Below given are some statements about electronic devices:
(A) Diodes can be used for rectifying an ac voltage.
(B) For semiconductors, band gap energy \( E_g \gt 3\,\mathrm{eV} \).
(C) By changing the external applied voltage, junction barriers can be changed.
(D) p-n junction is the 'key' to all semiconductor devices.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A), (C) and (D) only
(A) Diodes can be used for rectifying an ac voltage. — Correct. A p-n junction diode conducts in one direction only, so it passes one half of the ac cycle and blocks the other.
(B) For semiconductors, band gap energy \( E_g \gt 3\,\mathrm{eV} \). — Incorrect. A semiconductor has a small band gap, below about \( 3\,\mathrm{eV} \) (for example about \( 1.1\,\mathrm{eV} \) for silicon). A gap above \( 3\,\mathrm{eV} \) belongs to an insulator.
(C) By changing the external applied voltage, junction barriers can be changed. — Correct. Forward bias lowers the barrier and reverse bias raises it.
(D) p-n junction is the 'key' to all semiconductor devices. — Correct. Diodes, transistors and many other devices are built from p-n junctions.
NCERT (2026-27):
Summary – “… 11. For insulators Eg > 3 eV, for semiconductors Eg is 0.2 eV to 3 eV, while for metals Eg ≈ 0.
12. p-n junction is the ‘key’ to all semiconductor devices. …
13. By changing the external applied voltage, junction barriers can be changed. …
14. Diodes can be used for rectifying an ac voltage (restricting the ac voltage to one direction).”