Metal semiconductor junctions: schottky barriers
When negative charges are brought near the metal surface, positive (image) charges are induced in the metal. When this image force is combined with an applied electric field, the effective work function is somewhat reduced. Such barrier lowering is called the Schottky effect, and this terminology is carried over to the discussion of potential barriers arising in metal-semiconductor contacts.
Schottky barrier on n type:
- When a metal with work function qΦm is brought in contact with a semiconductor having a work function qΦs, charge transfer occurs until the Fermi levels align at equilibrium (Fig. given below).
- For example, when Φm > Φs the semiconductor Fermi level is initially higher than that of the metal before contact is made.
To align the two Fermi levels, the electrostatic potential of the semiconductor must be raised (i.e., the electron energies must be lowered) relative to
that of the metal.
- In the n-type semiconductor of Fig. given below a depletion region W is formed near the junction.
- The positive charge due to uncompensated donor ions within W matches the negative charge on the metal.
- The electric field and the bending of the bands within W are similar to effects already discussed for p-n junctions.
- For example, the depletion width W in the semiconductor can be calculated from by using the p -n approximation (i.e., by assuming the negative charge in the dipole is a thin sheet of charge to the left of the junction).
- Similarly, the junction capacitance is Aξs/W, as in the p -n junction.
Schottky barrier on p type:
- Figure given below illustrates a Schottky barrier on a p-type semiconductor, with Φm < Φs.
- In this case aligning the Fermi levels at equilibrium requires a positive charge on the metal side and a negative charge on the semiconductor side of the junction.
- The negative charge is accommodated by a depletion region W in which ionized acceptors (Na-) are left uncompensated by holes.
- The potential barrier V0 retarding hole diffusion from the semiconductor to the metal is Φs - Φm, and as before this barrier can be raised or lowered by the application of voltage across the junction.