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