High-performance broadband photodetectors offering spectral tunability and a high gain-bandwidth product are crucial in many applications. Here, we report on a detailed experimental and theoretical study of three-terminal phototransistors comprised of three million InP nanowires with 20 embedded InAsP quantum discs in each nanowire. A global, transparent ITO gate all around the nanowires facilitates a radial control of the carrier concentration by more than two orders of magnitude. The transfer characteristics reveal two different transport regimes. In the subthreshold region, the photodetector operates in a diffusion mode with a distinct onset at the bandgap of InP. At larger gate biases, the phototransistor switches to a drift mode with a strong contribution from the InAsP quantum discs. Besides an unexpected spectral tunability, the detector exhibits a state-of-the-art responsivity, reaching around 100 A/W (638 nm/20 μW) @ V_GS = 1.0 V/V_DS = 0.5 V with a gain-bandwidth product of around 1 MHz, in excellent agreement with a comprehensive real-device model. © 2023 The Authors. Published by American Chemical Society.