Semiconductor Electronics Class 12 Notes (2026-27) — CBSE
Class 12 Physics Chapter 14 notes: energy bands, intrinsic and extrinsic semiconductors, p-n junction, diode biasing, rectifiers and special purpose diodes.
Semiconductor Electronics — Class 12 Physics Notes
Chapter Snapshot
This chapter explains how semiconductors work and why they are the basis of all modern electronics. It covers energy bands, intrinsic and extrinsic semiconductors (doping to make n-type and p-type), the p-n junction with its depletion region, forward and reverse bias, the diode as a rectifier, and special purpose diodes.
Board relevance: the Electronic Devices unit is ~7 marks. Expect a p-n junction/rectifier question and a doping question. Circuit diagrams and the I–V characteristic earn marks.
Syllabus note (rationalised): transistors and logic gates have been removed. The chapter now ends at diodes and their applications.
Key Concepts & Definitions
Energy bands in solids
Electrons in a solid occupy bands; the valence band holds bound electrons and the conduction band holds free ones, separated by a forbidden energy gap (Eg).
Material Energy gap Behaviour
Conductor No gap (bands overlap) Many free electrons; conducts readily
Insulator Large ( 3 eV) No electrons can cross; does not conduct
Semiconductor Small (~1 eV) Some electrons cross at room temperature
Silicon has Eg ≈ 1.1 eV and germanium ≈ 0.7 eV.
Temperature effect: heating a semiconductor gives more electrons enough energy to cross the gap, so its conductivity increases and resistance decreases — the opposite of a metal (negative temperature coefficient of resistance).
Intrinsic and extrinsic semiconductors
Intrinsic — pure semiconductor. Electrons and holes are created in pairs, so ne = nh = ni. Conductivity is low.
Extrinsic — deliberately doped with impurity to raise conductivity:
Type Dopant Majority carriers Minority carriers
n-type Pentavalent (P, As, Sb) — a donor Electrons Holes
p-type Trivalent (B, Al, In) — an acceptor Holes Electrons
Both types remain electrically neutral overall. In all cases ne·nh = ni².
The p-n Junction
When p and n materials are joined, electrons diffuse from n→p and holes from p→n, recombining near the junction. This leaves behind immobile ions, creating:
- A depletion region — a thin layer with no free charge carriers.
- A barrier potential (potential barrier) opposing further diffusion: about 0.3 V for germanium and 0.7 V for silicon.
Biasing
Bias Connection Depletion region Barrier Current
Forward p to +, n to − Narrows Decreases Large (mA)
Reverse p to −, n to + Widens Increases Tiny leakage (μA)
So a diode acts as a one-way valve for current. In the I–V characteristic, forward current rises sharply after the knee (threshold) voltage; in reverse, a small constant leakage current flows until breakdown.
Rectifiers and Special Purpose Diodes
Rectifier — converts AC into DC using the diode's one-way conduction.
- Half-wave rectifier: one diode; conducts during only one half of each AC cycle. Output frequency = input frequency; less efficient.
- Full-wave rectifier: two diodes with a centre-tapped transformer (or four diodes in a bridge); conducts during both halves. Output frequency = twice the input; higher and smoother output.
- A capacitor filter across the output smooths the pulsating DC.
Special purpose diodes:
Device Principle / use
Zener diode Heavily doped; operated in reverse breakdown at a constant voltage → used as a voltage regulator
LED Forward-biased; recombination of electrons and holes emits light
Photodiode Reverse-biased; incident light generates carriers → used to detect light
Solar cell Generates EMF from light without any external bias
Key Facts
Quick facts boards ask directly:
Topic Fact to remember
Band gap, silicon ≈ 1.1 eV
Band gap, germanium ≈ 0.7 eV
Insulator band gap 3 eV
Semiconductor resistance vs temperature Decreases (negative coefficient)
Intrinsic relation ne = nh = ni
Mass-action law ne · nh = ni²
n-type dopant Pentavalent (P, As, Sb) — donor
p-type dopant Trivalent (B, Al, In) — acceptor
Barrier potential, germanium ≈ 0.3 V
Barrier potential, silicon ≈ 0.7 V
Forward bias Depletion narrows, large current
Reverse bias Depletion widens, μA leakage
Half-wave output frequency Same as input
Full-wave output frequency Twice the input
Zener diode bias Reverse breakdown (voltage regulator)
Photodiode bias Reverse
LED bias Forward
Solar cell bias None required
Two definitions to quote: Doping — the deliberate addition of a small amount of impurity to a pure semiconductor to increase its conductivity. Depletion region — the thin layer near a p-n junction that has no free charge carriers, containing only immobile ions.
Worked Examples
Example 1 — Doping: Silicon is doped with arsenic. What type of semiconductor results and what are the majority carriers?
Arsenic is pentavalent, so it donates an extra electron → n-type, with electrons as majority carriers.
Example 2 — Carrier concentration: In an n-type sample, ni = 1.5 × 10¹⁶ /m³ and ne = 5 × 10²² /m³. Find nh.
ne·nh = ni² → nh = (1.5 × 10¹⁶)²/(5 × 10²²) = (2.25 × 10³²)/(5 × 10²²) = 4.5 × 10⁹ /m³.
Example 3 — Rectifier output: A full-wave rectifier is fed 50 Hz AC. What is the output ripple frequency?
It conducts on both halves, so the output frequency = 2 × 50 = 100 Hz.
Example 4 — Barrier: Why does the depletion region widen under reverse bias?
The external field pulls electrons and holes away from the junction, exposing more immobile ions, so the region widens and the barrier increases — blocking current.
Important Question Patterns
1. Energy bands (2–3 marks): distinguish conductor/insulator/semiconductor by band gap; why semiconductor resistance falls with temperature.
2. Doping (2–3 marks): n-type vs p-type, dopant valency, majority/minority carriers; why the doped crystal stays neutral; ne·nh = ni².
3. p-n junction (3 marks): formation of the depletion region and barrier potential; effect of forward and reverse bias; draw the I–V characteristic.
4. Rectifier (3–5 marks): circuit diagram and working of a half-wave or full-wave rectifier; input/output waveforms; output frequency.
5. Special diodes (2–3 marks): Zener as a voltage regulator (reverse breakdown); LED, photodiode (reverse-biased) and solar cell.
⚡ Quick Revision
- Band gap: conductor (overlap), insulator ( 3 eV), semiconductor (~1 eV; Si 1.1, Ge 0.7).
- Semiconductor resistance falls as temperature rises (negative temperature coefficient).
- Intrinsic: ne = nh = ni. Extrinsic: n-type = pentavalent donor (electrons), p-type = trivalent acceptor (holes); both neutral; ne·nh = ni².
- p-n junction: diffusion → depletion region + barrier potential (Ge 0.3 V, Si 0.7 V).
- Forward bias: depletion narrows, large current. Reverse bias: widens, tiny leakage. Diode = one-way valve.
- Half-wave: 1 diode, one half-cycle, output f = input f. Full-wave: 2 diodes (centre-tap) or bridge, both halves, output f = 2 × input.
- Zener = voltage regulator (reverse breakdown); LED emits light (forward); photodiode detects light (reverse); solar cell needs no bias.
- Removed from syllabus: transistors and logic gates.
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