A method for making a CMOS image sensor may include forming a first semiconductor chip including an array of image sensor pixels and readout circuitry electrically connected thereto, forming a second semiconductor chip comprising image processing circuitry electrically connected to the readout circuitry, and coupling the first semiconductor chip and the second semiconductor chip together in a stack. The readout circuitry may include a plurality of transistors each including spaced apart source and drain regions, a superlattice channel extending between the source and drain regions, and a gate including a gate insulating layer on the superlattice channel and a gate electrode on the gate insulating layer.