22.1 P–N Junction
- Formed by joining p-type and n-type semiconductors.
- At junction:
- Electrons & holes combine → depletion layer formed.
- Potential barrier created → prevents further flow.
22.2 Semiconductor Diode: Characteristics
(a) Forward Bias
- P connected to +, N to –.
- Barrier potential decreases → current flows easily.
- Current rises sharply after threshold (≈ 0.7 V for Si, 0.3 V for Ge).
(b) Reverse Bias
- P connected to –, N to +.
- Barrier potential increases → only small leakage current flows.
- At high reverse voltage → breakdown occurs.
22.3 Full Wave Rectification
- Converts AC → DC using two diodes (center tap transformer) or bridge rectifier.
- Output: Pulsating DC during both half cycles.
- Efficiency ≈ 81% (greater than half-wave rectifier).
22.5 Logic Gates
- NOT gate: Output is inverse of input.
- OR gate: Output 1 if any input is 1.
- AND gate: Output 1 if all inputs are 1.
- NAND gate: Output is inverse of AND. (Universal gate).
- NOR gate: Output is inverse of OR. (Universal gate).
Short Questions and Answers in Short
Q1. What is a depletion layer?
👉 Region around p–n junction with immobile ions, no free charge carriers.
Q2. Define barrier potential.
👉 Small potential across junction that opposes further carrier diffusion.
Q3. What happens when diode is forward biased?
👉 Current flows after threshold voltage.
Q4. What is the condition of diode in reverse bias?
👉 Only a small leakage current flows.
Q5. Why is a diode called a rectifier?
👉 Because it converts AC into DC.
Q6. Write one advantage of full-wave over half-wave rectifier.
👉 Higher efficiency and smoother output.
Q7. What is the threshold voltage for Si and Ge diodes?
👉 Si = 0.7 V, Ge = 0.3 V.
Q8. What are universal gates? Why called so?
👉 NAND and NOR → because any logic function can be implemented using them.
Q9. Write Boolean expression for OR gate.
👉 Y=A+B
Q10. Write Boolean expression for AND gate.
👉 Y=AB
Q11. What is the output of NOT gate if input is 0?
👉 Output = 1.
Q12. State the truth table of NAND gate (for 2 inputs).
👉 Output is 0 only when both inputs are 1, otherwise 1.
Numerical Examples
✅ Formula Sheet (Quick Revision)
