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