Zinc oxide (ZnO) and zinc sulfide (ZnS) are II-VI compound semiconductors with notable piezoelectric properties, making them attractive for strain-gated transistors. These materials generate piezopotentials when subjected to mechanical stress due to their non-centrosymmetric crystal structures. The magnitude and polarity of the piezopotential depend on crystallographic orientation, strain direction, and material composition. Understanding these properties is critical for optimizing strain-gated transistor performance.

ZnO crystallizes in the wurtzite structure (hexagonal, P6₃mc space group), which lacks inversion symmetry along the c-axis. This asymmetry enables strong piezoelectric coupling. The polar c-axis is the primary direction for piezopotential generation. When uniaxial stress is applied parallel to the c-axis, a piezoelectric potential arises due to the displacement of Zn²⁺ and O²⁻ ions relative to each other. The piezoelectric coefficients for ZnO are well-documented: d₃₃ ≈ 12.4 pm/V and d₃₁ ≈ -5.0 pm/V. The d₃₃ coefficient governs longitudinal piezoelectric response along the c-axis, while d₃₁ describes transverse effects. For strain-gated transistors, aligning the c-axis with the applied strain maximizes piezopotential output. Off-axis orientations reduce efficiency due to weaker polarization contributions.

ZnS exists in two polymorphs: zincblende (cubic, F-43m) and wurtzite (hexagonal, P6₃mc). The zincblende phase is centrosymmetric and thus non-piezoelectric under normal conditions. However, the wurtzite phase of ZnS exhibits piezoelectricity similar to ZnO but with lower coefficients. The d₃₃ value for wurtzite ZnS is approximately 6.0 pm/V, roughly half that of ZnO. Despite this, ZnS offers advantages in certain applications due to its wider bandgap (3.7 eV for ZnS vs. 3.4 eV for ZnO), which may reduce leakage currents in strain-gated devices. The crystallographic orientation effects in wurtzite ZnS mirror those of ZnO, with the c-axis being the preferred direction for piezopotential generation.

Piezopotential characterization involves measuring the voltage output under controlled strain conditions. For ZnO nanowires grown along the c-axis, experiments have demonstrated piezopotentials exceeding 0.5 V under 1% tensile strain. The relationship between strain and potential is generally linear at low strains but may exhibit saturation or nonlinearity at higher deformations due to electromechanical coupling limits. ZnS, with its lower piezoelectric coefficients, typically generates smaller potentials under equivalent strain conditions. However, its higher stiffness (Young’s modulus ~100 GPa for ZnS vs. ~140 GPa for ZnO) influences strain transfer efficiency in composite structures.

Strain-gated transistors leverage piezopotentials to modulate carrier transport without external gate voltages. In a typical configuration, a piezoelectric ZnO or ZnS layer is integrated into the transistor channel or gate region. Applied strain induces a piezopotential that locally alters the energy bands, effectively gating the device. The performance depends on the alignment between the strain direction and the polar axis of the material. For example, a ZnO nanowire transistor with the c-axis perpendicular to the strain direction will exhibit minimal gating effect, whereas alignment parallel to the strain maximizes modulation.

Crystallographic defects influence piezoelectric response. Oxygen vacancies in ZnO, for instance, can screen piezopotentials by introducing free carriers that neutralize polarization charges. Reducing defect concentrations through optimized growth conditions (e.g., high-temperature annealing or controlled atmospheres) enhances piezopotential retention. ZnS is less prone to vacancy-related screening but may suffer from sulfur vacancy-induced deep traps that affect carrier mobility. Doping strategies can mitigate these effects; for example, magnesium doping in ZnO has been shown to improve piezoelectric performance by reducing free carrier concentrations.

Temperature stability is another consideration. The piezoelectric coefficients of ZnO decrease by about 0.5% per degree Celsius above room temperature due to thermal expansion effects. ZnS exhibits better thermal stability owing to its higher bond strength, but its lower baseline piezoelectric coefficients may offset this advantage. For strain-gated transistors operating in variable environments, these thermal dependencies must be accounted for in device design.

The choice between ZnO and ZnS depends on application requirements. ZnO’s higher piezoelectric coefficients make it suitable for low-strain, high-sensitivity applications, while ZnS may be preferred for high-strain or high-temperature environments where material stability is critical. Hybrid structures combining both materials could exploit complementary properties, such as using ZnS as a passivation layer over a ZnO active region to reduce environmental degradation while maintaining strong piezoresponse.

Recent advances in nanostructuring have further enhanced piezoelectric performance. Vertically aligned ZnO nanorod arrays exhibit collective piezopotential amplification under uniform strain, enabling higher output voltages for strain-gated logic circuits. Similarly, ZnS nanowires with controlled defect densities demonstrate improved consistency in piezopotential generation. The scalability of these nanostructures is critical for integrating strain-gated transistors into large-area electronics.

Characterization techniques such as Kelvin probe force microscopy (KPFM) and piezoresponse force microscopy (PFM) provide nanoscale resolution of piezopotential distributions. These methods reveal localized variations in piezoelectric activity due to grain boundaries, defects, or inhomogeneous strain fields. For strain-gated transistors, uniformity in piezopotential generation across the active region is essential for predictable device behavior.

In summary, ZnO and ZnS offer distinct yet complementary piezoelectric properties for strain-gated transistors. Crystallographic orientation, defect engineering, and nanostructuring play pivotal roles in determining device performance. While ZnO provides higher piezoresponse, ZnS contributes greater thermal and environmental stability. Advances in material synthesis and characterization continue to refine their applicability in next-generation strain-sensitive electronics.