 IEEE is probably the best resource for looking into differences between CMOS architecture for active pixel sensors. This whitepaper in particular is a brief overview of differences in fill-factor and peak QE between FSI and BSI designs (but there are plenty more): https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=0000000 In practice the gains of BSI are realized with very small pixel sizes - usually between 0-0 micrometers per side. That's why almost every smartphone sensor (0/0.0" or smaller) and most premium compacts have all adopted BSI designs. 