 1968 weight model, not a measurementcomposition not stated | The study used this value for weight only. The Ioffe density fit for Si1-xGex gives a lower value at x = 0.2 (about 3.0 g/cm3 by arithmetic), so do not treat 4.31 as a measured density. |
Corrosion and compatibility
What happens when it meets other materials and environments.
| Environment | Concern | What happens |
|---|---|---|
| Air or vacuum at high temperature (RTG operation) | low | NASA says SiGe is stable in air or vacuum at high temperature.1 |
| Dry oxygen at 850 to 1000 °C (thermal oxidation of Si1-xGex, x = 0.13) | not rated | Ge atoms are pushed out of the oxide at the interface and do not enter the oxide layer.2 |
| Wet and dry oxidation at high temperature | not rated | Wet oxidation of SiGe is faster than for Si, while dry oxidation is about equal or slightly faster.3 |
| # | Environment | Details | Test conditions |
|---|---|---|---|
| 1 | Air or vacuum at high temperature (RTG operation) | The NASA report lists SiGe as a popular RTG choice for high temperature, mechanically robust, stable in air or vacuum, with reasonable ZT. No oxidation rate, mass change or temperature limit is given. | basis qualitative statement in the source; no rate or temperature number printed |
| 2 | Dry oxygen at 850 to 1000 °C (thermal oxidation of Si1-xGex, x = 0.13) | The Osaka study of a fully strained Si0.87Ge0.13 layer on Si oxidized in dry O2 at 850 to 1000 °C. Ordered SiO2 formed, and the authors describe the characteristic of SiGe oxidation as the ejection of Ge atoms from the interface between the surface oxide and the SiGe layer, without their incorporation into the oxide layer. | x 0.13oxidant dry O2temperature 850 to 1000 °C |
| 3 | Wet and dry oxidation at high temperature | The Osaka paper's introduction says that SiGe oxidation is faster than that of Si under wet oxidation conditions, but almost equal or slightly higher under dry oxidation. No rate constants are given in the text read. | basis literature summary in the introduction of the Osaka paper; no rate number read |
Risks and side effects
Hazards to people, what happens when a cell overheats, and how the material is handled and stored.
For people
The only SDS read is for germanium (1989), and it gives low toxicity. No SDS for the SiGe alloy was read.
- Germanium SDS
- The 1989 SDS says germanium is considered to be of low toxicity and may be a mild skin irritant. Inhalation effects are unknown.
- Handling
- Avoid heating germanium with bromine, chlorine, fluorine, oxygen or potassium chlorate, per the same SDS.
If it fails
The failure routes in the sources are grain growth at high temperature and germanium supply.
- Heat
- NASA says thermally induced grain growth can hinder nanostructured SiGe, because the ZT gain from small grains is lost.
- Supply
- USGS reports that US imports of germanium metal fell 67 percent in 2025, to an estimated 7,000 kg, after China's ban on exports to the United States in 2024.
Test details
The original wording and numbers from the sources, for readers who want them.
Failure risks
| Failure | Trigger and consequence |
|---|---|
| Grain growth in nanostructured SiGemoderate severity | Long operation at high temperatureNASA says thermally induced grain growth can hinder the practical use of nano-sized SiGe grains, because the ZT gain from small grains is lost as the grains grow. |
| Germanium supply interruptionmoderate severity | China's ban on germanium exports to the United States in 2024USGS reports that US imports of germanium metal fell 67 percent in 2025 to an estimated 7,000 kg, and that net import reliance was above 50 percent in every year from 2021 to 2025e. Germanium is about 20 percent of a Si80Ge20 alloy, so germanium supply sets the cost and availability of the alloy, while silicon is far more abundant. |
Handling and storage
- The germanium SDS lists germanium as a material that fractures mechanically, so SiGe wafers and ingots should be handled as brittle solids.
- The germanium SDS lists incompatibilities with bromine, chlorine, fluorine, oxygen and potassium chlorate when germanium is heated.
