effect of doping on contact potential

[harshit_agarwal]

hi everybody
the equation V=vlog(na*nd)/ni2 depicts that V increases with the doping concentration.However it appears (infact logical as in tunnel) that it should decrease with the doping.
How is it possible?please clearfy....

 

[Sceadwian]

I think it increases with doping concentration up to the point where the tunneling effect become dominant.