This paper presents a novel hybrid graphene-quantum dot phototransistor with ultrahigh gain and high quantum efficiency, enabling high-sensitivity and gate-tunable photodetection. The device combines the strong light absorption of quantum dots with the two-dimensionality and high mobility of graphene. The key functionality is provided by a layer of strongly light-absorbing colloidal quantum dots, from which photo-generated charges can transfer to graphene, while oppositely charged carriers remain trapped in the QD layer. These trapped carriers lead to a photogating effect, where the presence of these charges changes the graphene sheet resistance through capacitative coupling. The hybrid phototransistor devices reveal quantum efficiencies exceeding 25%, and photoconductive gain of 10^8 carriers per absorbed photon for a bias of 1V, corresponding to a responsivity of ~10^8 A/W, which is ten orders of magnitude larger than the responsivity of graphene photodetectors. Graphene's unique electronic properties offer gate-tunable carrier density and polarity, enabling the tuning of the sensitivity and operating speed of the detector. The device consists of a graphene sheet sensitized with colloidal quantum dots. Graphene is the carrier transport channel, and the quantum dots are employed as the photon absorbing material. The channel of the phototransistor, consisting of a monolayer or bilayer graphene sheet, is placed atop a Si/SiO2 wafer. The structure is photo-responsive over a large area, a feature important in most sensing applications. The spectral responsivities of two devices containing PbS QDs with first exciton peaks at ~950 nm and ~1450 nm are shown. The photocurrent response follows the absorption of the PbS QDs, and no photocurrent is observed for photon energies below the bandgap of the QD layer. The photoconductive gain is achieved through the recirculation of charge carriers through the prolonged lifetime of the carriers that remain trapped in the PbS QDs. The photo-induced shift of the Dirac peak can be reversed depending on the initial doping level of the graphene sheet. The device behavior is understood by considering the negatively charged quantum dot layer as a local photoinduced gate that modifies the graphene carrier concentration through capacitive coupling. A simple parallel plate capacitor model is used to estimate the internal quantum efficiency of the device. The results show an external quantum efficiency of 25%. The device exhibits ultrahigh gain, which originates from the high carrier mobility of the graphene sheet and the recirculation of charge carriers through the prolonged lifetime of the carriers that remain trapped in the PbS QDs. The photoconductive gain for this mechanism is given by G = τlifetime/τtransit, evidencing the importance of a long lifetime and a high carrier mobility. The device shows a responsivity of up to 10^8 A/W, which is ten orders of magnitudeThis paper presents a novel hybrid graphene-quantum dot phototransistor with ultrahigh gain and high quantum efficiency, enabling high-sensitivity and gate-tunable photodetection. The device combines the strong light absorption of quantum dots with the two-dimensionality and high mobility of graphene. The key functionality is provided by a layer of strongly light-absorbing colloidal quantum dots, from which photo-generated charges can transfer to graphene, while oppositely charged carriers remain trapped in the QD layer. These trapped carriers lead to a photogating effect, where the presence of these charges changes the graphene sheet resistance through capacitative coupling. The hybrid phototransistor devices reveal quantum efficiencies exceeding 25%, and photoconductive gain of 10^8 carriers per absorbed photon for a bias of 1V, corresponding to a responsivity of ~10^8 A/W, which is ten orders of magnitude larger than the responsivity of graphene photodetectors. Graphene's unique electronic properties offer gate-tunable carrier density and polarity, enabling the tuning of the sensitivity and operating speed of the detector. The device consists of a graphene sheet sensitized with colloidal quantum dots. Graphene is the carrier transport channel, and the quantum dots are employed as the photon absorbing material. The channel of the phototransistor, consisting of a monolayer or bilayer graphene sheet, is placed atop a Si/SiO2 wafer. The structure is photo-responsive over a large area, a feature important in most sensing applications. The spectral responsivities of two devices containing PbS QDs with first exciton peaks at ~950 nm and ~1450 nm are shown. The photocurrent response follows the absorption of the PbS QDs, and no photocurrent is observed for photon energies below the bandgap of the QD layer. The photoconductive gain is achieved through the recirculation of charge carriers through the prolonged lifetime of the carriers that remain trapped in the PbS QDs. The photo-induced shift of the Dirac peak can be reversed depending on the initial doping level of the graphene sheet. The device behavior is understood by considering the negatively charged quantum dot layer as a local photoinduced gate that modifies the graphene carrier concentration through capacitive coupling. A simple parallel plate capacitor model is used to estimate the internal quantum efficiency of the device. The results show an external quantum efficiency of 25%. The device exhibits ultrahigh gain, which originates from the high carrier mobility of the graphene sheet and the recirculation of charge carriers through the prolonged lifetime of the carriers that remain trapped in the PbS QDs. The photoconductive gain for this mechanism is given by G = τlifetime/τtransit, evidencing the importance of a long lifetime and a high carrier mobility. The device shows a responsivity of up to 10^8 A/W, which is ten orders of magnitude