Supply and geography
How common the raw materials are and where they come from.
Silicon (about 80 percent of a Si80Ge20 alloy)
The USGS silicon chapter gives no crustal abundance figure. It says world and domestic resources for making silicon metal are abundant.
- Top producers (2025)
- China 80%
- Critical mineral lists
- US: listed
- Recycling
- USGS silicon chapter: recycling is insignificant.
- Substitutes
- USGS: gallium arsenide and germanium are the principal substitutes for silicon in semiconductor and infrared applications.
Germanium (about 20 percent of a Si80Ge20 alloy)
No abundance figure was read in the germanium chapter. It says resources are linked to zinc and lead-zinc-copper sulfide ores, and that reserves were not widely reported.
- Recycling
- USGS: much germanium removed in machining infrared optics is routinely recycled as new scrap, and germanium wafers used as solar-cell substrates are recycled. The USGS says the information available was inadequate for reliable estimates of secondary germanium.
- Substitutes
- USGS: silicon or gallium arsenide substitute for germanium in certain electronic applications.
Documents
Datasheets, safety data sheets, papers and standards you can open.
Full record
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spore: profile
id: silicon-germanium
spore_version: 0.2
family: semiconductor-materials
name: Silicon-germanium alloy
abbreviation: SiGe
role: semiconductor
category: semiconductors
type: silicon
summary: SiGe is an alloy of silicon and germanium, written Si1-xGex, and it can be made in any ratio of the two (Wikipedia). NASA radioisotope generators on Voyager 1 and 2, Galileo, Ulysses, Cassini and New Horizons used SiGe thermoelectric couples. NASA calls SiGe a popular RTG choice because it is mechanically robust and stable in air or vacuum at high temperature. In a 2023 paper, p-type Si80Ge20B2 reached a figure of merit (zT) of 0.75 at 1000 K, and 0.8 with a MnTe addition. SiGe is also used in heterojunction bipolar transistors (HBTs), and IHP reports a SiGe HBT with fT/fmax of 505 GHz/720 GHz.
best_for: High-temperature thermoelectric legs for radioisotope generators, and SiGe transistors for fast RF chips
weakest_at: 'Thermoelectric efficiency: NASA says the figure of merit of Si-Ge is much lower than that of lead telluride'
identity:
formula: Si1-xGex, a silicon-germanium alloy in any ratio; the thermoelectric composition in the sources read is Si80Ge20
source: wiki-sige
where: Lead section, composition sentence (an alloy of silicon and germanium in any molar ratio)
tags:
environments: []
stresses: []
applications:
- electronics-and-chips
- batteries
properties:
- benchmark: figure_of_merit_zt
label: Thermoelectric figure of merit, p-type Si80Ge20B2
value: 0.75
unit: '-'
scope: material
conditions:
composition: Si80Ge20B2, pristine, p-type
temperature: 1000 K
method: Value printed in the paper abstract
status: ground
source: ivanova2023
where: Abstract, zT values
- benchmark: figure_of_merit_zt
label: Thermoelectric figure of merit, Si80Ge20B2 with 4 wt% MnTe
value: 0.8
unit: '-'
scope: material
conditions:
composition: Si80Ge20B2 plus 4 wt% MnTe, p-type
temperature: 1000 K
method: Value printed in the paper abstract
status: ground
source: ivanova2023
where: Abstract, zT values
- benchmark: figure_of_merit_zt
label: Thermoelectric figure of merit, p-type SiGe (up to)
value: 0.7
unit: '-'
scope: material
conditions:
temperature: 1000 °C
bound: up to, as printed
method: Value printed on a reference page (Wikipedia); zT is dimensionless and no unit is printed
status: ground
source: wiki-sige
where: Thermoelectric application section, p-SiGe bullet
- benchmark: figure_of_merit_zt
label: Thermoelectric figure of merit, n-type SiGe (up to)
value: 1
unit: '-'
scope: material
conditions:
temperature: 1000 °C
bound: up to, as printed
method: Value printed on a reference page (Wikipedia); zT is dimensionless and no unit is printed
status: ground
source: wiki-sige
where: Thermoelectric application section, n-SiGe bullet
- benchmark: seebeck_coefficient
label: Seebeck coefficient, p-type Si80Ge20B2 (first value in abstract)
value: 252
unit: uV/K
scope: material
conditions:
composition: Si80Ge20B2, pristine, p-type
temperature: 1000 K
method: Value printed in the paper abstract; the abstract reads it as a fall from 252 to 197 when MnTe is added
status: ground
source: ivanova2023
where: Abstract, Seebeck coefficient sentence
- benchmark: seebeck_coefficient
label: Seebeck coefficient, p-type Si80Ge20B2 with 4 wt% MnTe (second value in abstract)
value: 197
unit: uV/K
scope: material
conditions:
composition: Si80Ge20B2 plus 4 wt% MnTe, p-type
temperature: 1000 K
method: Value printed in the paper abstract
status: ground
source: ivanova2023
where: Abstract, Seebeck coefficient sentence
- benchmark: hot_junction_temperature
label: Hot-junction temperature, SiGe stage of a 1968 generator study
value: 1089
value_high: 1255
bound: range
unit: K
scope: material
conditions:
basis: design hot-junction temperatures in a parametric study, not measured material limits
method: Values stated in a NASA Lewis parametric analysis (the study gives 811 K for lead telluride)
status: ground
note: Design temperatures for a generator model. The property name hot_junction_temperature is not in the canonical list (see gaps).
source: nasa-tmx1453
where: Summary, sentence on hot-junction temperatures for lead telluride and Si-Ge
- benchmark: hot_junction_temperature
label: Hot-junction temperature, MHW-RTG SiGe couples
value: 1273
unit: K
scope: material
conditions:
device: MHW-RTG, Voyager era
couples: 312 SiGe thermoelectric couples
method: Value printed on a reference page (Wikipedia)
status: ground
source: wiki-rtg-app
where: MHW-RTG section, SiGe couples paragraph
- benchmark: cold_junction_temperature
label: Cold-junction temperature, MHW-RTG SiGe couples
value: 573
unit: K
scope: material
conditions:
device: MHW-RTG, Voyager era
couples: 312 SiGe thermoelectric couples
method: Value printed on a reference page (Wikipedia)
status: ground
source: wiki-rtg-app
where: MHW-RTG section, SiGe couples paragraph
- benchmark: density
label: Density assumed for Si-Ge elements in a generator weight model
value: 4.31
unit: g/cm3
scope: material
conditions:
basis: input to a 1968 weight model, not a measurement
composition: not stated
method: Value stated in a NASA Lewis analysis
status: ground
note: The study used this value for weight only. The Ioffe density fit for Si1-xGex gives a lower value at x = 0.2 (about 3.0 g/cm3 by arithmetic), so do not treat 4.31 as a measured density.
source: nasa-tmx1501
where: Weight analysis paragraph, sentence on the Si-Ge element density
corrosion:
- environment: Air or vacuum at high temperature (RTG operation)
headline: NASA says SiGe is stable in air or vacuum at high temperature.
effect: The NASA report lists SiGe as a popular RTG choice for high temperature, mechanically robust, stable in air or vacuum, with reasonable ZT. No oxidation rate, mass change or temperature limit is given.
rating: low
conditions:
basis: qualitative statement in the source; no rate or temperature number printed
source: nasa-wsi2
where: Silicon Germanium Alloys slide, Popular choice for RTG systems bullet
- environment: Dry oxygen at 850 to 1000 °C (thermal oxidation of Si1-xGex, x = 0.13)
headline: Ge atoms are pushed out of the oxide at the interface and do not enter the oxide layer.
