Metal-Semiconductor Junctions

Fig. 5.2(b): Band profiles of semiconductor-metal junction in equilibrium

When metal and semiconductor are separate from each other, the Fermi levels will look like in fig. 5.2(a). When they are in electronic contact, the Fermi levels will line up. This is achieved by the exchange of charge carriers across the junction, with the consequence that the layers approach the thermal equilibrium. The energy at the conduction band edge at the interface between semiconductor and metal is higher than in the bulk of the semiconductor. The electrostatic potential energy is shown in fig. 5.2(b) by the change in Evac.

The space charge region or depletion region is the region where there is a net charge.

As Evac changes by a certain amount, so must the conduction and valence band energies, and by the same amount. This happens because the electron affinity and band gap are invariant in the semiconductor, and is called band bending.

5.4.1 Behaviour in the light

Illumination of the semiconductor with photons of energy greater than Eg, accumulates the electrons in semiconductor side and holes in the metal side of the depletion region. There occurs an electron-hole pair generation. The light splits the Fermi level (fig. 5.3) and creates a photovoltage V, equal to the difference in the Fermi levels of semiconductor and metal far from the junction.

Fig. 5.3: Under illumination, photogenerated electrons accumulate in the n-type semiconductor. This raises the electron Fermi level and generates a photovoltage, V.

5.4.2 Behaviour in the dark

In the dark, the interface between semiconductor and metal works as a barrier to electron flow from -type semiconductor.

• At equilibrium, drift of holes from semiconductor is balanced by a small current due to the thermal activation of electrons over barrier.
• When a forward bias is applied the barrier reduces and electrons pass from semiconductor to metal.
• A reverse bias increases the barrier height and there is only a small "leakage" current in the reverse direction.

Fig. 5.4: Current-voltage characteristic of Schottky barrier junction in the dark.

5.4.2.1 n and p type metal-semiconductor junction

The contact of metal with p-type semiconductor with work function Φm < Φp causes bands to bend down toward the interface. The majority carriers in this case are holes. (fig. 5.5)

Fig. 5.5: Band profiles for the -type semiconductor-metal junction (a) at equilibrium and (b) under illumination at open circuit.

5.4.3 Ohmic contacts

The opposite case to the Schottky barrier is Ohmic contact, where for the -type semiconductor-metal junction, Φm < Φn, and for the -type semiconductor-metal junction, Φm > Φp.

Fig. 5.6: Band profiles for ohmic metal-semiconductor contacts for (a) an -type semiconductor and (b) a -type semiconductor. The difference in work function between the two layers is caused by the build-up of majority carriers near the interface.

The majority carrier current passes easily in either direction because the semiconductor bands bend a certain way, and we have a low resistance contact. The mechanism which contributes to the photovoltage in a barrier junction is absent.