Deep-level defect profiling in AlGaN heterojunction phototransistors using low-frequency noise spectroscopy

This study investigates the spatial distribution and density of deep-level defects in AlGaN heterojunction phototransistors using low-frequency noise (LFN) spectroscopy. Unlike the conventional 1/fγ noise with γ ≈ 1, the photocurren noise under 260 nm illumination exhibits power-dependent frequency exponents (γ = 1.4-1.9) , reflecting defect redistribution effects. By combining the carrier number fluctuation (CNF) model with photogenerated voltage analysis, we quantitatively extract the bulk trap density (Nt) over an optical power range of 20 nW/cm² to 200 μW/cm². The non-passivated device shows Nt =1.35×10²⁰ cm⁻³eV⁻¹, while the in-situ SiNx passivated device exhibits Nt = 2.36×10¹⁹ cm⁻³eV⁻¹. The 82% reduction in Nt and the γ-shift under strong illumination reveal both density suppression and spatial redistribution of deep-level defects. This work provides a novel methodology for defect characterization in wide-bandgap optoelectronic devices.