Semiconductor Electronics explains how the energy band gap sorts solids into conductors, insulators and semiconductors, and how doping pure silicon or germanium creates n-type and p-type material that combine into the p-n junction diode. It builds up to real devices like rectifiers, special diodes, transistors and logic gates, making it a high-yield, formula-and-concept chapter for NEET.
Key Concepts
1. Energy Bands in Solids
| Type | Band Gap | Examples |
|---|---|---|
| Conductor | No gap (bands overlap) | Copper, silver, aluminium |
| Semiconductor | Small gap (~1 eV) | Silicon (1.1 eV), Germanium (0.7 eV) |
| Insulator | Large gap (> 3 eV) | Diamond, rubber, glass |
2. Intrinsic and Extrinsic Semiconductors
Intrinsic: Pure semiconductor (equal electrons and holes). Conductivity is low.
Extrinsic: Doped with impurities to increase conductivity.
| Type | Dopant | Majority Carriers | Example |
|---|---|---|---|
| n-type | Pentavalent (P, As, Sb) | Electrons | Si doped with As |
| p-type | Trivalent (B, Al, Ga, In) | Holes | Si doped with B |
3. P-N Junction Diode
When p-type and n-type are joined, a depletion region forms at the junction (thin region with no free carriers).
- Forward bias: P connected to +, N to −. Current flows (barrier reduced). Diode conducts above ~0.7 V (Si) or ~0.3 V (Ge).
- Reverse bias: P to −, N to +. Very small reverse current (barrier increased). Diode blocks.
Applications of Diode
- Rectifier: Converts AC to DC (half-wave or full-wave)
- Zener diode: Voltage regulator (maintains constant output voltage)
- LED: Emits light when forward biased
- Photodiode: Generates current when light falls on it (reverse biased)
- Solar cell: Converts light to electricity (no external bias needed)
4. Transistor
A transistor has three layers (NPN or PNP) and three terminals: Emitter (E), Base (B), Collector (C).
Current relation: IE = IB + IC (emitter current = base + collector)
Current gain: β = IC/IB (typically 20–200)
Transistor as Amplifier (Common Emitter)
Small change in base current → large change in collector current.
Voltage gain: Av = −βRL/Ri
Input: base-emitter circuit; Output: collector-emitter circuit.
Transistor as Switch
Cutoff region: both junctions reverse biased → transistor OFF (switch open)
Saturation region: both junctions forward biased → transistor ON (switch closed)
5. Logic Gates
| Gate | Symbol | Boolean | Output |
|---|---|---|---|
| AND | A · B | Y = A·B | 1 only if both inputs are 1 |
| OR | A + B | Y = A+B | 1 if any input is 1 |
| NOT | Ā | Y = Ā | Inverts input |
| NAND | (A·B)̄ | Y = (A·B)̄ | 0 only if both inputs are 1 |
| NOR | (A+B)̄ | Y = (A+B)̄ | 1 only if both inputs are 0 |
NAND and NOR are universal gates - any logic function can be built using only NAND or only NOR gates.
Solved Examples
Example 1
In a transistor, IB = 50 μA and IC = 5 mA. Find IE and β.
Answer: IE = IB + IC = 0.05 + 5 = 5.05 mA. β = IC/IB = 5/0.05 = 100.
Example 2
A Zener diode has breakdown voltage 5 V. Input voltage varies from 7–12 V with 500 Ω series resistance. Find current range.
Answer: I = (V_in − V_z)/R. Min: (7−5)/500 = 4 mA. Max: (12−5)/500 = 14 mA. Output stays at 5 V.
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Quick Revision Points
- Semiconductor band gap: Si = 1.1 eV, Ge = 0.7 eV
- n-type: pentavalent dopant (majority: electrons); p-type: trivalent (majority: holes)
- Diode: forward bias → conducts; reverse bias → blocks
- Zener: voltage regulator; LED: light emission; Solar cell: light → electricity
- Transistor: IE = IB + IC; β = IC/IB; amplifier (CE mode): Av = −βRL/Ri
- Logic gates: AND, OR, NOT, NAND (universal), NOR (universal)
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Energy Bands of Solids
In a solid, atomic levels split into closely-spaced energy bands, and the gap between two key bands decides if it conducts.
E_g = forbidden energy gap, measured in eV.
- Valence band holds bonding electrons; conduction band carries current
- Free electrons can carry current only after crossing the gap E_g
- An electron promoted to the CB leaves a hole in the VB
Metals vs Insulators vs Semiconductors
The size of E_g cleanly sorts every solid into conductor, insulator or semiconductor.
