Conductivity and Resistivity of Semiconductor Materials

The value of conductivity of a material gives us an estimate of the extent to which a material supports the flow of current through it. Electrical conductivity depends upon the number of electrons available in the conduction process. The concept of conductivity is useful in many engineering applications including medical electronics.

J = nqμE

Equation (2.17) derived in the previous section can also be written as

is called as conductivity of the material.

Thus, electrical conductivity of a material is defined as the ratio of current density J and electric field intensity E.

Conductivity of semiconductor materials increases with temperature, as an increase in temperature causes increase in conduction current. This is due to increase in broken covalent bonds that result in more charge carriers for current flow. So more electrons from Valence Band jump to Conduction Band with increase in temperature. The conductivity of semiconductors varies completely in the opposite way to that of metals.

Here it is found that current density (J) and field strength (E) are proportional to each other with σ as the constant of proportionality: J ∝ I and E ∝ v.

So σ has the dimensions of Siemens/m as shown below:

As already explained, semiconductors contain two types of mobile charge carriers, electrons and Holes. In semiconductors, the conductivity depends upon the concentrations and mobility of both electrons and Holes (Fig. 2.11).

Fig. 2.11 Electrons in a conducting medium

where n is the concentration (number) of electrons, p is the concentration (number) of Holes, μ_n is the mobility of electrons and μ_p = mobility of Holes.

In an intrinsic semiconductor n = p = n_i

If the values for the mobility and concentrations of electrons and Holes are known, the conductivity of the materials can be estimated.