Strain-controlled power devices as inspired by human reflex

The study addresses the need for power devices that can directly and rapidly modulate output power in response to mechanical stimuli without complex sensor-actuator chains. Conventional systems rely on sensors, conversion (A/D, D/A), isolation, and CPU control, introducing latency and complexity, which is problematic for AI applications like self-driving cars and robotics that require real-time, unsupervised control, while still allowing supervised intervention. The authors propose a bioinspired strain-controlled power device (SPD) that emulates human knee-jerk reflex: external strain directly modulates device output (unsupervised), while a gate bias allows ultimate control (supervised). Leveraging the piezotronic effect in AlGaN/AlN/GaN HEMTs, strain-induced piezoelectric polarization modulates 2DEG density, enabling direct, fast power control suitable for compact, stable AI systems.

Prior work has shown that III–V nitride heterostructures, particularly AlGaN/AlN/GaN HEMTs, are promising for high-power electronics due to high 2DEG density, mobility, and large breakdown fields. The piezotronic effect—coupling semiconductor transport with piezoelectric polarization—has been used to modulate transport in nanowires, sensors, and HEMTs by altering interfacial charges and 2DEG concentration under strain. Reports have demonstrated strain-tuned heterojunction electron gases and piezotransistive cantilevers that change conductivity with applied strain, suggesting an avenue for seamless, real-time interaction between mechanical stimuli and electronic output. However, integrating such effects into high-power devices that perform direct power modulation for AI-relevant tasks with both unsupervised (reflex-like) and supervised (brain-like) control remains challenging.

Device design and fabrication: The SPD is a cantilever-structured AlGaN/AlN/GaN HEMT on Si(111). Layer stack: AlGaN (30 nm, 30% Al) / AlN (1 nm) / GaN (4.3 µm) / AlGaN buffer / Si substrate; initial 2DEG sheet density 8 × 10^12–1 × 10^13 cm−2. A fully dry ICP etching process (SENTECH SI 500) creates the cantilever: Step 1 (anisotropic etch of GaN/Si) using BCl3/Cl2/Ar (10/32/5 sccm), 550 W, 20 min to define mesa and trenches; Step 2 (isotropic etch of Si) using SF6/O2/Ar (30/5/10 sccm), 800 W, 25 min to release the cantilever. Ohmic contacts: Ti/Al/Ni/Au (20/120/45/55 nm) evaporated (Denton Explorer 14) and annealed at 850 °C in N2 for 30 s (RTP LABSYS RTP-1200). Gate Schottky: Ni/Au (80/50 nm). Final device dimensions: cantilever 350 × 50 × 5 µm^3; embedded HEMT mesa 27 × 27 µm^2; gate length 5 µm. Characterization: SEM (Nova Nano SEM 450) imaged device morphology; FIB (FEI Helios NanoLab 600i) prepared TEM lamellae; HRTEM (TECNAI F20) with EDS mapped AlGaN/AlN/GaN heterostructures; SAED verified crystallinity; confocal Raman (HORIBA LabRAM HR Evolution) probed strain; COMSOL simulated strain distribution under load. Electrical measurements used Keysight B1500A. Strain-modulated tests: External strain was applied along c-axis by bending the cantilever using a 10 µm tip probe in a probe station (step ~5 µm). The depression depth was converted to strain/force (0–16 mN; 0–20 µm deflection) following Eliza et al.’s method (Supplementary Note 1). Ids–Vds and Ids–Vgs were recorded under various strains and gate biases. Acceleration-feedback-control: A linear motor generated reciprocating accelerations of 1–5 G while monitoring device power with Keysight B1500A and LCR E4980A. Vds was set between 5–15 V and Vgs typically 0–15 V to assess real-time power response to acceleration changes. Data were collected over time to extract ΔP versus acceleration and bias.

• The SPD directly modulates output power via strain (piezotronic effect) in a reflex-like manner, with gate bias providing supervisory control.
• Output characteristics: The SPD exhibits gate-controlled Ids–Vds with large output currents. Maximum transconductance gm,max ≈ 7.5 mS mm−1 (at Vds = 6 V). Raman E2(high) of GaN blue-shifts from 566.8 to 569.2 cm−1 after cantilever etching, indicating tensile strain relaxation.
• Strain-enhanced conduction: Under 16 mN strain, saturated current increases from 32.83 to 40.43 mA mm−1 at Vgs = −5 V, and from 59.87 to 70.36 mA mm−1 at Vgs = 1 V. Transconductance increases with strain, evidencing stronger gate control.
• Output power density (P): Increases with strain (0–16 mN). At 16 mN, P reaches 1.39 × 10^3 W cm−2 (Vgs = −5 V) and 2.72 × 10^3 W cm−2 (Vgs = 1 V). With Vgs tuning, relative P at 16 mN spans ≈1.51–2.54 × 10^3 W cm−2 across different gate biases. The abstract reports modulation in the 2.30–2.72 × 10^3 W cm−2 range at Vgs = 1 V.
• Reproducibility: Cyclic loading/unloading (e.g., 0 ↔ 10–16 mN) shows repeatable, reversible power modulation with small variance.
• Mechanism: Simulations indicate that tensile strain raises Ec in AlGaN and lowers it in GaN, deepening the AlN/GaN quantum well, increasing carrier confinement; integrated 2DEG sheet density increases with strain, consistent with measured Ids and power trends.
• Acceleration-feedback control: Real-time power varies with acceleration (1–5 G). At 5 G, achievable P under Vds = 5, 10, 15 V are ≈866.8, 2765.9, and 4304.2 W cm−2, respectively. ΔP increases with acceleration and bias: at Vgs (or high-bias operation) ≈15 V, ΔP rises from 72.78 to 132.89 W cm−2 (1→5 G); at 5 V, ΔP rises from 5.23 to 11.64 W cm−2. Fast response and recovery enable effective feedback control.

The SPD meets the core objective of enabling direct, rapid modulation of output power by mechanical stimuli without the latency and complexity of conventional sensor–A/D–CPU–D/A–actuator chains. Emulating a reflex arc, strain applied to the cantilever immediately alters the interfacial piezoelectric polarization and 2DEG density in the AlGaN/AlN/GaN heterostructure, thus modulating channel conduction and power output. The gate bias affords higher-level supervisory control, allowing programmable sensitivity and operating point selection. Experimental data and simulations consistently show that tensile strain increases 2DEG confinement and carrier density, increasing current and power. The device demonstrates high power densities, sensitive strain dependence, reproducibility over cycles, and real-time acceleration-feedback capability, validating its relevance for AI applications that require unsupervised rapid control with optional supervision, such as emergency braking power management and robotic balance control.

This work demonstrates a bioinspired strain-controlled power device based on a cantilevered AlGaN/AlN/GaN HEMT that directly translates mechanical strain into controllable output power via the piezotronic effect. The SPD achieves ultra-high output power densities up to 2.72 × 10^3 W cm−2 under weak forces (≤16 mN), programmable via gate bias, and supports real-time acceleration-feedback control with measurable ΔP scaling with acceleration and bias. The findings establish a compact, fast, and stable platform for reflex-like unsupervised power modulation with supervised override, promising for AI applications in autopilot, robotics, and human–machine interfaces. Future work could focus on optimizing fabrication to minimize etch-induced defects and residual strain, scaling to arrays and integrated systems, improving durability and environmental robustness, and extending to multi-axis or multi-parameter feedback for complex control tasks.

• The cantilever release and dry-etching process partially relaxes tensile strain and introduces defects, reducing 2DEG concentration and degrading electrical performance compared to non-released HEMTs (e.g., lower current, gm).
• The demonstrated strain range (0–16 mN) and probe-based loading are laboratory conditions; robustness under long-term cyclic loading and in practical environments was not reported.
• Larger power modulation for acceleration-feedback requires operation at higher gate/bias voltages (saturation region), which may entail higher power consumption and potential reliability concerns not addressed here.
• Detailed thermal management, long-term stability, and packaging/integration with system-level electronics were not explored.