Physics 2009 JAMB Past Questions - Question 50: When a pentavalent impurity is added to a pure semiconductor, it becomes

Correct Answer

B

Explanation

When a pentavalent (having 5 valence electrons) impurity is added to a pure semiconductor, it becomes an N-type semiconductor.

Here's how it works:

1. Pure Semiconductor: A pure semiconductor, like silicon or germanium, has a crystal lattice structure where each atom forms four covalent bonds with its neighboring atoms. This means that each atom has four valence electrons.

2. Pentavalent Impurity: When you introduce a pentavalent impurity (such as phosphorus or arsenic) into the crystal lattice, these impurity atoms have one extra valence electron compared to the host semiconductor atoms. As a result, they can't form all of their five valence electron bonds with neighboring atoms. This extra electron is loosely held, and it can move through the crystal lattice.

3. Electron Donation: The pentavalent impurity atoms, with their extra electron, become donors of free electrons. These free electrons are mobile charge carriers in the crystal lattice. They are not bound to any specific atom and can move through the lattice when influenced by an external electric field or thermal energy.

4. N-type Semiconductor: As a result of the introduction of these electron-donor impurities, the semiconductor becomes N-type. It has an excess of free electrons, which are negative charge carriers. N stands for "negative."

N-type semiconductors are important in electronic devices because they provide a surplus of charge carriers (electrons) that can conduct current. This makes them useful in the construction of various electronic components like diodes, transistors, and integrated circuits.