Which of the following should be doped with pure silicon to get an n-type and p-type semiconductors respectively?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Phosphorous and Boron
Silicon is tetravalent. The type of an extrinsic semiconductor depends on the valency of the dopant.
n-type: a pentavalent atom such as phosphorus uses four electrons in bonds with the neighbouring silicon atoms; the fifth electron is loosely held and becomes a free electron. Phosphorus is a donor.
p-type: a trivalent atom such as boron leaves one bond short of an electron, creating a hole. Boron is an acceptor.
The order asked is n-type first, then p-type, so phosphorus and boron. Indium and aluminium are both trivalent and would both give p-type; sodium and magnesium are not used as dopants for silicon.
NCERT (2026-27):
Extrinsic Semiconductor – “… There are two types of dopants used in doping the tetravalent Si or Ge: (i) Pentavalent (valency 5); like Arsenic (As), Antimony (Sb), Phosphorous (P), etc.”
Extrinsic Semiconductor – “(ii) Trivalent (valency 3); like Indium (In), Boron (B), Aluminium (Al), etc.”
n-type Semiconductor – “Suppose we dope Si or Ge with a pentavalent element as shown in Fig. 14.7.”
p-type Semiconductor – “This is obtained when Si or Ge is doped with a trivalent impurity like Al, B, In, etc.”