Arsenic (As) is added as an impurity to a silicon crystal. The resulting crystal will have
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → more majority carriers
Arsenic is a pentavalent atom. In the silicon lattice, four of its electrons bond with the four Si neighbours. The fifth is so weakly bound that it is free to move at room temperature. So each As atom acts as a donor, and the crystal becomes an n-type semiconductor.
Option (1) a higher resistance — Incorrect. Adding a small amount of a suitable impurity increases the conductivity of the semiconductor many times, so the resistance of the crystal falls.
Option (2) more minority carriers — Incorrect. The extra electrons raise the rate of recombination with holes, so the number of holes, which are now the minority carriers, is reduced further.
Option (3) more majority carriers — Correct. Each donor adds one free electron, so with proper doping the conduction electrons far outnumber the holes, \( n_e\gg n_h \). The electrons are the majority carriers, and doping increases their number.
Option (4) an increased junction height — Incorrect. A uniformly doped crystal has no p-n junction, so there is no junction barrier to change. A barrier height belongs to a junction between a p-region and an n-region.
NCERT (2026-27):
Extrinsic Semiconductor – “In a doped semiconductor the total number of conduction electrons ne is due to the electrons contributed by donors and those generated intrinsically … Thus, with proper level of doping the number of conduction electrons can be made much larger than the number of holes. Hence in an extrinsic semiconductor doped with pentavalent impurity, electrons become the majority carriers and holes the minority carriers.”