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