effect: The Osaka study of a fully strained Si0.87Ge0.13 layer on Si oxidized in dry O2 at 850 to 1000 °C. Ordered SiO2 formed, and the authors describe the characteristic of SiGe oxidation as the ejection of Ge atoms from the interface between the surface oxide and the SiGe layer, without their incorporation into the oxide layer.
rating: unknown
conditions:
x: 0.13
oxidant: dry O2
temperature: 850 to 1000 °C
source: osaka-prb2010
where: Introduction and oxidation results, paragraph on Ge ejection from the interface
- environment: Wet and dry oxidation at high temperature
headline: Wet oxidation of SiGe is faster than for Si, while dry oxidation is about equal or slightly faster.
effect: The Osaka paper's introduction says that SiGe oxidation is faster than that of Si under wet oxidation conditions, but almost equal or slightly higher under dry oxidation. No rate constants are given in the text read.
rating: unknown
conditions:
basis: literature summary in the introduction of the Osaka paper; no rate number read
source: osaka-prb2010
where: Introduction, paragraph comparing wet and dry oxidation
risks:
hazards:
- text: The only SDS read for this profile is for germanium (Pure Tech, revised 10/89). It says germanium is considered to be of low toxicity and may be a mild skin irritant. Inhalation effects are listed as unknown. No SDS for a SiGe alloy was read for this profile.
ghs: []
source: ge-msds
where: Section 4, Health hazard data
failure_risks:
- mode: Grain growth in nanostructured SiGe
trigger: Long operation at high temperature
consequence: NASA says thermally induced grain growth can hinder the practical use of nano-sized SiGe grains, because the ZT gain from small grains is lost as the grains grow.
severity: moderate
source: nasa-wsi2
where: Enhancing Si/Ge slide, last bullet
- mode: Germanium supply interruption
trigger: China's ban on germanium exports to the United States in 2024
consequence: USGS reports that US imports of germanium metal fell 67 percent in 2025 to an estimated 7,000 kg, and that net import reliance was above 50 percent in every year from 2021 to 2025e. Germanium is about 20 percent of a Si80Ge20 alloy, so germanium supply sets the cost and availability of the alloy, while silicon is far more abundant.
severity: moderate
source: usgs-ge-2026
where: Events, Trends, and Issues; Salient Statistics, net import reliance row
side_effects: []
handling:
- text: The germanium SDS lists germanium as a material that fractures mechanically, so SiGe wafers and ingots should be handled as brittle solids.
source: ge-msds
where: Protection section, MECHANICAL (General) line
- text: The germanium SDS lists incompatibilities with bromine, chlorine, fluorine, oxygen and potassium chlorate when germanium is heated.
source: ge-msds
where: Section 6, Reactivity data, incompatibility line
mitigations: []
uses:
- application: electronics-and-chips
device: SiGe heterojunction bipolar transistors (HBTs) in RF and mixed-signal chips
example: IHP presented a SiGe HBT with fT/fmax of 505 GHz/720 GHz at the 2016 International Electron Devices Meeting (IEDM).
source: ihp-2016
where: Press release title and first paragraph
- application: electronics-and-chips
device: SiGe strain layer in CMOS transistors
example: Wikipedia says SiGe is used as a strain-inducing layer for CMOS transistors, and that HBTs allow CMOS logic to be integrated into mixed-signal circuits.
source: wiki-sige
where: Lead section, Uses paragraph
- application: batteries
device: Radioisotope thermoelectric generators (MHW-RTG and GPHS-RTG; a power source, not a battery)
example: MHW-RTG units with SiGe couples flew on Voyager 1 and 2 (launched 1977). GPHS-RTG units with the same SiGe technology flew on Galileo (October 18, 1989), Ulysses (October 6, 1990), Cassini (October 15, 1997) and New Horizons (January 19, 2006).
