A first portion of the intrinsic base layer is masked while {HEAD ~ a second portion} of {TAIL ~ an intrinsic base layer} is etched 
of
A first portion of the intrinsic base layer is masked while { HEAD ~ a second portion } of { TAIL ~ an intrinsic base layer } is etched
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH