Overview

Semiconductor Devices is a high-frequency chapter with two distinct halves: P-N junction diodes and rectification, and digital logic gates. Nearly every board paper draws a question from each half rather than favoring one — the diode side almost always asks you to explain full wave rectification using two junction diodes, often paired with a conceptual question about why the depletion layer shrinks under forward bias, while the logic side asks you to derive a truth table for a combination of AND, OR, and NOT gates and identify which composite gate the combination forms (usually NAND or NOR).

The logic gate questions look intimidating with their circuit diagrams, but they’re mechanical once you know the individual truth tables for AND, OR, and NOT — build the intermediate outputs (like A’ and B’) column by column, then combine them for the final output Y. The Zener diode questions form a smaller but recurring third theme, usually asking you to explain how it differs from an ordinary diode and how it’s used as a voltage regulator, which hinges on understanding controlled reverse breakdown rather than the usual forward-conduction behavior of a diode.

Below is the full set of MCQs, theory questions, and numerical problems from past NEB board exams, organized by course and question type.

⚠️ Common Mistakes Students Make

• Mixing up forward and reverse bias effects on the depletion layer. Forward bias narrows the depletion layer (allowing current to flow); reverse bias widens it. Students frequently reverse this when justifying “why current flows” in forward bias.
• Building truth tables in the wrong order. For multi-gate circuits, skipping the intermediate outputs (like A’ from a NOT gate) and trying to jump straight to the final column Y is the most common source of wrong truth tables — work through each gate’s output as its own column first.
• Confusing avalanche breakdown with Zener breakdown. Both occur under reverse bias, but Zener breakdown happens in heavily doped junctions at lower reverse voltages due to the electric field itself pulling electrons free, while avalanche breakdown involves collision-triggered multiplication at higher reverse voltages in lightly doped junctions.