source: wiki-rtg-app
where: Application history, MHW-RTG and GPHS-RTG subsections
- application: batteries
device: SiGe thermoelectric legs in radioisotope generators (power source, not a battery)
example: NASA's report on SiGe with tungsten silicide calls SiGe a popular choice for RTG systems, citing its high temperature, mechanical robustness and stability in air or vacuum.
source: nasa-wsi2
where: Silicon Germanium Alloys slide, first bullet group
supply:
- subject: Silicon (about 80 percent of a Si80Ge20 alloy)
abundance: The USGS silicon chapter gives no crustal abundance figure. It says world and domestic resources for making silicon metal are abundant.
producers:
- country: China
share_percent: 80
year: 2025
us_position: 'USGS: China accounted for almost 80 percent of estimated global production of silicon materials in 2025 (this figure covers all silicon materials, not silicon metal alone). World production of silicon metal in 2025 was 4,600 thousand t, with China 4,000, Brazil 180, Norway 130 and Russia 35 (thousand t).'
critical_lists:
- list: USGS 2025 List of Critical Minerals (silicon added in 2025)
listed: true
recycling: 'USGS silicon chapter: recycling is insignificant.'
substitutes:
- 'USGS: gallium arsenide and germanium are the principal substitutes for silicon in semiconductor and infrared applications.'
source: usgs-si-2026
where: World Production table (silicon metal, 2024 and 2025 columns); Salient Statistics; Recycling; Substitutes; Final 2025 critical list paragraph
- subject: Germanium (about 20 percent of a Si80Ge20 alloy)
abundance: No abundance figure was read in the germanium chapter. It says resources are linked to zinc and lead-zinc-copper sulfide ores, and that reserves were not widely reported.
producers: []
us_position: 'USGS: US imports of germanium metal were 22,000 kg in 2023, 21,000 kg in 2024 and an estimated 7,000 kg in 2025e. Import sources 2021 to 2024 for metal: China 41 percent, Belgium 27 percent, Germany 25 percent. Net import reliance was above 50 percent in each year from 2021 to 2025e.'
recycling: 'USGS: much germanium removed in machining infrared optics is routinely recycled as new scrap, and germanium wafers used as solar-cell substrates are recycled. The USGS says the information available was inadequate for reliable estimates of secondary germanium.'
substitutes:
- 'USGS: silicon or gallium arsenide substitute for germanium in certain electronic applications.'
source: usgs-ge-2026
where: Salient Statistics; Import Sources (2021 to 2024); Recycling; Substitutes
media: []
documents:
- title: Ivanova et al., p-type SiGe-based composite produced by mechanical alloying and spark plasma sintering, Materials Letters 357 (2023), 135746 (NIMS repository record)
type: paper
url: https://mdr.nims.go.jp/datasets/8ac554e7-8275-4a96-90f5-99a2ea57ef18
year: 2023
- title: 'Mackey, Sehirlioglu and Dynys, WSi2 in Si1-xGex composites: processing and thermoelectric properties (NASA NTRS 20150002719)'
type: report
url: https://ntrs.nasa.gov/citations/20150002719
year: 2015
- title: Parametric analysis of radioisotope-thermoelectric generators, NASA Lewis TM X-1453 (1968)
type: report
url: https://ntrs.nasa.gov/citations/19680005161
year: 1968
- title: Ward and Ruch, Parametric analysis of radioisotope cascaded thermoelectric generators, NASA TM X-1501 (1968)
type: report
url: https://ntrs.nasa.gov/citations/19680005116
year: 1968
- title: Shimura et al., Residual order in the thermal oxide of a fully strained SiGe alloy on Si, Physical Review B 81, 033308 (2010)
type: paper
url: https://hdl.handle.net/11094/85489
year: 2010
- title: USGS Mineral Commodity Summaries 2026, Silicon
type: report
url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-silicon.pdf
year: 2026
- title: USGS Mineral Commodity Summaries 2026, Germanium
type: report
url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-germanium.pdf
year: 2026
- title: Pure Tech Material Safety Data Sheet, Germanium (last revision 10/89)
type: sds
url: https://cores.research.utdallas.edu/files/2023/04/germanium.pdf
year: 1989
gaps:
- 'Thermal conductivity of SiGe at a stated composition and temperature: not found in a page read for this profile. The Basu et al. 2014 RSC page returned 403, the AIP Advances 2019 record on DOAJ returned a bot check, and the Inorganics 2025 MDPI paper returned 403. The Materials Letters abstract gives no absolute thermal conductivity.'
