Read the following passage and answer the questions.
A thin wafer of p-type silicon is made by adding boron, a trivalent element, to pure silicon. About one boron atom is added for every million silicon atoms. Pure silicon has about \(5\times10^{28}\) atoms per cubic metre. Its intrinsic carrier concentration \(n_i\) at room temperature is about \(1.5\times10^{16}\ \mathrm{m^{-3}}\). Next, a larger and carefully measured amount of phosphorus, a pentavalent element, is added to one part of the wafer. This part becomes n-type. The wafer now has a p-region and an n-region with a junction between them. As soon as the junction forms, charge carriers start to cross it. After a short time, the net flow of carriers stops and a potential difference appears across the junction. Finally, metal contacts are fixed at the two ends, and the device is joined to a dc supply.
Match List-I with List-II for the junction formed in the wafer.
| List-I | List-II |
|---|---|
| (A) Diffusion | (I) A layer near the junction that has lost its free electrons and holes |
| (B) Drift | (II) Motion of carriers caused by the difference in their concentration across the junction |
| (C) Depletion region | (III) Tends to stop electrons moving from the n-region into the p-region |
| (D) Barrier potential | (IV) Motion of carriers caused by the electric field at the junction |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A)-(II), (B)-(IV), (C)-(I), (D)-(III)
Two processes go on while a p-n junction forms. Carriers first spread from the side where they are many to the side where they are few. The fixed charges left behind then set up a field, and this field moves carriers back across the junction.
(A) Diffusion — This matches (II). Electrons are many on the n-side and holes are many on the p-side. So each kind of carrier spreads across the junction to the side where it is scarce.
(B) Drift — This matches (IV). The field at the junction pushes carriers across it. The drift current is opposite in direction to the diffusion current.
(C) Depletion region — This matches (I). The carriers that crossed first leave behind a thin layer with only fixed ions in it. Its thickness is about one-tenth of a micrometre.
(D) Barrier potential — This matches (III). The potential difference across the junction acts against the flow of carriers. It holds back the electrons that try to move from the n-region into the p-region, so a balance is reached.
NCERT (2026-27):
p-n junction formation – “During the formation of p-n junction, and due to the concentration gradient across p-, and n- sides, holes diffuse from p-side to n-side (p → n) and electrons diffuse from n-side to p-side (n → p).”
p-n junction formation – “The motion of charge carriers due to the electric field is called drift.”
p-n junction formation – “This space-charge region on either side of the junction together is known as depletion region as the electrons and holes taking part in the initial movement across the junction depleted the region of its free charges …”
p-n junction formation – “… Since this potential tends to prevent the movement of electron from the n region into the p region, it is often called a barrier potential.”