Two photodiodes are fabricated with {HEAD ~ essentially the same structure} in {TAIL ~ close enough proximity} to each other on a single semiconductor substrate so that they both experience the same environment 
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Two photodiodes are fabricated with { HEAD ~ essentially the same structure } in { TAIL ~ close enough proximity } to each other on a single semiconductor substrate so that they both experience the same environment
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH