When a piece of pure silicon is doped with aluminium (a trivalent impurity), what type of semiconductor is formed?

Introduction / Context:
Doping is the process of adding small amounts of impurity atoms to pure semiconductors like silicon to control their electrical properties. Depending on whether the dopant has three or five valence electrons, the resulting material becomes p-type or n-type. Understanding this classification is crucial for learning how diodes, transistors, and integrated circuits work in modern electronics.

Given Data / Assumptions:
• The base material is pure silicon, which has four valence electrons. • Aluminium is used as the dopant. • Aluminium has three valence electrons (it is trivalent). • The amount of dopant is small compared to the number of silicon atoms.

Concept / Approach:
In a silicon crystal, each silicon atom forms four covalent bonds with neighbouring silicon atoms. When a trivalent impurity such as aluminium, boron, or gallium is introduced, it has only three valence electrons to form bonds. As a result, one covalent bond remains incomplete, creating a "hole" (an effective positive charge carrier). These holes can move through the lattice when an electric field is applied, and they act as majority carriers. A semiconductor in which holes are the majority carriers is called a p-type semiconductor.

Step-by-Step Solution:
Step 1: Note that silicon is tetravalent and forms four covalent bonds in its crystal structure. Step 2: Aluminium is trivalent, having only three valence electrons available for bonding. Step 3: When an aluminium atom replaces a silicon atom in the lattice, it forms only three complete bonds, leaving one bond incomplete and creating a hole. Step 4: These holes act as positive charge carriers that can move when an external electric field is applied. Step 5: Since holes become the majority carriers, the doped semiconductor is classified as p-type.

Verification / Alternative check:
Standard semiconductor physics categorises trivalent dopants (boron, aluminium, gallium, indium) as acceptor impurities because they can accept an electron into the incomplete bond, leaving behind a mobile hole. In contrast, pentavalent dopants (phosphorus, arsenic, antimony) donate extra electrons and produce n-type materials. As aluminium is trivalent, silicon doped with aluminium must behave like a p-type semiconductor, confirming our conclusion.

Why Other Options Are Wrong:
Option a: Doping significantly changes conductivity by increasing the number of charge carriers; it does not leave conductivity unchanged. Option b: n-type semiconductors are formed by pentavalent dopants that donate extra electrons, not by trivalent aluminium. Option d: Doping with aluminium introduces more charge carriers (holes) and typically reduces resistivity, increasing conductivity. Option e: The doped silicon does not become an insulator; it actually becomes more conductive than pure silicon.

Common Pitfalls:
A common confusion is mixing up donor and acceptor impurities. To avoid this, remember that trivalent impurities (three valence electrons) cannot fully bond with four neighbours, so they create holes and lead to p-type behaviour. Pentavalent impurities (five valence electrons) have one extra electron and therefore produce n-type behaviour. A quick memory trick is "p-type → positive (holes), n-type → negative (electrons)." Aluminium, with three valence electrons, clearly belongs on the p-type side.

Final Answer:
When pure silicon is doped with aluminium, it becomes a p-type semiconductor.