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