A low-threshold avalanche effect in stepwise van-der-Waals homojunction photodiodes is reported, achieving room-temperature performance with a threshold voltage of -1.6 V, significantly lower than traditional avalanche diodes. This effect is enabled by weak electron-phonon scattering and high electric fields, leading to efficient carrier acceleration and multiplication. The threshold energy approaches the semiconductor bandgap, Eₐ ≈ E₉, enabling high sensitivity and low dark current. The device structure, featuring a stepwise geometry, enhances electric field and reduces scattering, improving performance. The photodiode exhibits high photogain, detecting signals down to 24 fW, and operates efficiently at low voltages. The study highlights the potential of van-der-Waals materials for future photovoltaic and photodetection applications, with low energy loss and compatibility with standard digital circuits. The findings offer a new perspective for designing high-performance avalanche and hot-carrier devices.A low-threshold avalanche effect in stepwise van-der-Waals homojunction photodiodes is reported, achieving room-temperature performance with a threshold voltage of -1.6 V, significantly lower than traditional avalanche diodes. This effect is enabled by weak electron-phonon scattering and high electric fields, leading to efficient carrier acceleration and multiplication. The threshold energy approaches the semiconductor bandgap, Eₐ ≈ E₉, enabling high sensitivity and low dark current. The device structure, featuring a stepwise geometry, enhances electric field and reduces scattering, improving performance. The photodiode exhibits high photogain, detecting signals down to 24 fW, and operates efficiently at low voltages. The study highlights the potential of van-der-Waals materials for future photovoltaic and photodetection applications, with low energy loss and compatibility with standard digital circuits. The findings offer a new perspective for designing high-performance avalanche and hot-carrier devices.