- 'Melting point of SiGe: the Ioffe page gives solidus Ts = 1412 - 738x + 263x2 degC and liquidus Tl = 1412 - 80x - 395x2 degC (Stohr and Klemm 1954). These are formulas, not values at a stated composition, so no value is given. The Ioffe page was read over plain HTTP because its HTTPS certificate failed.'
- 'Density of SiGe at a stated composition: the Ioffe fit (2.329 + 3.493x - 0.499x2 g/cm3 at 300 K) was read but is a formula. The only density value given is the 1968 weight-model input of 4.31 g/cm3, which does not match the fit.'
- Thermal-conductivity formula on the Ioffe page (0.046 + 0.084x W/cm/K) gives values that do not match the pure-Si and pure-Ge entries printed on the same page (1.3 and 0.58 W/cm/K), so it was not used.
- 'Band gap, electron and hole mobility, lattice constant, dielectric constant and elastic constants of SiGe at a stated composition: not read as printed values in this run. Wikipedia gives only the qualitative statement that SiGe has a lower band gap than silicon.'
- 'Hazard data: the PubChem GHS entry for germanium returned a browser check and could not be read. No SDS for a SiGe alloy was read. The germanium SDS is dated 1989 and lists no exposure limit.'
- 'Occupational exposure limits (OSHA PEL, NIOSH REL, ACGIH TLV) for germanium or SiGe: not found. The NIOSH search returned only germanium tetrahydride.'
- 'Critical-mineral status of germanium on the 2025 USGS list: not confirmed in the germanium chapter read for this profile. Silicon is confirmed as added to the 2025 list in the USGS silicon chapter. EU critical-materials status: not checked.'
- 'Crustal abundance of silicon and germanium: not found in the USGS chapters read.'
- 'Dopant levels and growth conditions for the RTG couples: the NASA report says p-type is doped with boron and n-type with phosphorus, and that traditional samples were solidified and homogenized by zone-levelling (citing Dismukes 1964). No dopant level or growth parameter is given.'
- 'Conflict in sources: the Wikipedia RTG page says the n-leg is doped with boron and the p-leg with phosphorus. The NASA report says n-type with P and p-type with B. This profile follows the NASA report.'
- 'Radiation, vacuum and flight data for the SiGe couples in the RTGs: not checked in this profile. Voyager power status in 2026 was not verified.'
- 'Property names not in spores/property-canon.yaml and used here: figure_of_merit_zt, seebeck_coefficient, hot_junction_temperature, cold_junction_temperature.'
- 'Type: the taxonomy has a SiGe type under semiconductors, which fits better than silicon. The profile uses silicon, as the brief asked, to match germanium.yaml.'
sources:
ivanova2023:
title: Ivanova et al., p-type SiGe-based composite produced by mechanical alloying and spark plasma sintering, Materials Letters 357 (2023), 135746
url: https://mdr.nims.go.jp/datasets/8ac554e7-8275-4a96-90f5-99a2ea57ef18
kind: paper
license: In Copyright (repository record); values cited, not reproduced
wiki-sige:
title: Wikipedia, Silicon-germanium (composition, uses, thermoelectric and RTG sections)
url: https://en.wikipedia.org/wiki/Silicon%E2%80%93germanium
kind: web
license: page content, cited as published
wiki-rtg-app:
title: Wikipedia, Application of silicon-germanium thermoelectrics in space exploration
url: https://en.wikipedia.org/wiki/Application_of_silicon-germanium_thermoelectrics_in_space_exploration
kind: web
license: page content, cited as published
nasa-wsi2:
title: 'Mackey, Sehirlioglu and Dynys, WSi2 in Si1-xGex composites: processing and thermoelectric properties (NASA NTRS 20150002719)'
url: https://ntrs.nasa.gov/citations/20150002719
kind: report
license: public domain (NASA)
nasa-tmx1453:
title: Parametric analysis of radioisotope-thermoelectric generators, NASA Lewis TM X-1453 (1968)
url: https://ntrs.nasa.gov/citations/19680005161
kind: report
license: public domain (NASA)
nasa-tmx1501:
title: Ward and Ruch, Parametric analysis of radioisotope cascaded thermoelectric generators, NASA TM X-1501 (1968)
url: https://ntrs.nasa.gov/citations/19680005116
kind: report
license: public domain (NASA)
osaka-prb2010:
title: Shimura et al., Residual order in the thermal oxide of a fully strained SiGe alloy on Si, Physical Review B 81, 033308 (2010)
url: https://hdl.handle.net/11094/85489
kind: paper
license: Copyright 2010 American Physical Society (repository record); values cited, not reproduced
ihp-2016:
title: IHP press release, fastest silicon-based transistor (IEDM 2016, SiGe HBT with fT/fmax of 505 GHz/720 GHz)
url: https://nachrichten.idw-online.de/2016/12/05/ihp-presents-the-fastest-silicon-based-transistor-in-the-world
kind: web
license: page content, cited as published
usgs-si-2026:
title: USGS Mineral Commodity Summaries 2026, Silicon
url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-silicon.pdf
kind: report
license: public domain (USGS)
usgs-ge-2026:
title: USGS Mineral Commodity Summaries 2026, Germanium
url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-germanium.pdf
kind: report
license: public domain (USGS)
ge-msds:
title: Pure Tech Material Safety Data Sheet, Germanium (revision 10/89)
url: https://cores.research.utdallas.edu/files/2023/04/germanium.pdf
kind: datasheet
license: supplier document, linked not reproduced
license: CC-BY-4.0
checked_values: 10
updated: '2026-10-26'
review:
status: draft
method: assistant
checked_values: 10
note: Written by an AI assistant on 2026-10-26 from pages read for this profile. Ten property values are sourced. Thermal conductivity, melting point and density at a stated composition were not found as printed values.
story:
risk_groups:
- title: For people
lead: The only SDS read is for germanium (1989), and it gives low toxicity. No SDS for the SiGe alloy was read.
level: 1
verdict: Low toxicity for germanium; no SiGe SDS
points:
- label: Germanium SDS
line: The 1989 SDS says germanium is considered to be of low toxicity and may be a mild skin irritant. Inhalation effects are unknown.
- label: Handling
line: Avoid heating germanium with bromine, chlorine, fluorine, oxygen or potassium chlorate, per the same SDS.
- title: If it fails
lead: The failure routes in the sources are grain growth at high temperature and germanium supply.
points:
- label: Heat
line: NASA says thermally induced grain growth can hinder nanostructured SiGe, because the ZT gain from small grains is lost.
- label: Supply
line: USGS reports that US imports of germanium metal fell 67 percent in 2025, to an estimated 7,000 kg, after China's ban on exports to the United States in 2024.
recipe:
line: SiGe is an alloy of silicon and germanium in any molar ratio, written Si1-xGex (Wikipedia). The RTG thermoelectric material in the NASA report is Si1-xGex, doped with boron for p-type and with phosphorus for n-type. The 2023 paper's p-type composition is Si80Ge20B2. NASA's report describes powder processing with spark plasma sintering. Traditional samples were solidified and homogenized by zone-levelling (NASA report, citing Dismukes 1964).
parts:
- name: Silicon
symbol: Si
share_percent: 80
role: Host lattice of the alloy. Phonon scattering from the Si and Ge mass difference lowers lattice conductivity (NASA report, citing Abeles).