Si: E_g ≈ 1.1 eV, Ge ≈ 0.7 eV, diamond ≈ 6 eV.
- Metals: VB and CB overlap, so plenty of free electrons
- Semiconductor conductivity rises with temperature (resistance falls)
- This is opposite to a metal, whose resistance rises with temperature
Intrinsic Semiconductor
A perfectly pure Si or Ge crystal where thermal energy breaks bonds to make electron-hole pairs.
n_i = intrinsic carrier concentration; conductivity is tiny.
- Electrons and holes are always created in equal pairs
- At 0 K it behaves like an insulator
- Heating it generates more pairs and raises conductivity
Doping: n-type & p-type
Adding a controlled impurity (doping) makes an extrinsic semiconductor with far higher conductivity.
Doped crystal stays electrically neutral overall.
- Pentavalent dopant (As, P, Sb) → n-type → electrons majority
- Trivalent dopant (B, Al, In, Ga) → p-type → holes majority
- Donor level sits just below CB; acceptor level just above VB
p-n Junction & Depletion Layer
Joining p and n material lets carriers diffuse and recombine, leaving a carrier-free depletion layer with a barrier potential.
At equilibrium, diffusion current balances drift current.
- Fixed ions in the depletion region create the barrier potential
- Diffusion = majority carriers; drift = minority carriers
- No external voltage applied → net current is zero
Forward vs Reverse Bias
Biasing changes the barrier width, so a diode conducts strongly one way and blocks the other.
Reverse bias passes only a tiny reverse saturation current.
- Forward (p to +): barrier and width decrease, large current above V_b
- Reverse (p to −): barrier and width increase, high resistance
- A diode is a one-way valve; current rises sharply past the knee voltage
Rectifiers (AC → DC)
A diode’s one-way conduction converts alternating current into direct current.
A capacitor filter smooths the pulsating DC output.
- Half-wave: one diode, uses only one half of each AC cycle
- Full-wave: two diodes (centre-tap) or a 4-diode bridge, more efficient
- Full-wave output frequency is double the input frequency
Zener, LED, Photodiode, Solar Cell
Specially designed diodes regulate voltage, emit light, detect light or generate power.
LED needs E_g ≥ photon energy → GaAs/GaAsP, not Si/Ge.
- Zener: works in reverse breakdown as a voltage regulator
- Photodiode: reverse-biased; light increases the reverse current
- Solar cell: produces emf from light with no external bias
Transistor (BJT) & Gains
A bipolar junction transistor uses a thin lightly-doped base so emitter carriers reach the collector, enabling amplification and switching.
Active mode: emitter-base forward, collector-base reverse biased.
- Common-emitter gain β is large (~50–300); α is slightly less than 1
- CE amplifier gives a 180° phase shift between input and output
- As a switch: cutoff = OFF, saturation = ON
Logic Gates
Gates are the digital building blocks that combine binary inputs into one output.
NAND and NOR are universal gates.
- OR outputs 1 if any input is 1; AND outputs 1 only if all inputs are 1
- NOT (inverter) flips the input
- Any logic circuit can be built using only NAND or only NOR gates
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Frequently Asked Questions
An intrinsic semiconductor is a perfectly pure crystal of silicon or germanium where electrons and holes are created in equal pairs by heat, so n_e equals n_h. An extrinsic semiconductor is one that has been doped with impurity atoms to greatly increase conductivity, making it either n-type or p-type.
Doping with a pentavalent impurity such as arsenic, phosphorus or antimony gives an n-type semiconductor where electrons are the majority carriers. Doping with a trivalent impurity such as boron, aluminium, indium or gallium gives a p-type semiconductor where holes are the majority carriers.
At the junction, diffusion and recombination leave a carrier-free depletion layer with a built-in barrier potential. This barrier is about 0.7 volts for a silicon diode and about 0.3 volts for a germanium diode.
A half-wave rectifier uses one diode and conducts during only one half of each AC cycle, so its output frequency equals the input frequency f. A full-wave rectifier uses two diodes with a centre tap or a four-diode bridge to use both halves, giving an output frequency of 2f and higher efficiency.
Yes. It is a Class 12 chapter in the NEET syllabus and is considered high-yield because questions on band gaps, diodes, rectifiers, transistors and logic gates are largely conceptual and quick to solve, making it a reliable scoring chapter.