Boron Arsenide (BAs)

Boron arsenide (also called arsenic boride) is a chemical compound of boron and arsenic, usually written BAs. It is a III–V semiconductor. The source dossier on boron arsenide treats three distinct forms of the material, which should not be conflated:

• Cubic boron arsenide (c-BAs) — the zinc blende (sphalerite) form, the main subject of the thermal-conductivity and semiconductor literature.
• Boron subarsenide (B₁₂As₂) — a rhombohedral icosahedral boride, a wide-bandgap semiconductor.
• Hexagonal boron arsenide (h-BAs) — an ultrathin nanosheet allotrope reported in 2025.

Claims about one form do not transfer to the others. The thermal-conductivity record, in particular, belongs to c-BAs.

Cubic boron arsenide (c-BAs)

Cubic BAs has a zinc blende crystal structure in space group F4̄3m (cF8, No. 216), with a lattice constant of a = 0.4777 nm and four formula units per cell. Each boron atom is tetrahedrally bonded to four arsenic atoms and vice versa. It is an indirect-band-gap semiconductor with a band gap of 1.82 eV.

The dossier reproduces the Wikipedia infobox for c-BAs verbatim:

Chemical formula     BAs
Molar mass           85.733 g/mol
Appearance           Brown cubic crystals
Density              5.22 g/cm3
Melting point        1,100 °C (2,010 °F; 1,370 K) decomposes
Solubility in water  Insoluble
Band gap             1.82 eV
Thermal conductivity 1300 W/(m·K) (300 K)
Crystal structure    Cubic (sphalerite), cF8, No. 216
Space group          F43m
Lattice constant     a = 0.4777 nm
Formula units (Z)    4
CAS Number           12005-70-8

Basic measured properties reported in the dossier (from the arXiv preprint by Kang, Li, Wu, Nguyen and Hu):

• Optical band gap: 1.82 eV
• Optical refractive index: 3.29 (at 657 nm)
• Elastic modulus: 326 GPa (twice that of silicon)
• Linear thermal expansion coefficient: 3.85×10⁻⁶/K (close to GaN)
• Thermal conductivity: 1300 W/(m·K) at 300 K (defect-free crystals)

The dossier's Wikipedia excerpt reports that cubic BAs decomposes to the subarsenide B₁₂As₂ above 920 °C. The excerpt is internally inconsistent on melting point: the infobox says 1,100 °C (decomposes) while the prose says 2076 °C. This inconsistency is carried in the source and is not resolved by the dossier.

Chemical synthesis of cubic BAs is described as very challenging, and single crystals usually contain defects. The dossier's central scientific narrative is that these defects — specifically antisite pairs — limited early thermal-conductivity measurements, and that purer synthesis has since produced values approaching the theoretical prediction. See Ultrahigh Thermal Conductivity in Boron Arsenide for the measurement history and the theory-versus-measurement reconciliation, and Antisite Defects in Boron Arsenide.

Boron subarsenide (B₁₂As₂)

B₁₂As₂ is an icosahedral boride belonging to space group R3̄m with a rhombohedral structure (hR42, No. 166) based on clusters of boron atoms and two-atom As–As chains. The dossier reproduces its Wikipedia infobox verbatim:

Chemical formula     B12As2
Molar mass           279.58 g/mol
Density              3.56 g/cm3
Solubility in water  Insoluble
Band gap             3.47 eV
Crystal structure    Rhombohedral, hR42, No. 166
Space group          R3m
Lattice constant     a = 0.6149 nm, b = 0.6149 nm, c = 1.1914 nm,
                     α = 90°, β = 90°, γ = 120°
Formula units (Z)    6

It is a wide-bandgap semiconductor (3.47 eV) with a reported ability to "self-heal" radiation damage. It can be grown on substrates such as silicon carbide, and has been proposed for solar-cell fabrication, though the dossier notes it is not currently used for that purpose. Patent CN115259174 describes preparing B₁₂As₂ nanocrystal powder from arsenic trioxide, boron oxide and magnesium via a mechanochemical reaction — a patent reference, which under the wiki's Patent as Evidence convention is a claim of method, not proof of performance.

Hexagonal boron arsenide (h-BAs)

A 2025 Nature Communications paper (Zenghui Wu et al.) reports synthesis of ultrathin crystalline hexagonal boron arsenide nanosheets in large quantities via an in-situ chemical reaction of sodium borohydride with elemental arsenic in a low-pressure hydrogen atmosphere. The h-BAs-based memory devices showed ON/OFF current ratios up to 10⁹, energy consumption below 4.65 pJ, and good stability; flexible h-BAs memristors were also demonstrated. These are device-level results for h-BAs specifically and should not be read as properties of c-BAs.

Semiconductor properties

BAs is notable for high ambipolar mobility — high mobility for both electrons and holes — unlike silicon, which has high electron mobility but low hole mobility. Reported values include:

• Intrinsic electron mobility of 1,400 cm²/V·s and hole mobility of 2,110 cm²/V·s (attributed to UC Santa Barbara), per the dossier.
• An experimentally confirmed ambipolar mobility of 1600 cm²/V·s reported in Advanced Science.
• The Wikipedia excerpt states more generally that BAs has high electron and hole mobility, >1000 cm²/V/second.

See Ambipolar Mobility in Boron Arsenide.

The material's thermal expansion coefficient is close to GaN's, and a GaN-on-BAs structure shows a thermal boundary conductance of 250 MW/m²·K, higher than GaN-on-diamond. The dossier reports that GaN–BAs heterostructures outperform the best GaN HEMT devices on silicon carbide or diamond substrates. These are dossier assertions; the source text does not supply the underlying measurement conditions.

Related pages

• Ultrahigh Thermal Conductivity in Boron Arsenide
• Four-Phonon Scattering
• Antisite Defects in Boron Arsenide
• Isotope Enrichment and Thermal Conductivity
• Ambipolar Mobility in Boron Arsenide
• Boron Arsenide Claims vs. Evidence: Measured Values, Patent Claims and the Theory–Experiment Gap
• the source archive
• Patent as Evidence