Aluminum nitride (AlN) is a III-nitride semiconductor with a direct wide bandgap of approximately 6.2 eV at room temperature. This property makes it highly suitable for applications in deep-ultraviolet optoelectronics, high-power electronics, and high-frequency devices. The electronic properties of AlN are primarily governed by its band structure, carrier transport mechanisms, and the ability to modify its bandgap through alloying, strain, and doping. Understanding these properties is essential for optimizing its performance in advanced semiconductor technologies.

The bandgap of AlN is one of the largest among III-nitride semiconductors, contributing to its high breakdown electric field, which exceeds 10 MV/cm. This characteristic is critical for high-voltage and high-power applications, as it allows devices to operate at larger electric fields without undergoing avalanche breakdown. The wide bandgap also results in a low intrinsic carrier concentration, reducing leakage currents at elevated temperatures. The conduction band minimum of AlN is located at the Γ-point, making it a direct bandgap material, which is advantageous for optoelectronic applications such as light-emitting diodes (LEDs) and laser diodes in the ultraviolet spectrum.

Bandgap engineering in AlN is achieved through several approaches, including alloying with other nitrides, applying strain, and intentional doping. Alloying AlN with gallium nitride (GaN) to form AlGaN is a common method to tune the bandgap across a wide range, from 6.2 eV (AlN) to 3.4 eV (GaN). The bandgap of AlGaN follows a nonlinear relationship with composition, described by the equation Eg(x) = x·Eg(AlN) + (1−x)·Eg(GaN) − b·x·(1−x), where b is the bowing parameter. For AlGaN, the bowing parameter is typically around 1.0 eV, though reported values vary slightly depending on growth conditions and measurement techniques. This tunability enables the design of heterostructures with tailored electronic and optical properties for specific applications.

Strain also plays a significant role in modifying the electronic properties of AlN. Biaxial strain, commonly induced by lattice mismatch in epitaxial growth on substrates such as sapphire or silicon carbide, shifts the band edges and alters the bandgap. Tensile strain reduces the bandgap, while compressive strain increases it. Strain can also split the valence band degeneracy, affecting the effective masses of holes and the optical polarization properties of emitted light. For instance, compressive strain in AlN layers grown on sapphire can enhance the transverse-electric (TE) polarized emission, which is desirable for certain optoelectronic devices.

Doping is another critical aspect of bandgap engineering and carrier control in AlN. N-type doping is typically achieved using silicon (Si) or germanium (Ge), which substitute for aluminum (Al) sites and donate electrons to the conduction band. However, achieving high n-type conductivity in AlN is challenging due to the high ionization energy of donors, often exceeding 100 meV. P-type doping is even more difficult, with magnesium (Mg) being the most common acceptor. The ionization energy of Mg in AlN is around 0.5 eV, leading to low hole concentrations at room temperature. Oxygen (O) is another unintentional donor in AlN, originating from impurities during growth, and can influence the electrical properties if not carefully controlled.

Carrier transport in AlN is characterized by high effective masses for both electrons and holes. The electron effective mass in the Γ-valley is approximately 0.48 m0, while the hole effective masses are anisotropic, with values ranging from 3.5 m0 to 6.0 m0 depending on the crystallographic direction. These large effective masses result in low carrier mobilities, particularly for holes, which exhibit room-temperature mobilities below 10 cm²/V·s. Electron mobilities are higher but still limited to around 300 cm²/V·s in high-quality bulk AlN. The low mobilities are attributed to strong phonon scattering, particularly from longitudinal optical (LO) phonons, which have high energies in AlN due to its stiff lattice and strong ionic bonding.

The breakdown field of AlN is among the highest of all known semiconductors, theoretically predicted to exceed 15 MV/cm. Experimental measurements often yield slightly lower values due to material defects and impurities, but they still surpass those of silicon carbide (SiC) and gallium nitride (GaN). This exceptional breakdown strength, combined with the high thermal conductivity of AlN (up to 320 W/m·K), makes it an ideal material for high-power electronic devices operating under extreme conditions.

Polarization effects are another important consideration in AlN due to its wurtzite crystal structure. Spontaneous polarization arises from the non-centrosymmetric nature of the lattice, while piezoelectric polarization occurs under strain. These polarization fields can induce large built-in electric fields in AlN-based heterostructures, influencing carrier confinement and transport. For example, in AlGaN/AlN quantum wells, polarization fields can lead to spatial separation of electrons and holes, reducing radiative recombination efficiency but enabling unique device designs for sensors and modulators.

The electronic properties of AlN can be further tailored through quantum confinement in nanostructures such as quantum wells, wires, and dots. Reducing the dimensionality of AlN increases the energy levels of charge carriers, effectively widening the bandgap and modifying the density of states. Quantum confinement also enhances the oscillator strength for optical transitions, which is beneficial for light-emitting applications. However, surface states and defects in nanostructures can adversely affect carrier lifetimes and mobilities, necessitating careful passivation and growth optimization.

In summary, the electronic properties of aluminum nitride are defined by its wide bandgap, high breakdown field, and strong polarization effects. Bandgap engineering through alloying, strain, and doping allows for precise control over its electronic and optical behavior. Despite challenges such as low carrier mobilities and high dopant ionization energies, AlN remains a cornerstone material for high-power, high-frequency, and deep-ultraviolet technologies. Continued research into defect reduction, doping efficiency, and heterostructure design will further unlock its potential in next-generation semiconductor devices.