{TAIL ~ Double heterojunctions} are employed in {HEAD ~ two different modes}: bulk mode or quantum well mode 
{ TAIL ~ Double heterojunctions } are employed in { HEAD ~ two different modes } : bulk mode or quantum well mode
TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH