In a Germanium crystal equal number of aluminum and indium atoms are added. Then,
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → it becomes a p-type semiconductor
Germanium is tetravalent. Aluminium and indium are both trivalent (group 13): each atom forms only three bonds with the neighbouring germanium atoms and leaves a vacancy, a hole. Both are therefore acceptor impurities.
Adding them in equal numbers does not cancel anything, because both create holes. The holes become the majority carriers, so the crystal becomes a p-type semiconductor.
Why the other options are incorrect
Option (1) it remains an intrinsic semiconductor — Incorrect. Equal amounts of two dopants cancel only when one is a donor and the other an acceptor; here both are acceptors, so their effects add.
Option (2) it becomes a n-type semiconductor — Incorrect. An n-type semiconductor needs a pentavalent (donor) impurity such as phosphorus, arsenic or antimony.
Option (4) it becomes an insulator — Incorrect. Doping increases the number of charge carriers; it cannot turn a semiconductor into an insulator.
NCERT (2026-27):
p-type Semiconductor – “This is obtained when Si or Ge is doped with a trivalent impurity like Al, B, In, etc. … Therefore, extrinsic semiconductors doped with trivalent impurity are called p-type semiconductors.”