AbstractAmorphous metal oxide semiconductor phototransistors (MOTPs) integrated with colloidal quantum dots (QDs) (QD‐MOTPs) are promising infrared photodetectors owing to their high photoconductive gain, low off‐current level, and high compatibility with pixel circuits. However, to date, the poor mobility of conventional MOTPs, such as indium gallium zinc oxide (IGZO), and the toxicity of lead (Pb)‐based QDs, such as lead sulfide and lead selenide, has limited the commercial applications of QD‐MOTPs. Herein, an ultrasensitive QD‐MOTP fabricated by integrating a high‐mobility zinc oxynitride (ZnON)–based MOTP and lead‐free indium arsenide (InAs) QDs is demonstrated. A new gradated bandgap structure is introduced in the InAs QD layer that absorbs infrared light, which prevents carriers from moving backward and effectively reduces electron–hole recombination. Chemical, optical, and structural analyses confirm the movement of the photoexcited carriers in the graded band structure. The novel QD‐MOTP exhibits an outstanding performance with a responsivity of 1.15 × 105A W−1and detectivity of 5.32 × 1016Jones at a light power density of 2 µW cm−2under illumination at 905 nm.