Semiconductor Electronics — Physics Class 12 Notes (CBSE & HBSE)
Free NCERT Physics notes for Semiconductor Electronics (Class 12) on Siksha Sarovar, aligned to CBSE and Haryana Board (HBSE). This chapter is broken into 3 topics with clear explanations, formulas, solved examples and board-pattern practice — free to read, no sign-up required.
Board exam focus — Semiconductor Electronics (CBSE & HBSE)
CBSE focuses on energy band theory, p-n junction formation, depletion layer, I-V characteristics, rectification, transistor action in CE configuration, and logic gates. HBSE emphasizes semiconductor types, diode as rectifier, transistor basics, and truth tables of logic gates.
Energy Bands and Semiconductors
Energy Bands in Solids
In isolated atoms, electrons occupy discrete energy levels. When atoms bond together in a solid, these levels broaden into energy bands due to interactions between atoms.
Three important bands:
- Valence band: Highest filled band (occupied by valence electrons)
- Conduction band: Next higher band (electrons here are free to conduct)
- Forbidden gap (Band gap E_g): Energy gap between valence and conduction bands
Classification by Band Gap
| Material | Band Gap E_g | Characteristics |
|---|---|---|
| Conductor (metals) | Zero (overlap) | Always conducts; electrons free |
| Semiconductor | Small (~1 eV) | Conducts with energy; variable |
| Insulator | Large (>3 eV) | Rarely conducts; band gap too large |
Examples:
- Conductor: Cu, Ag, Al (E_g ≈ 0)
- Semiconductor: Si (E_g = 1.12 eV), Ge (E_g = 0.67 eV)
- Insulator: Glass (~10 eV), Diamond (~5.5 eV)
Intrinsic Semiconductors
Intrinsic semiconductor: Pure semiconductor without any impurities.
- At 0K: No free electrons; insulator
- At room temperature: Thermal energy breaks some bonds → electron-hole pairs
- Charge carriers: Electrons (n) and holes (p)
- n_e = n_h = n_i (intrinsic carrier concentration)
- Conductivity increases with temperature (α < 0, unlike metals)
Hole: Absence of electron in valence band; behaves as positive charge carrier.
Extrinsic Semiconductors (Doped)
Adding impurities (dopants) dramatically increases conductivity.
n-type semiconductor:
- Dopant: Pentavalent element (P, As, Sb) with 5 valence electrons
- Extra electron donated to conduction band → majority carriers: electrons
- n_e >> n_h; donor energy level just below conduction band
p-type semiconductor:
- Dopant: Trivalent element (B, Al, Ga) with 3 valence electrons
- Creates hole in valence band → majority carriers: holes
- n_h >> n_e; acceptor energy level just above valence band
Mass Action Law
n_e × n_h = n_i² (at constant temperature)
Doping increases majority carriers but decreases minority carriers.
Temperature Dependence
- Semiconductors: Conductivity increases with T (more electron-hole pairs)
- Metals: Conductivity decreases with T (more collisions)
Diagram Indicator: [Energy band diagrams for conductor (overlapping bands), semiconductor (small gap), insulator (large gap); also n-type and p-type doped semiconductor band diagrams with donor/acceptor levels labeled.]
p-n Junction Diode and Rectifier
p-n Junction Formation
A p-n junction is formed when a p-type semiconductor is joined with an n-type semiconductor.
Diffusion and Drift:
- Holes from p-side diffuse to n-side; electrons from n-side diffuse to p-side
- Recombination near junction → creates depletion region (depleted of mobile carriers)
- Depletion region has fixed ion charges → creates built-in potential barrier V₀
- Barrier prevents further diffusion (equilibrium)
Built-in voltage: V₀ ≈ 0.3V (Ge), 0.7V (Si)
Forward Biasing
- External battery: p-side to +ve, n-side to −ve
- Applied V opposes built-in potential → depletion region narrows
- Above threshold voltage (≈0.7V for Si): Current flows
- Current increases exponentially with voltage
Reverse Biasing
- External battery: p-side to −ve, n-side to +ve
- Applied V aids built-in potential → depletion region widens
- Only small reverse current (minority carriers) flows
- At breakdown voltage: Large reverse current (Zener or avalanche breakdown)
I-V Characteristics
| Condition | Behavior | Current |
|---|---|---|
| Forward bias > V₀ | Conducts well | mA range |
| Forward bias < V₀ | Little current | Negligible |
| Reverse bias | Blocked | μA (leakage) |
| Beyond V_br | Breakdown | Large |
Ideal diode: Zero resistance in forward bias; infinite resistance in reverse bias.
