Explain the working of a PN junction diode
Quick Answer
A PN junction diode joins P-type and N-type semiconductors. A depletion region forms with 0.7V barrier (Si). Forward bias reduces depletion, allowing current. Reverse bias widens depletion, blocking current. Applications: rectifiers, LEDs, voltage regulators.
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Why Interviewers Ask This
Fundamental building block of all semiconductor devices
Tests understanding of semiconductor physics
Basis for understanding transistors, LEDs, solar cells
Shows grasp of core ECE concepts
Foundation for analog and digital electronics
Concept Explanation
Simple Explanation (Start Here)
A PN junction diode is like a one-way door for current. When you push from the P-side (forward bias), the door opens and current flows. When you push from the N-side (reverse bias), the door stays closed. The "doorframe" is the depletion region that controls this behavior.
Real-World Analogy
Imagine two rooms separated by a door with a spring. Room P has people wanting to go to Room N. Forward bias = pushing the door in the direction it opens (easy). Reverse bias = pushing against the spring (hard, door stays closed). The spring represents the depletion region's barrier potential.
Detailed Technical Explanation
Formation: When P-type (excess holes) and N-type (excess electrons) semiconductors are joined, electrons diffuse from N to P, and holes from P to N. This creates a depletion region with immobile ions and no free charge carriers, establishing a barrier potential (0.7V for Si, 0.3V for Ge).
Forward Bias: Positive terminal to P-side, negative to N-side. External voltage opposes barrier potential, reducing depletion width. When V > barrier potential, current flows exponentially.
Reverse Bias: Positive terminal to N-side, negative to P-side. External voltage adds to barrier potential, widening depletion region. Only tiny leakage current flows (minority carriers). At breakdown voltage, current increases sharply (avalanche/Zener breakdown).
Key Facts to Remember
- Depletion Region: Created at junction, no free carriers, acts as insulator until forward biased
- Barrier Potential: 0.7V for Silicon, 0.3V for Germanium
- Forward Bias: P to +ve, N to -ve, current flows when V > 0.7V
- Reverse Bias: P to -ve, N to +ve, depletion widens, only leakage current
- Breakdown: At high reverse voltage, diode conducts (Zener/Avalanche effect)
Visual Explanation
Draw PN junction showing: P-region (holes), N-region (electrons), Depletion region in middle with +/- ions. Show forward bias connection (battery + to P) and reverse bias connection (battery + to N). Draw VI characteristic curve showing exponential rise in forward, flat region with small leakage in reverse, and breakdown.
Pro tip: Draw this diagram while explaining to leave a strong impression.
Common Mistakes to Avoid
- ✗Forgetting to mention the barrier potential value (0.7V for Si)
- ✗Confusing forward and reverse bias terminal connections
- ✗Not explaining why depletion region forms
- ✗Saying current is zero in reverse bias (small leakage current exists)
- ✗Not mentioning applications like rectifiers, LEDs
Pro Tips for Success
- ✓Always draw a diagram—it shows you understand the concept visually
- ✓Mention the 0.7V barrier potential for silicon—interviewers expect this specific value
- ✓Know at least one application: rectifier, LED, voltage regulator (Zener)
- ✓Be ready to explain why minority carriers cause leakage current in reverse bias
Expected Follow-up Questions
Key Takeaways
- Depletion region = insulating layer at junction with barrier potential
- Forward bias: P to +ve, current flows after 0.7V (Si)
- Reverse bias: P to -ve, blocks current, only leakage
- Breakdown occurs at high reverse voltage (Zener/Avalanche)
- Applications: Rectifiers, LEDs, Zener regulators
Related Questions You Should Know
What is the difference between analog and digital communication?
Analog communication is like a live concert—sound waves travel continuously as they are. Digital communication is like sending a text message—the voice is converted to 0s and 1s, sent, then reconstructed. Digital is like sending LEGO instructions instead of the actual sculpture—easier to fix if pieces are missing.
Explain Kirchhoff's laws with examples
KCL (Current Law): At any junction, what goes in must come out—like cars at an intersection. KVL (Voltage Law): Around any closed loop, the total voltage rise equals total voltage drop—like climbing a hill and coming back down, net elevation change is zero.
Research Foundations
Our Electronics & Communication Engineering interview guides are built on established pedagogical research and industry best practices. Here are the key sources that inform our approach:
Dr. HC Verma
Concepts of Physics (1992)
“Understanding fundamentals deeply enables solving complex problems by breaking them into basic principles.”
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When answering technical questions, always start from first principles. Interviewers value candidates who understand WHY, not just WHAT.
Gayle Laakmann McDowell
Cracking the Coding Interview (2022)
“Technical interviews test problem-solving process, not just memorized answers.”
How We Apply This:
Think out loud, explain your reasoning, and show how you approach unfamiliar problems systematically.
Richard Feynman
The Feynman Technique
“If you cannot explain something simply, you do not understand it well enough.”
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NPTEL Faculty
National Programme on Technology Enhanced Learning
“Strong fundamentals in core subjects differentiate exceptional engineers from average ones.”
How We Apply This:
Revisit core subjects from your curriculum. Most technical questions test fundamental concepts, not advanced topics.
George Pólya
How to Solve It (1945)
“A systematic approach to problem-solving works across all engineering domains.”
How We Apply This:
Use a structured approach: Understand → Plan → Execute → Verify. Interviewers notice methodical thinking.
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