Low-frequency noise characteristics of waveguide-integrated lateral and vertical Ge-on-Si p–i–n photodiodes

We report forward bias and temperature-dependent low-frequency (LF) noise characteristics of waveguide-integrated Ge-on-Si p–i–n photodiodes across three architectures: doped-Si lateral, doped-Ge and doped-Si lateral, and vertical heterojunctions. Forward I–V measurements and activation energy analysis reveal mixed Shockley–Read–Hall recombination and diffusion transport below 0.4 V, with transport ratios evolving more gradually with temperature in vertical devices than in lateral devices. At room temperature, the noise spectra exhibit broad generation-recombination (G–R) features near V=0.2 V, aligning with the transition from G–R-dominated (SI∼I2 for V≤0.2 V) to diffusion-limited (SI∼I for 0.2V≤V≤0.5V) current noise. Technology computer-aided design simulations indicate that in lateral devices the defect-rich Ge/Si interface lies within the depletion region, where interface traps strongly enhance G–R noise. In vertical devices, the Ge/Si interface is located outside the depletion zone, so the noise response is dominated by bulk Ge traps. Temperature-dependent measurements reveal a smooth transition from G–R noise below 300 K to diffusion-type flicker noise at higher temperatures. Notably, normalized noise SI/I2 peaks around 300 K in lateral devices, marking a crossover in dominant noise mechanisms. Observed increases of low-temperature noise after total-ionizing-dose irradiation in lateral devices result from the electrostatic activation of traps within the narrow depletion region, whereas the increase of forward current originates mainly from radiation-induced additional leakage paths. These results establish LF noise spectroscopy as an effective tool for probing interfacial defect dynamics in Ge-on-Si photodiodes.