A Schottky barrier is the potential barrier that prevents charge carriers from being injected from the semiconductor into the metal or vice versa.
For an ohmic contact, you want a very low (or no) Schottky barrier because you desire a linear current with no potential barrier in either bias direction. A Schottky barrier contributes to the contact resistivity, which is undesirable in an ohmic contact. In the thermionic emission regime, the Schottky barrier height is the primary impediment for current flow in an ohmic contact. However, if your underlying semiconductor is doped to a high enough degree (thinning the depletion region) or you are at a low enough temperature (not enough thermal energy to excite carriers over the Schottky barrier), thermionic-field emission (thermally assisted tunneling) or field emission (tunneling) will become the dominant transport mechanism.
For a rectifying (Schottky) contact, you want to have as high current drive in forward bias and as low current drive in reverse bias as possible. A high Schottky barrier in a rectifying contact prevents leakage current in reverse bias. In forward bias, the quasi-Fermi level (in an M/n-type contact) will rise, increasing the carrier concentration in the conduction band exponentially. Eventually, a large concentration of carriers will be able to surmount the Schottky barrier, allowing current to flow. In reverse bias, the quasi-Fermi level will decrease (same case as before), decreasing the carrier concentration in the SC conduction band. This effect can be seen in the diode equation, where at large reverse bias the total current density simply goes to a small negative constant.
A few other notes about the Schottky barrier:
- The barrier HEIGHT does NOT depend on doping, only the shape (width) of the barrier, which can lead to tunneling.
- Fermi level pinning can either cause a high OR low Schottky barrier depending upon where the pinning location (near the so-called "charge neutrality level") is. For example, if the charge neutrality level is near the valence band for an n-type semiconductor, a positive Schottky barrier is almost inevitable unless the pinning is extremely weak (very low density of interface states). On a p-type semiconductor, this would almost ubiquitously yield an ohmic contact (negligible/negative Schottky barrier) A good example of this is metal/Ge contacts. However, be warned that the causes of Fermi level pinning are very complicated. You'll be going down quite a rabbit hole if you seek to learn more about that. The simplest explanation is that interface states, coming about from many factors, cause the charge neutrality condition of the junction to change, thus modifying the band bending of the semiconductor, "pinning" the Fermi Level.
