Why Does Electron Flow Differ in Direction at a Metal-Semiconductor Junction?

[abomination5]

Hi,

I have a questions regarding semiconductor-metal junctions that has been left unresolved. I hope that you can offer me an explanation.

I know that when a metal and n-doped semiconductor are in contact upward band bending occurs. This is because the Fermi levels in the two materials must be the same. This causes a diffusion force on the electrons. Eventually positive charge builds up near the junction which halts the diffusion current, thus establishing equilibrium.

My question is: what causes the shape of the potential? Why is it easier for electrons to flow in one direction than the other? The potential seems to undergo a steep drop on the metal side and a gradual drop on the semiconductor side. What causes this?

Thanks,
abomination5

 

[DrDu]

I would say because the screening length in the metal is much shorter than in the semiconductor due to the higher electron density.