in a monotonically varying bandgap white light coupled through the surface the layers with the widest bandgap is progressively stripped of the shorter wavelengths which are absorbed as the white light beam progresses further into the layer stack in situ doping can be engineered in conjunction with the variation in bandgap to produce a continuous and monotonic built in drift electric field across the entire absorption region of the photo diode driving photo generated carriers towards a charge collecting region such built in field is crucial for carriers photo generated near the surface from which light is coupled to be driven away from said surface and thus avoid fast annihilation due to the typically high density of surface recombination centers this is particularly relevant to increase the qe for the shorter wavelengths such as blue and uv the aforementioned stacking of superlattices with multiple compositions and periods ordered in decreasing or increasing magnitude of the minibandgap is the superlattice equivalent of a sige and or sigec random alloy with graded ge and or c concentration producing in a triangular potential profile in order to insure the possibility of drift diffusion transport perpendicularly to the stack of superlattices care must be taken in order to avoid a u201c broken minigap u201d in any of its configurations as shown in fig1.