Silicon doped with electron elements of group-13 forms.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → p-type semi conductor
Silicon has four valence electrons. A group-13 element (B, Al, Ga) has only three, so when it replaces a silicon atom in the lattice one bond is left short of an electron. This place is called an electron hole.
Under an electric field an electron from a neighbouring atom moves into the hole and leaves a new hole behind, so the holes appear to move like positive charges. Conduction by positive holes makes the crystal a p-type semiconductor.
Why the other options are incorrect
Option (1) Conductor — Incorrect. Doping with a few ppm of impurity only raises the small conductivity of silicon; it does not turn it into a metallic conductor.
Option (2) Intrinsic semiconductor — Incorrect. Pure silicon is the intrinsic semiconductor; a doped crystal is an extrinsic one.
Option (4) n-type semi conductor — Incorrect. n-type silicon is made by doping with a group-15 element (P, As), which brings a fifth, extra electron.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Electron-deficit Impurities
