Step 1: Concept Assessment: This question evaluates fundamental understanding of semiconductors and p-n junction diodes. Each statement requires validation.
Step 2: Detailed Analysis: (A) Diodes can be used for rectifying an ac voltage. A diode permits current flow in one direction (forward bias) and impedes it in the opposite direction (reverse bias). This unidirectional characteristic is essential for converting alternating current (AC) to direct current (DC), a process known as rectification. This statement is correct. (B) For semiconductors, band gap energy \(E_g\) > 3 eV. Materials are categorized by their band gap energy (\(E_g\)). Semiconductors possess a small band gap, typically between 0.2 eV and 3 eV (e.g., Silicon \(E_g \approx 1.1\) eV, Germanium \(E_g \approx 0.7\) eV). Materials with \(E_g\) > 3 eV are generally classified as insulators (e.g., Diamond \(E_g \approx 5.5\) eV). Consequently, this statement is incorrect. (C) By changing the external applied voltage, junction barriers can be changed. A p-n junction has a potential barrier (depletion region). An applied external voltage (biasing) modifies this barrier. Forward bias reduces the barrier, enabling current flow. Reverse bias increases the barrier, blocking current flow. Thus, the junction barrier is controllable via external voltage. This statement is correct. (D) p-n junction is the 'key' to all semiconductor devices. The p-n junction serves as the fundamental component for most semiconductor devices, including diodes, transistors, solar cells, and LEDs. Its current-controlling capability is vital to modern electronics. This statement is generally accepted as correct in basic electronics contexts.
Step 3: Conclusion: Statements (A), (C), and (D) are accurate; statement (B) is inaccurate. The correct option is (D).