Neutron radiation presents unique challenges and opportunities in semiconductor materials, particularly in applications involving nuclear reactors, space electronics, and radiation-hardened systems. The interaction of neutrons with semiconductors leads to two primary effects: bulk damage due to atomic displacements and transmutation doping through nuclear reactions. Understanding these phenomena is critical for optimizing material performance in high-radiation environments. This article examines neutron-induced damage mechanisms, compares the neutron tolerance of silicon (Si), germanium (Ge), and compound semiconductors, and discusses transmutation doping effects based on experimental data from nuclear reactor tests.

Neutron interactions with semiconductors occur primarily through elastic and inelastic scattering, resulting in atomic displacements that create lattice defects. The extent of damage depends on neutron energy, fluence, and the material's displacement threshold energy. Fast neutrons (E > 1 MeV) are particularly damaging, as they transfer sufficient kinetic energy to knock atoms from their lattice sites, generating vacancy-interstitial pairs (Frenkel defects). Over time, these defects aggregate into clusters, dislocations, or other extended defects that degrade electrical properties by acting as carrier traps or recombination centers.

Transmutation doping occurs when neutrons are absorbed by the semiconductor nucleus, leading to nuclear reactions that produce dopant atoms. For example, thermal neutrons (E < 1 eV) interact with Si-30 (4.67% natural abundance) via the (n,γ) reaction, producing Si-31, which decays to phosphorus (P-31) with a half-life of 2.62 hours. This process introduces n-type doping uniformly throughout the material. Similarly, Ge-74 (36.5% natural abundance) undergoes neutron capture to form Ge-75, which decays to arsenic (As-75), another n-type dopant. The doping concentration depends on neutron fluence, isotopic abundance, and cross-section values.

Silicon exhibits moderate neutron tolerance due to its relatively high displacement threshold energy (12–15 eV) and stable diamond cubic structure. Studies show that Si retains functionality up to neutron fluences of 10^15–10^16 n/cm², beyond which carrier lifetime degradation becomes severe. At 10^17 n/cm², resistivity increases by orders of magnitude due to defect clustering. However, Si benefits from well-established defect engineering techniques, such as oxygen precipitation gettering, which mitigates damage effects.

Germanium, with a lower displacement threshold energy (~9 eV), is more susceptible to neutron damage than Si. At equivalent fluences, Ge experiences higher defect densities and faster carrier lifetime degradation. For instance, a fluence of 10^14 n/cm² reduces minority carrier lifetime in Ge by 50%, whereas Si requires 10^15 n/cm² for comparable degradation. However, Ge's higher atomic number enhances gamma radiation shielding, making it useful in mixed radiation environments.

Compound semiconductors, such as GaAs, GaN, and SiC, exhibit varied responses to neutron irradiation. GaAs suffers severe lattice disorder at fluences above 10^14 n/cm² due to its lower displacement energy (~10 eV) and ionic bonding. In contrast, GaN and SiC demonstrate superior radiation hardness, withstanding fluences up to 10^17 n/cm² before significant property degradation. This resilience stems from their high bond strength (GaN: ~9.1 eV/atom; SiC: ~11 eV/atom) and defect-tolerant crystal structures. SiC, in particular, maintains stable operation at high temperatures and radiation levels, making it ideal for nuclear and aerospace applications.

Transmutation doping rates vary significantly among materials. In Si, a thermal neutron fluence of 10^17 n/cm² produces a phosphorus concentration of ~10^15 cm⁻³, assuming a capture cross-section of 0.11 barns for Si-30. For Ge, the same fluence yields arsenic concentrations of ~10^16 cm⁻³ due to Ge-74's higher cross-section (0.5 barns). Compound semiconductors like GaAs and SiC have negligible transmutation doping effects because their constituent isotopes either lack suitable neutron capture reactions or produce non-dopant elements.

The following table summarizes key parameters for neutron tolerance and transmutation doping in selected semiconductors:

Material | Displacement Energy (eV) | Critical Fluence (n/cm²) | Transmutation Dopant | Doping Efficiency (cm⁻³ per n/cm²)
Si | 12–15 | 10^16 | P | ~10^-2
Ge | ~9 | 10^14 | As | ~10^-1
GaAs | ~10 | 10^14 | None | N/A
GaN | ~9.1 | 10^17 | None | N/A
SiC | ~11 | 10^17 | None | N/A

Radiation hardening strategies include defect engineering, doping optimization, and material selection. Silicon remains the most practical choice for low-to-medium radiation environments due to its balanced performance and manufacturability. For extreme conditions, wide bandgap materials like SiC and GaN offer superior resilience but face higher production costs and integration challenges. Future research should focus on defect dynamics in emerging materials, such as ultra-wide bandgap oxides and 2D semiconductors, to expand the radiation-hardened electronics portfolio.

In conclusion, neutron-induced damage and transmutation doping are critical considerations for semiconductor applications in radiation-intensive settings. While Si and Ge serve as benchmarks, compound semiconductors like SiC and GaN provide unparalleled radiation tolerance. The choice of material depends on the specific radiation environment, performance requirements, and economic constraints. Advances in defect characterization and mitigation will further enhance the viability of semiconductors in nuclear and space technologies.