Below given are some statements about electronic devices:

(A) Diodes can be used for rectifying an ac voltage.
(B) For semiconductors, band gap energy $E_g > 3 eV$.
(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:

(A), (C) and (D) only

The correct answer is Option (4) → (A), (C) and (D) only

Analysis of the given statements:

(A) True — Diodes allow current in one direction and are used for rectifying AC voltage.

(B) False — For semiconductors, the band gap energy is typically less than 3 eV (about 1 eV).

(C) True — The applied external voltage (forward or reverse bias) changes the junction barrier potential.

(D) True — The p–n junction is fundamental to almost all semiconductor devices.

Correct statements: (A), (C), and (D)