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 {HEAD ~ a diffusion barrier} for {TAIL ~ the thermally excited holes} 
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 { HEAD ~ a diffusion barrier } for { TAIL ~ the thermally excited holes }
OTH 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 REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL