Pentavalent Impurity in Semiconductors

Introduction:
When impurities are added to a semiconductor material, it can modify its electrical properties. This process is known as doping and is commonly used in semiconductor device fabrication. Pentavalent impurities have five valence electrons, which means they have one extra electron compared to the atoms in the semiconductor crystal lattice.

Explanation:
When a pentavalent impurity is added to a semiconductor, such as silicon, it replaces some of the atoms in the crystal structure. The impurity atom has one extra electron that cannot form covalent bonds with the surrounding atoms. This extra electron becomes a "donor" electron, and its presence creates more free electrons in the semiconductor material.

Formation of Free Electrons:
The extra electron from the pentavalent impurity is loosely bound to the impurity atom and requires only a small amount of energy to break the bond. This energy can be provided by thermal energy or external sources, such as an electrical field. Once the bond is broken, the electron becomes free and can move more easily through the crystal lattice. These free electrons contribute to the conductivity of the semiconductor material.

Contribution to Electrical Conductivity:
The presence of more free electrons in the semiconductor increases its electrical conductivity. These electrons can move freely in response to an applied electric field, creating a flow of current. The concentration of free electrons in the semiconductor depends on the level of pentavalent impurity doping. Higher doping levels result in a higher concentration of free electrons and, thus, higher conductivity.

Formation of Holes:
In addition to creating free electrons, the addition of pentavalent impurities also creates "holes" in the semiconductor material. When the impurity atom replaces a semiconductor atom, it leaves behind a vacant spot in the crystal lattice. This vacancy, or hole, behaves as if it were a positive charge carrier. It can accept an electron from a neighboring atom, effectively creating a mobile positive charge carrier.

Conclusion:
In summary, the addition of pentavalent impurities to a semiconductor material creates more free electrons and holes. The extra electron from the impurity becomes a donor electron, increasing the concentration of free electrons and the electrical conductivity of the material. At the same time, the impurity also creates holes, which behave as positive charge carriers. Both free electrons and holes play important roles in the operation of semiconductor devices.