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