Semiconductor Electronics — NEET Physics Notes
Semiconductors form the backbone of modern electronic devices. This topic covers the physics of semiconductors, diodes, transistors, and digital circuits — essential for NEET.
Quick Revision
- Intrinsic Semiconductor: Pure silicon/germanium (Si: 1.1 eV, Ge: 0.72 eV)
- Doping: Adding trivalent (p-type) or pentavalent (n-type) atoms
- p-n Junction: Forms the basis of diode, rectifier, LED
- Forward Bias: p-side positive relative to n-side — current flows
- Reverse Bias: p-side negative relative to n-side — no current (small leakage)
- Rectifier: Converts AC to DC using diode property
- Transistor: Current controlled device — npn and pnp types
- Logic Gates: AND, OR, NOT, NAND, NOR — form digital circuits
Standard Study
Energy Bands
- Conduction Band: Higher energy states where electrons are free
- Valence Band: Lower energy states with bound electrons
- Band Gap (Eg): Forbidden energy region between conduction and valence bands
- Conductor: Overlapping conduction and valence bands
- Insulator: Large band gap (> 3 eV) — electrons cannot jump
- Semiconductor: Small band gap (~1 eV) — electrons can jump with thermal energy
Intrinsic Semiconductors
- Pure semiconductors at room temperature
- Electrons and holes are equal in number (n = p = ni)
- Electrical conductivity increases with temperature (unlike metals)
- Silicon (Si): Eg = 1.12 eV, ni ≈ 10¹⁰/cm³ at 300K
- Germanium (Ge): Eg = 0.72 eV, ni ≈ 10¹³/cm³ at 300K
Extrinsic Semiconductors
n-Type (Donor doping):
- Add group V elements (Phosphorus, Arsenic)
- Majority carriers: electrons
- Minority carriers: holes
- Fermi level moves towards conduction band
p-Type (Acceptor doping):
- Add group III elements (Boron, Gallium)
- Majority carriers: holes
- Minority carriers: electrons
- Fermi level moves towards valence band
p-n Junction
- Depletion Region: No mobile charge carriers on either side of junction
- Barrier Potential: ~0.7V for Si, ~0.3V for Ge at 300K
- Forward Bias: External voltage reduces barrier → current flows
- Reverse Bias: External voltage increases barrier → no conduction (only leakage)
- Breakdown: When reverse voltage exceeds critical value — Zener or Avalanche breakdown
Diode Applications
- Rectifier: Converts AC to DC (half-wave and full-wave bridge)
- Zener Diode: Voltage regulation — operates in reverse breakdown
- LED: Emits light when forward biased — GaAs, GaP used
- Photodiode: Reverse biased, converts light to current
- Solar Cell: Converts light energy to electrical energy
Bipolar Junction Transistor (BJT)
- npn Transistor: Two n-regions separated by p-region
- pnp Transistor: Two p-regions separated by n-region
- Regions: Emitter (heavily doped), Base (thin, lightly doped), Collector
- Current Relations: IE = IB + IC, IC ≈ β × IB, α ≈ β/(β+1)
- Common Base Configuration: α ~ 0.98, no current gain but voltage gain
- Common Emitter Configuration: β varies from 20 to 200, current gain
Transistor as Amplifier
- Input is in forward-biased emitter-base circuit
- Output is in reverse-biased collector-base circuit
- Small base current controls large collector current
- Voltage gain: Av = β × (Rc/Re)
Transistor as Switch
- Cut-off region: Base current = 0 → Transistor OFF
- Saturation region: Base current large enough → Transistor ON
- Used in digital logic circuits
Logic Gates
| Gate | Symbol | Output |
|---|---|---|
| AND | A·B | 1 only if both inputs 1 |
| OR | A+B | 1 if either input 1 |
| NOT | A’ | Inverts input |
| NAND | (A·B)‘ | NOT of AND |
| NOR | (A+B)‘ | NOT of OR |
- Boolean algebra basics: Associative, Distributive, Commutative laws
- De Morgan’s Theorems: (A+B)’ = A’·B’ and (A·B)’ = A’+B’
Deep Study
Drift and Diffusion Currents
- Drift Current: Due to electric field in semiconductor
- Diffusion Current: Due to concentration gradient of carriers
- Total current = drift + diffusion for each carrier type
Fermi Level
- Represents energy level with 50% probability of occupation at 0K
- In intrinsic semiconductor, Fermi level is mid-gap
- In n-type, Fermi level is above mid-gap (closer to conduction band)
- In p-type, Fermi level is below mid-gap (closer to valence band)
Transistor Characteristics
- Input Characteristic: IB vs VBE at constant VCE
- Output Characteristic: IC vs VCE at constant IB
- Active region: Used for amplification
- Saturation and cut-off: Used as switch in digital circuits
Feedback in Amplifiers
- Negative feedback stabilizes gain, reduces distortion
- Emitter bypass capacitor improves gain at high frequencies
Oscillator Circuit
- An amplifier with positive feedback produces oscillations
- LC or RC network determines frequency
- Barkhausen criterion: Total phase shift = 360° (or 0°), Loop gain = 1
Exam Tips
- Diode is forward biased when p-side is at higher potential than n-side
- Zener diode is always reverse biased during operation
- Transistor amplifies current (β) and voltage simultaneously
- For amplification, base-emitter junction is forward biased; base-collector is reverse biased
- Logic gates are building blocks of digital circuits — NAND and NOR are universal gates
- Semiconductor physics: remember Si vs Ge differences (band gap, barrier potential, ni)
- BJT current relation: IE = IB + IC, and IC = αIE
Common Pitfalls
- Confusing donor and acceptor impurities in doping
- Forgetting direction of conventional current (opposite to electron flow)
- Not applying correct biasing conditions for transistor operation
- Confusing transistor configurations (CB vs CE vs CC)
- Miscounting number of valence electrons when determining doping type
Suggested Study Order
- Energy bands in solids (conductor, insulator, semiconductor)
- Intrinsic semiconductors and conduction mechanism
- Extrinsic semiconductors (n-type and p-type)
- p-n junction formation and characteristics
- Diode types and applications
- BJT structure and working
- Transistor as amplifier and switch
- Logic gates and Boolean algebra