Statement I : By doping silicon semiconductor with pentavalent material, the electrons density increases.
Statement II : The n-type semiconductor has net negative charge.
In the light of the above statements, choose the most appropriate answer from the options given below :

Solution

Physics sir · 4 steps

Doping adds electrons, but every electron leaves a positive ion behind, so the crystal stays neutral.

1. Statement I. A pentavalent atom such as P or As has 5 valence electrons. Four of them bond with neighbouring Si atoms and the fifth is loosely held, so it becomes a free electron at room temperature. The electron density rises, so Statement I is true.
2. Count the charges. Each donor atom that gives away its extra electron becomes a fixed positive ion, P⁺. So every extra free electron is matched by one positive donor ion in the lattice.

   nₑ = nₕ + N_D⁺

3. Statement II. Total negative charge (electrons) equals total positive charge (holes plus donor ions). The n-type crystal is electrically neutral, so Statement II is false. The "n" tells you the majority carriers are electrons, not that the crystal carries a net negative charge.
4. So. Statement I is true and Statement II is false, which is option (a).

The idea to keep. Doping never adds net charge, because you are adding neutral atoms. n-type and p-type only name the majority carrier; both crystals stay electrically neutral.

Answer from the NTA final answer key. Solution by PrepWiser.