Ohmic contacts

OHMIC CONTACTS

Definition

An ohmic contact is a type of metal semiconductor junction. It is formed by a contact of a metal with a heavily doped semiconductor.

When the semiconductor has a higher work function than that of metal, then the junction formed is called the Ohmic junction.

Here, the current is conducted equally in both directions and there is a very little voltage drop across the junction.

Before contact, Fermi levels of the metal and semiconductor are at different positions as shown in fig. 3.27(a).

Working

After contact, the ohmic junction is shown in figure 3.27 (b).

At equilibrium, the electrons move from the metal to the empty states in the conduction band of semiconductor. Thus, there is an accumulation region near the interface (on the semiconductor side).

It results in line up of Fermi levels of metal and semiconductor as shown in Fig. 3.27 (b).

The accumulation region has a higher conductivity than the bulk semiconductor due to higher concentration of electrons.

Thus, a ohmic contact behaves as a resistor conducting in both forward and reverse bias (Fig. 3.28). The resistivity is determined by the bulk resistivity of the semiconductor.

V-I characteristics

The volt-ampere (V-I) characteristic of the ohmic contact is shown in Fig. 3.29.

The current is directly proportional to the potential across the junction and it is symmetric about the origin, as shown in fig. 3.29.

Thus, ohmic contacts are non-rectifying and show negligible voltage drop and resistance irrespective of the direction and magnitude of current.

Applications

The use of ohmic contacts is to connect one semiconductor device to another, an IC, or to connect an IC to its external terminals.

Table 3.8

Differences between schottky diode and ohmic contact