The study reports a room-temperature low-threshold avalanche effect in a stepwise van-der-Waals (vdW) homojunction photodiode using WSe2. The combination of weak electron-phonon scattering and high electric fields in the stepwise WSe2 structure leads to low-loss carrier acceleration and multiplication, reducing the threshold energy to approach the fundamental limit, \(E_{\text{thre}} \approx E_g\), where \(E_g\) is the bandgap of the semiconductor. This results in a significantly reduced avalanche threshold voltage of -1.6 V, compared to traditional avalanche diodes with threshold voltages up to 42 V. The device exhibits low background dark current (10–100 fA) and high sensitivity, capable of detecting signals down to 24 fW, equivalent to 7.7 × 10^4 photons. The findings offer a new perspective for designing and fabricating future avalanche and hot-carrier photovoltaic devices, potentially extending their application in various scenarios.The study reports a room-temperature low-threshold avalanche effect in a stepwise van-der-Waals (vdW) homojunction photodiode using WSe2. The combination of weak electron-phonon scattering and high electric fields in the stepwise WSe2 structure leads to low-loss carrier acceleration and multiplication, reducing the threshold energy to approach the fundamental limit, \(E_{\text{thre}} \approx E_g\), where \(E_g\) is the bandgap of the semiconductor. This results in a significantly reduced avalanche threshold voltage of -1.6 V, compared to traditional avalanche diodes with threshold voltages up to 42 V. The device exhibits low background dark current (10–100 fA) and high sensitivity, capable of detecting signals down to 24 fW, equivalent to 7.7 × 10^4 photons. The findings offer a new perspective for designing and fabricating future avalanche and hot-carrier photovoltaic devices, potentially extending their application in various scenarios.