Re: Is a 'neutral' metal-semiconductor junction possible?

Thanks for the insight Bill. It would seem so that, ignoring possible
impurity and surface effects, only a true neutral junction can be
formed between two identical materials, and that its conductivity will
only be as good as these materials.
John.

William R. Frensley wrote:
> olongdomango@xxxxxxxxxxx wrote:
> > Hello,
> >
> > While reviewing literature on metal-semiconductor junctions, I came
> > across a definition for a "neutral contact", which was described as the
> > junction that is formed when two materials with identical work
> > functions are joined, so that no band bending occurs. This makes sense
> > for two identical materials, but if for example, an n-type
> > semiconductor with a wide forbidden band gap and a metal somehow had
> > the same work functions and were joined, I appreciate that there would
> > be no built-in potential barrier, but wouldn't a Schottky barrier be
> > formed due to the position of the conduction band edge over the common
> > Fermi level? Would electrons still not have to overcome this barrier if
> > the metal was biased negative wrt the semiconductor? Is it that metal
> > electrons simply tunnel into the forbidden gap, where they are swept to
> > the conduction band under the influence of the external bias? If so,
> > wouldn't there not be a minimum bias required to achieve this, which
> > would be proportional to the size of the energy gap between the
> > Fermi-level and the conduction band? Sorry for all the questions, but I
> > would be glad if someone could help me to clarify my understanding.
> >
> This sort of "neutral contact" doesn't say anything about the transport
> properties [I(V) curve]. If you have, say a typical semiconductor like
> Si lightly doped p-type or GaAs very lightly doped n-type, you can get
> a flat energy-band diagram. The Fermi level lies near the middle
> of the band gap, and with very precise control might be made to line up
> with the pinned value of the metal Fermi level at the semiconductor surface.
> In principle, these junctions should behave like normal Schottky diodes,
> turning on with a voltage that pushes the semiconductor majority carriers
> toward the junction. In practice, the resistivity of the semiconductor
> will be so high that all you are likely to see is leakage currents.
>
> For a junction on a highly ionic (wide gap) material, where the electron
> affinity rule actually works, the situation is the same, except that the
> Schottky barrier depends on the metal work function, and has to be taken
> into account. But here the equilibrium carrier concentration is so low in the
> neutral case that you probably won't have a carrier in any reasonable device
> volume.
>
> A very interesting case is InAs, whose bands lie so low in energy that the
> conduction band edge is a bit below the Fermi level of metal contacts made
> to it. The neutral junction condition thus requires degenerate n-type doping,
> and there is no energy barrier at all. Since any reasonable carrier
> concentration in InAs is degenerate (because the effective mass is very low)
> putting just about any metal on n-type InAs gives an excellent Ohmic contact.
>
> - Bill Frensley

.

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