Floating-surface single-junction type photodiodes are mostly used in solar cell applications for simplicity and cost. On the other hand, pinned-surface and double-junction type photodiodes are used now in super high-performance image sensor applications.

This paper explains the difference between the floating-surface single-junction type photodiode and the pinned-surface double-junction type photodiode. It is shown that silicon-based thin-film amorphous surface-pinned P+PNP+P double junction type solar cell has a good figure-of-merit in cost-performance and simplicity for a solar cell application.

A real-time AI smart robot vision chip is described as an example of application, which is composed of an array of N x N pinned-surface buried-channel P+PNPP+ double junction type photo diodes, N x N analog-data stream mask-and-match comparators, digital processing and SRAM cache buffer memory units. All of them are integrated in a 3-D multichip architecture. In the external power-off mode, the image sensor array of N x N pinned-surface buried-channel P+PNPP+ double junction type photo diodes also function as a solar cell unit for the AI self-energy robot vision chip.