Below given are some statements about electronic devices:
(A) Diodes can be used for rectifying an ac voltage.
(B) For semiconductors, band gap energy Eg 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:

The correct answer is (A), (C) and (D) only.

Let us analyze each statement step-by-step to understand why this option is correct:

Statement (A): Diodes can be used for rectifying an ac voltage.
A diode allows electric current to flow primarily in one direction (during forward bias) while blocking it in the opposite direction (during reverse bias). This unidirectional conduction property makes diodes ideal for converting alternating current (AC), which periodically reverses direction, into direct current (DC), which flows in a single direction. This process is called rectification. Thus, Statement (A) is correct.

Statement (B): For semiconductors, band gap energy E_g < 3 eV.
Typically, semiconductors have a relatively small energy band gap between their valence band and conduction band, which is generally less than 3 eV (for example, Silicon has an E_g of about 1.1 eV and Germanium has about 0.72 eV). Materials with a band gap E_g > 3 eV are classified as insulators (such as Diamond with E_g ≈ 5.4 eV). Since the statement in the question writes "Eg 3 eV" without a clear inequality symbol, or if we look at standard classifications, a semiconductor bandgap is strictly less than 3 eV. Since this statement is excluded from the correct option combination, it is considered incorrect or improperly stated in this context. Thus, Statement (B) is not included in the correct set.

Statement (C): By changing the external applied voltage, junction barriers can be changed.
When an external voltage is applied across a p-n junction, it alters the potential barrier height. In forward bias (positive terminal connected to p-side, negative to n-side), the external voltage opposes the built-in barrier potential, decreasing the barrier height and width. In reverse bias, the external voltage aids the built-in barrier potential, increasing the barrier height and width. Thus, junction barriers are indeed changed by external applied voltage, making Statement (C) correct.

Statement (D): p-n junction is the 'key' to all semiconductor devices.
The p-n junction is the fundamental building block of semiconductor technology. Devices such as diodes, transistors (BJTs, FETs), solar cells, LEDs, and integrated circuits rely on the physics and control of one or more p-n junctions to function. Therefore, the p-n junction is rightfully referred to as the 'key' to all semiconductor devices. Thus, Statement (D) is correct.

Combining our findings, statements (A), (C), and (D) are correct, which matches the option (A), (C) and (D) only.