The semiconductor filled portion of each trench then serves as a drain and {HEAD ~ source} of {TAIL ~ a field effect transistor} 
of
The semiconductor filled portion of each trench then serves as a drain and { HEAD ~ source } of { TAIL ~ a field effect transistor }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL