Question

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. (A), (B) and (D) only
• B. (A), (B) and (C) only
• C. (B), (C) and (D) only
• D. (A), (C) and (D) only

Hint:

Memorize the approximate band gap energies for conductors, semiconductors, and insulators. - Conductors: Overlapping bands (\(E_g \approx 0\)). - Semiconductors: Small band gap (\(E_g < 3\) eV). - Insulators: Large band gap (\(E_g > 3\) eV). This is a common point of confusion tested in exams.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
This question tests fundamental knowledge about semiconductors and p-n junction diodes. We need to evaluate each statement's validity.

Step 2: Detailed Explanation:
(A) Diodes can be used for rectifying an ac voltage.
A diode allows current to flow easily in one direction (forward bias) and offers very high resistance in the opposite direction (reverse bias). This unidirectional property is exactly what is needed to convert alternating current (AC) into direct current (DC), a process called rectification. This statement is correct.
(B) For semiconductors, band gap energy E\(_g\) > 3 eV.
Materials are classified based on their band gap energy (\(E_g\)). For semiconductors, the band gap is relatively small, typically in the range of 0.2 eV to 3 eV (e.g., Silicon \(E_g \approx 1.1\) eV, Germanium \(E_g \approx 0.7\) eV). Materials with a band gap \(E_g > 3\) eV are generally classified as insulators (e.g., Diamond \(E_g \approx 5.5\) eV). Therefore, this statement is incorrect.
(C) By changing the external applied voltage, junction barriers can be changed.
A p-n junction has a potential barrier (or depletion region). Applying an external voltage (biasing) alters this barrier. A forward bias voltage reduces the barrier height, allowing current to flow. A reverse bias voltage increases the barrier height, blocking current flow. Thus, the junction barrier can be controlled by an external voltage. This statement is correct.
(D) p-n junction is the 'key' to all semiconductor devices.
The p-n junction is the fundamental building block for a vast majority of semiconductor devices, including diodes, transistors (which can be seen as two back-to-back p-n junctions), solar cells, LEDs, etc. Its ability to control current flow is central to modern electronics. This statement is generally considered correct in the context of introductory electronics.

Step 3: Final Answer:
Statements (A), (C), and (D) are correct, while (B) is incorrect. The correct option is (D).