Broadband photodetectors with a wide ultraviolet (UV)–visible (vis)–near-infrared (NIR) spectral response reduce cost and form factors in smart sensing systems by using one rather than multiple devices for versatile applications. However, simultaneously achieving broadband detection, high responsivity, and fast response time remains challenging. This study demonstrates a high-performance broadband amorphous In–Ga–Zn–O (a-IGZO) phototransistor using an engineered Ni/Ti bilayer metal contact. This device differs drastically from the conventional a-IGZO phototransistors in several aspects: 1) Pronounced photoresponse occurs at the device ON-state rather than the OFF-state. 2) A broadband response of 385–980 nm rather than a narrowband UV response. 3) Fast sub-ms response time with no persistent photocurrent. 4) High responsivity of exceeding 105 A W−1. The external quantum efficiency exceeding 107% and specific detectivity exceeding 1015 Jones are also among the best of any UV–vis–NIR broadband photodetectors. The photoresponse is attributed to the light-sensitive source–drain series resistance at the ON-state. The Ni diffusion during postmetal annealing induces photoactive subgap states and modulates the conductivity of a-IGZO at the contact region. The superior performance using low-temperature processes and simple device structures as the widely available a-IGZO technology suggests a tremendous application potential of this broadband phototransistor.