{TAIL ~ The wide band gap layer} has a band gap shifted relatively to valence band direction by n type doping and thereby more effectively functions as a diffusion barrier for {HEAD ~ the thermally excited holes} 
{ TAIL ~ The wide band gap layer } has a band gap shifted relatively to valence band direction by n type doping and thereby more effectively functions as a diffusion barrier for { HEAD ~ the thermally excited holes }
TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD