This paper presents a novel hybrid graphene-quantum dot phototransistor that demonstrates ultrahigh gain and sensitivity for photodetection. The device combines the strong light absorption and high mobility of graphene with the high quantum efficiency of quantum dots. By exploiting charge transfer between graphene and quantum dots, the phototransistor achieves a photoconductive gain of \(10^8\) and a responsivity of \(10^8\) A/W, which is ten orders of magnitude higher than that of pristine graphene photodetectors. The device exhibits gate-tunable sensitivity and spectral selectivity from the shortwave infrared to the visible range, making it suitable for integration with current circuit technologies. The key functionality is provided by a layer of strongly light-absorbing colloidal quantum dots, which transfer photo-generated charges to graphene, while oppositely charged carriers remain trapped in the quantum dot layer. This results in a photogating effect, where the presence of these charges changes the graphene sheet resistance through capacitive coupling. The device's performance is further enhanced by the high carrier mobility of graphene and the prolonged lifetime of carriers trapped in the quantum dots. The study highlights the potential of this hybrid platform for various optoelectronic applications, including integrated circuits, biomedical imaging, remote sensing, optical communications, and quantum information technology.This paper presents a novel hybrid graphene-quantum dot phototransistor that demonstrates ultrahigh gain and sensitivity for photodetection. The device combines the strong light absorption and high mobility of graphene with the high quantum efficiency of quantum dots. By exploiting charge transfer between graphene and quantum dots, the phototransistor achieves a photoconductive gain of \(10^8\) and a responsivity of \(10^8\) A/W, which is ten orders of magnitude higher than that of pristine graphene photodetectors. The device exhibits gate-tunable sensitivity and spectral selectivity from the shortwave infrared to the visible range, making it suitable for integration with current circuit technologies. The key functionality is provided by a layer of strongly light-absorbing colloidal quantum dots, which transfer photo-generated charges to graphene, while oppositely charged carriers remain trapped in the quantum dot layer. This results in a photogating effect, where the presence of these charges changes the graphene sheet resistance through capacitive coupling. The device's performance is further enhanced by the high carrier mobility of graphene and the prolonged lifetime of carriers trapped in the quantum dots. The study highlights the potential of this hybrid platform for various optoelectronic applications, including integrated circuits, biomedical imaging, remote sensing, optical communications, and quantum information technology.