Semiconductor Electronics
Class 12 · Physics
4 ideas in Class 12 Physics, explained in plain words, with games and quizzes to practise.
Energy Bands and Semiconductors
Part of: Bands and Doping
Electrons live in filled valence bands below empty conduction bands. Metals overlap the two, insulators separate them by a huge gap, and semiconductors keep a small gap near 1 eV that heat can push electrons across, so warming raises their conductivity.
Why it matters. Every chip, sensor and solar cell exploits the small gap. Calling band gap irrelevant, or heating effects backwards, breaks device reasoning.
Formulas
- Silicon gap about 1.1 eV; germanium about 0.7 eV
- Cutoff wavelength(nm) = 1240 / gap(eV)
- Heating raises semiconductor conductivity
Key words
- Valence band
- : Highest electron band filled at zero temperature.
- Conduction band
- : Empty band above where electrons move freely.
- Band gap
- : Energy jump between valence and conduction bands.
Try it in two minutes
- Sketch valence and conduction bands for the three classes.
- Compare gap sizes: zero, small, huge.
- Heat silicon in thought and watch carriers multiply.
Try at home. Explain why a cold thermistor resists more than a warm one.
Doping and Carriers
Part of: Bands and Doping
Pure silicon carries equal electrons and holes from heat. Adding pentavalent donors gives free electrons (n-type) and trivalent acceptors give free holes (p-type), boosting conductivity while staying neutral, with thermal pairs as minority carriers.
Why it matters. Diodes and transistors are built from n and p regions. Calling doped silicon charged, or mixing donors with acceptors, breaks every junction answer.
Formulas
- Intrinsic: electrons equal holes
- n-type: electrons far exceed holes
- p-type: holes far exceed electrons
Key words
- Hole
- : Missing bond electron acting as a positive carrier.
- Donor
- : Pentavalent atom giving a free electron.
- Acceptor
- : Trivalent atom giving a free hole.
- Majority carriers
- : The abundant carrier type set by doping.
Try it in two minutes
- Count thermal electron-hole pairs in pure silicon.
- Add a donor atom and free one electron.
- Add an acceptor atom and free one hole.
Try at home. List donor and acceptor elements from the periodic table.
p-n Junction and Diode Action
Part of: Junction Diode and Rectifiers
Joining p and n lets carriers diffuse and recombine near the face, leaving uncovered ions that build a barrier with the n side positive. Forward bias flattens the barrier so current surges past the knee, while reverse bias widens it, passing only a tiny minority current until breakdown.
Why it matters. Every diode, solar cell and rectifier depends on this one-sided flow. Reversing the battery logic predicts conduction exactly backwards.
Formulas
- Barrier points from n side to p side
- Forward conducts past the knee voltage
- Reverse passes only tiny saturation current
Key words
- Depletion region
- : Ion layer near the junction emptied of free carriers.
- Barrier potential
- : Voltage across the depletion layer opposing diffusion.
- Forward bias
- : p-positive connection that shrinks the barrier.
- Reverse bias
- : p-negative connection that widens the barrier.
Try it in two minutes
- Mark uncovered ions on each side of the face.
- Point the barrier from n to p.
- Apply p-positive to flatten it, p-negative to widen it.
Try at home. Test a diode with a cell and bulb in both directions.
Rectifiers and Smoothing
Part of: Junction Diode and Rectifiers
A rectifier diode passes only one polarity, so half-wave output keeps every other hump at the input frequency. Full-wave designs flip the missing half upright, doubling output frequency, and a capacitor filter fills the gaps toward smooth DC.
Why it matters. Chargers and power supplies all start here. Expecting pure DC without a filter, or the wrong output frequency, misreads every supply waveform.
Formulas
- Half-wave output frequency equals input frequency
- Full-wave output frequency is double the input
- Capacitor charges to peak and fills the gaps
Key words
- Rectifier
- : Circuit converting AC into pulsing one-way current.
- Half-wave rectifier
- : One-diode circuit passing alternate half cycles.
- Full-wave rectifier
- : Two or four-diode circuit using both half cycles.
- Filter capacitor
- : Capacitor smoothing pulses toward steady DC.
Try it in two minutes
- Trace which half each diode passes.
- Count output humps per input cycle.
- Add a capacitor and watch gaps fill.
Try at home. Hum the 100 Hz ripple of a full-wave 50 Hz supply.
What your child will practise
- Doping and Carriers: Desert Puzzle (Clue Puzzle)
- Doping and Carriers: Jungle Quick Fire (Quick Fire)
- Energy Bands and Semiconductors: Space Puzzle (Clue Puzzle)
- p-n Junction and Diode Action: Jungle Quest (Flip and Match)
- Rectifiers and Smoothing: Space Quick Fire (Quick Fire)
- Quiz: Doping and Carriers (8 questions)
- Quiz: Energy Bands and Semiconductors (8 questions)
- Quiz: Rectifiers and Smoothing (8 questions)
- Quiz: p-n Junction and Diode Action (8 questions)