NEB Class 12 Physics Semiconductor Devices Notes in PDF Complete Handwritten. Physics Notes 2081: All Chapters | New Curriculum | Class 12 Physics Notes download.
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NEB Class 12 Physics Semiconductor Devices Note Handwritten in PDF
PN Junction Formation
P-type semiconductor: Silicon doped with acceptors (Boron, Aluminium) β extra holes (positive charge carriers)
N-type semiconductor: Silicon doped with donors (Phosphorus, Arsenic) β extra electrons (negative charge carriers)
PN junction formed when: P and N regions are joined.
Depletion region: At the junction, electrons from N diffuse into P and recombine with holes. This creates:
- Positive ions on N side (donor atoms lost electron)
- Negative ions on P side (acceptor atoms gained electron)
- A built-in electric field pointing NβP
- A potential barrier (~0.7V for Silicon, ~0.3V for Germanium)
No free carriers in depletion region β acts as insulator.
Forward and Reverse Bias
Forward bias:
- terminal to P-side, β terminal to N-side
- External field opposes built-in field β depletion layer narrows
- At ~0.7V (Si), barrier overcome β large current flows
- Used in: LED, regular diodes for current flow
Reverse bias:
- terminal to N-side, β terminal to P-side
- External field aids built-in field β depletion layer widens
- Only tiny leakage current (microamperes) due to minority carriers
- At very high reverse voltage β breakdown (avalanche or Zener)
- Used in: Zener diodes (voltage regulation), photodiodes (detection)
I-V Characteristics of Diode
Forward bias curve:
- Very small current until threshold voltage (~0.7V for Si)
- After threshold: current rises steeply (exponentially)
Reverse bias curve:
- Very small constant reverse saturation current
- At breakdown voltage (V_BR): current suddenly increases sharply
Zener diode:
- Specially designed to operate in reverse breakdown
- Breakdown voltage (Zener voltage V_Z) is sharp and stable
- Uses: Voltage regulator (keeps output at V_Z regardless of input variation)
Rectification
Half-wave rectifier (one diode):
- One diode conducts only during positive half cycle
- Output: pulsating DC, frequency same as input
- Efficiency: ~40.6%
Full-wave rectifier (two diodes):
- D1 conducts positive half, D2 conducts negative half
- Both half cycles produce current in same direction through load
- Output: pulsating DC at double the input frequency
- More efficient, easier to filter
- Efficiency: ~81.2%
Adding a smoothing capacitor across the load significantly reduces ripple β smoother DC.
Logic Gates Truth Tables and Boolean Algebra
NOT gate: Output = Δ
- 0 β 1, 1 β 0
AND gate: Output = AΒ·B
- 1 only when BOTH inputs are 1
OR gate: Output = A+B
- 0 only when BOTH inputs are 0
NAND gate: Output = (AΒ·B)’ = NOT AND
- 0 only when BOTH inputs are 1 (opposite of AND)
NOR gate: Output = (A+B)’ = NOT OR
- 1 only when BOTH inputs are 0 (opposite of OR)
Universal gates:
- NAND and NOR are called universal gates because any logic gate can be made from only NANDs or only NORs
- NOT from NAND: connect both inputs together
- AND from NAND: NAND followed by NOT (another NAND with inputs tied)
- OR from NAND: requires 3 NAND gates
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Frequently Asked Questions
What is PN junction in Class 12 Physics?
PN junction forms when p-type (holes majority) and n-type (electrons majority) semiconductors are joined. At junction, holes and electrons diffuse and recombine creating a depletion region with no free carriers. A built-in electric field (potential barrier β 0.7V for silicon) prevents further diffusion across the junction.
What is the difference between forward bias and reverse bias?
Forward bias (+terminal to p-side): reduces depletion region, reduces potential barrier, large current flows above 0.7V. Reverse bias (+terminal to n-side): widens depletion region, increases barrier, only tiny leakage current flows until breakdown voltage. Forward bias allows current; reverse bias blocks it making diode a one-way valve.
What is Zener diode and its uses Class 12?
Zener diode is specially doped to operate in reverse breakdown. Above Zener voltage (V_Z), reverse current increases sharply but voltage stays constant at V_Z. This constant voltage property makes Zener ideal for voltage regulation. Other uses: overvoltage protection, reference voltage in circuits, and surge protection devices.
How does full wave rectifier work Class 12?
Full wave rectifier uses two diodes. In positive half cycle, D1 conducts and D2 blocks. In negative half cycle, D2 conducts and D1 blocks. Both half cycles produce current in same direction through load. Output frequency is double the input frequency. Adding a capacitor filter smooths the pulsating DC to nearly constant DC output.
What is NAND gate and why is it called universal gate?
NAND gate output: (AΒ·B)’. Output is zero only when both inputs are 1 opposite of AND. Called universal gate because NOT, AND, OR, and any other logic gate can be built using only NAND gates. NOT: tie both NAND inputs together. AND: NAND followed by NOT-NAND. OR: three NAND gates arranged appropriately.
What are the truth tables for all logic gates Class 12?
NOT: 0β1, 1β0. AND: output 1 only when both inputs are 1. OR: output 0 only when both inputs are 0. NAND: output 0 only when both inputs are 1 (opposite of AND). NOR: output 1 only when both inputs are 0 (opposite of OR). NAND and NOR are universal gates all other gates can be made from either one alone.
What is the difference between half wave and full wave rectifier?
Half wave rectifier: one diode, conducts only positive half cycle, wastes negative half, efficiency β 40.6%, output frequency = input frequency. Full wave rectifier: two diodes, uses both half cycles, efficiency β 81.2%, output frequency = double input frequency. Full wave has less ripple and is easier to filter to smooth DC.
Which semiconductor device topics are most important for Exam?
Most important: PN junction formation and bias (4 marks), full wave rectification with circuit diagram (4 marks), Zener diode uses (2 marks), truth tables for all five logic gates (4 marks), NAND as universal gate (2 marks). Logic gate truth tables and rectifier circuit diagrams appear in every NEB Class 12 board paper.
