Question

What is extrinsic semiconductor? How many types of these are? Write their names. Explain the processes which are occurred during the formation of a P-N junction. Determine the electric field produced at a P-N junction when width of depletion layer is $1$ micrometer and barrier potential is $0.7$ volt.

Answer 1

Extrinsic Semiconductor: When a small amount, say, a few parts per million (ppm), of a suitable impurity is added to the pure semiconductor, the conductivity of the semiconductor is increased manifold. Such materials are known as extrinsic semiconductors or impurity semiconductors.

There are two types of semiconductor p-type and n-type semiconductor.

p-type is obtained when Si or Ge is doped with a trivalent impurity like Al, B, In, etc. whereas in n-type Si or Ge is doped with a pentavalent element.

Formation of p-n junction:

Two important processes occur during the formation of a p-n junction: diffusion and drift.

Diffusion : During the formation of p-n junction, and due to the concentration gradient across p-, and n- sides, holes diffuse from p-side to n-side (p → n) and electrons diffuse from n-side to p-side (n → p). This motion of charge carries gives rise to diffusion current across the junction. As the electrons continue to diffuse from n → p, a layer of positive charge (or positive space-charge region) on n-side of the junction is developed. As the holes continue to diffuse, a layer of negative charge (or negative space-charge region) on the p-side of the junction is developed. This space-charge region on either side of the junction together is known as depletion region.

Drift : Due to the positive space-charge region on n-side of the junction and negative space charge region on p-side of the junction, an electric field directed from positive charge towards negative charge develops. Due to this field, an electron on p-side of the junction moves to n-side and a hole on n-side of the junction moves to p-side. The motion of charge carriers due to the electric field is called drift. Thus a drift current, which is opposite in direction to the diffusion current starts.

This process continues until the diffusion current equals the drift current. Thus a p-n junction is formed.

Problem: Given Width of depletion layer : $1\mu m$

Barrier potential :$0.7V$

Electric field $E=-\frac{dV}{dx}$

$=-\frac{0.7}{{10}^{-6}}=0.7\times {10}^{6}V/m$