Diode as Rectifier
Rectifier converts AC to DC (unidirectional current).
Half-wave rectifier (1 diode):
- Conducts only during positive half-cycle
- Output: Pulsating DC, only positive half
- Efficiency: 40.6%
Full-wave rectifier (bridge: 4 diodes or center-tap: 2 diodes):
- Both half-cycles utilized
- Output: Pulsating DC with double frequency
- Efficiency: 81.2%
Smoothing: LC filter added to reduce ripple → steady DC
Zener Diode
Operated in reverse breakdown region:
- Maintains constant voltage across it = Zener voltage V_Z
- Used as voltage regulator (stabilizes supply voltage)
LED (Light Emitting Diode)
In forward bias: Electrons and holes recombine → emit photons. Color depends on band gap of semiconductor.
Diagram Indicator: [p-n junction diagram showing depletion region, positive ions on n-side, negative ions on p-side; forward and reverse biased circuits; I-V characteristic curve with forward and reverse regions.]
Transistor, Logic Gates and Digital Electronics
Transistor
A transistor is a three-layer semiconductor device (p-n-p or n-p-n) with three terminals:
- Emitter (E): Heavily doped, emits majority carriers
- Base (B): Thin, lightly doped middle layer
- Collector (C): Moderately doped, collects carriers
Transistor Action (n-p-n in CE configuration)
- Emitter-Base (E-B) junction: Forward biased (low voltage ≈0.7V)
- Collector-Base (C-B) junction: Reverse biased (high voltage ≈5-10V)
- Electrons from emitter enter base → most diffuse across thin base to collector
- Only ~1-5% recombine in base → form base current I_B
Current relation: I_E = I_B + I_C
Current gain (β or h_FE): β = I_C/I_B (typical: 20-500)
Also: α = I_C/I_E; relation: β = α/(1−α)
Common Emitter (CE) Configuration
Input: between Base and Emitter Output: between Collector and Emitter
Characteristics:
- High voltage gain: A_V = −β·R_C/R_in
- High current gain (β)
- Phase reversal: 180° between input and output
- Most widely used configuration
Transistor as Switch
- Cutoff region: I_B = 0 → I_C ≈ 0; transistor OFF (open switch)
- Saturation region: I_B large → I_C maximum; transistor ON (closed switch)
- Logic gates use transistors in switching mode
Transistor as Amplifier
- Active region: I_B small → I_C = β·I_B; small I_B variation → large I_C variation
- Voltage gain in CE: A_V = −β(R_C/R_i)
Logic Gates
Digital electronics works with two states: 0 (LOW) and 1 (HIGH).
| Gate | Symbol | Boolean | Output |
|---|---|---|---|
| NOT | Triangle + bubble | Y = Ā | Inverts input |
| AND | D shape | Y = A·B | 1 only if A AND B are 1 |
| OR | Curved D | Y = A+B | 1 if A OR B is 1 |
| NAND | AND + bubble | Y = Ā·B̄ (NOT AND) | 0 only if both 1 |
| NOR | OR + bubble | Y = Ā+B̄ (NOT OR) | 1 only if both 0 |
| XOR | OR + curved input | Y = A⊕B | 1 if inputs differ |
Truth Tables for Basic Gates
AND Gate:
| A | B | Y=A·B |
|---|---|---|
| 0 | 0 | 0 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 1 |
OR Gate:
| A | B | Y=A+B |
|---|---|---|
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 1 |
NAND and NOR as Universal Gates:
- Any Boolean function can be implemented using ONLY NAND gates or ONLY NOR gates
- These are called universal gates
De Morgan's Theorems
- NOT(A AND B) = NOT(A) OR NOT(B): Ā·B̄ = Ā + B̄
- NOT(A OR B) = NOT(A) AND NOT(B): Ā+B̄ = Ā · B̄
Diagram Indicator: [n-p-n transistor CE amplifier circuit with biasing; truth table for AND, OR, NAND gates; circuit symbol for each gate with input/output labels.]
Frequently asked questions
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Do these notes follow CBSE and HBSE?
Yes. The Semiconductor Electronics notes are NCERT-aligned and include guidance for both CBSE and Haryana Board (HBSE), with important questions and MCQs for revision.
What does the Semiconductor Electronics chapter cover?
Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Semiconductor Electronics.