A p-n junction cannot be made by pressing a slab of p-type silicon firmly against a slab of n-type silicon. The main reason is that
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → the surface roughness of any slab is much larger than the spacing between atoms
Option (1) the p-type slab has no free electrons to send across the contact — Incorrect. In junction formation the p-side sends holes, its majority carriers, into the n-side. The electrons come from the n-side. So a lack of electrons in the p-slab is not the problem.
Option (2) the surface roughness of any slab is much larger than the spacing between atoms — Correct. Even a very flat slab has bumps much larger than the atomic spacing of about 2 to 3 Å. The two slabs touch only at a few points, so there is no continuous contact at the atomic level. The carriers meet a break at the contact.
Option (3) the p-type and n-type slabs have different crystal structures — Incorrect. Both slabs are silicon. Doping adds only a very small amount of impurity, so the crystal structure stays the same.
Option (4) the impurity atoms escape from both slabs at the contact — Incorrect. Each impurity atom is bonded to the silicon atoms around it. It stays fixed in the crystal.
NCERT (2026-27):
Example 14.3 – “… Any slab, howsoever flat, will have roughness much larger than the inter-atomic crystal spacing (~2 to 3 Å) and hence continuous contact at the atomic level will not be possible. The junction will behave as a discontinuity for the flowing charge carriers.”