Abstract
A photodetector's sensitivity is conventionally quantified by specific detectivity, which balances responsivity and noise. However, we reveal that the turn-on threshold power is fundamentally governed by photo-carrier injection rather than detectivity. In conventional phototransistors/diodes, incident light cannot generate photocurrent via thermionic injection until its intensity saturates the surface potential. To overcome this limit, we design a photo-tunneling transistor with a partially dual-gated black phosphorus channel. This device breaks the injection barrier, achieving a temperature-independent subthreshold swing of ~50 mV/dec up to 250 K and reducing the threshold power by over an order of magnitude. At 80 K, it detects mid-wave infrared light with a minimum power of ~35 pW, outperforming conventional phototransistors with higher detectivity by ~30-fold. Our work redefines the sensitivity criteria for photodetectors and highlights the potential of steep-slope transistors in low-power optoelectronics, offering a pathway to ultrasensitive infrared sensing and imaging technologies.