- name: Germanium
symbol: Ge
share_percent: 20
role: Alloying element. Its share is the Si80Ge20 composition of the 2023 paper; the alloy is made in any Si-Ge ratio (Wikipedia).
why: SiGe is the thermoelectric alloy used for deep-space radioisotope generators. NASA calls it a popular choice because it is high-temperature, mechanically robust and stable in air or vacuum. Alloying silicon with germanium scatters phonons and lowers lattice conductivity, which is the reason for the alloy. Its figure of merit is lower than that of lead telluride, but it tolerates hotter hot junctions. The same silicon-germanium material system is used in SiGe HBT transistors.
uses:
- where: Radioisotope generators
line: MHW-RTG units with SiGe couples flew on Voyager 1 and 2, and GPHS-RTG units with the same SiGe technology flew on Galileo, Ulysses, Cassini and New Horizons (Wikipedia).
- where: SiGe transistors
line: IHP presented a SiGe HBT with fT/fmax of 505 GHz/720 GHz at IEDM 2016. Wikipedia says the HBTs let CMOS logic be integrated into mixed-signal circuits.
- where: Strain layers in CMOS
line: Wikipedia says SiGe is used as a strain-inducing layer for CMOS transistors.
can_do:
- label: Thermoelectric figure of merit
value: 0.75 at 1000 K
benchmark: figure_of_merit_zt
line: A p-type Si80Ge20B2 sample reached zT of 0.75 at 1000 K, and 0.8 with 4 wt% MnTe (Ivanova et al., 2023).
- label: Hot junction
value: 1089 to 1255 K
benchmark: hot_junction_temperature
line: NASA's 1968 study used SiGe hot junctions of 1089 K and 1255 K in its generator designs.
- label: Density in a weight model
value: 4.31 g/cm3
benchmark: density
line: NASA's 1968 weight model used 4.31 g/cm3 for Si-Ge elements, against a higher value for lead telluride. The value is a model input, not a measurement.
compare: The NASA 1968 study says lead telluride has a higher figure of merit than Si-Ge, but a much higher density than Si-Ge (4.31 g/cm3), and it chose Si-Ge for generators with hotter hot junctions (1089 to 1255 K against 811 K for lead telluride). Against germanium on its own, the sources read here give no direct comparison of thermoelectric behaviour.
compare_rows:
- label: Density
benchmark: density
- label: Thermoelectric figure of merit at 1000 K
benchmark: figure_of_merit_zt
- label: Hot-junction temperature
benchmark: hot_junction_temperature
handling:
- Treat germanium-containing SiGe as brittle. The germanium SDS lists mechanical fracture.
- Keep it away from bromine, chlorine, fluorine, oxygen and potassium chlorate when heated, per the germanium SDS.
- Check a SiGe-specific SDS before handling, because none was read for this alloy.
good_for:
- High-temperature thermoelectric legs. NASA and Wikipedia report SiGe couples in RTGs with hot junctions of 1089 to 1273 K.
- Fast RF transistors. IHP reports a SiGe HBT with fT/fmax of 505 GHz/720 GHz.
watch_out:
- 'Figure of merit: zT is about 0.75 at 1000 K for the p-type sample read, and NASA says it is much lower than lead telluride.'
- 'Germanium supply: about 20 percent of a Si80Ge20 alloy is germanium, and USGS reports high US import reliance.'
- Grain growth at high temperature can erase the gain from nanostructuring.
verdict: SiGe is well established as a high-temperature thermoelectric alloy for deep-space radioisotope generators, and as the base of SiGe transistors for fast RF chips. Ten values are sourced here, covering zT, Seebeck coefficient, junction temperatures and one design density. Thermal conductivity, melting point and density at a stated composition were not found as printed values in the pages read, and no SiGe-specific hazard data were read.
References
Every number above carries a reference number. Click one to see the source and where in it the value comes from.