On doping germanium metal with phosphorus we would get:-

The correct answer is Option (3) → n-type semiconductor

Germanium belongs to group 14 and has four valence electrons, all used in four covalent bonds with its neighbours. Phosphorus (group 15) has five valence electrons. When P occupies a lattice site of germanium, four of its electrons form the four bonds and the fifth electron is extra and becomes delocalised. Conduction is now due to these negatively charged electrons, so the doped crystal is an n-type semiconductor.

Why the other options are incorrect

• Option (1) Insulator — Incorrect. Doping increases the conductivity of germanium; it does not make it an insulator.

• Option (2) Rectifier — Incorrect. A rectifier (diode) is a device made by joining n-type and p-type material, not the doped crystal itself.

• Option (4) p-type semiconductor — Incorrect. This is obtained by doping with a group-13 element such as B, Al or Ga, which creates electron holes.

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-Rich Impurities – “Silicon and germanium belong to group 14 of the periodic table and have four valence electrons each. In their crystals each atom forms four covalent bonds with its neighbours (Fig. 1.30 a). When doped with a group 15 element like P or As, which contains five valence electrons, they occupy some of the lattice sites in silicon or germanium crystal (Fig. 1.30 b). Four out of five electrons are used in the formation of four covalent bonds with the four neighbouring silicon atoms. The fifth electron is extra and becomes delocalised. These delocalised electrons increase the conductivity of doped silicon (or germanium). Here the increase in conductivity is due to the negatively charged electron, hence silicon doped with electron-rich impurity is called n-type semiconductor.”