Various embodiments of the present disclosure are directed towards an image sensor including a first chip stacked with a second chip. The first chip comprises a first substrate and a photodetector disposed in the first substrate. A first transistor is disposed on the first substrate and neighbors the photodetector. A plurality of second transistors is disposed within or on the stacked first and second chips. The plurality of second transistors comprises a first readout transistor having a first readout gate electrode over a first readout gate dielectric structure. The first readout gate dielectric structure comprises a lower dielectric layer stacked with an upper dielectric structure. A relative permittivity of the upper dielectric structure is greater than a relative permittivity of the lower dielectric layer.