Light-Emitting Diode
An LED is a heavily doped forward-biased p-n junction made from a suitable direct-band-gap semiconductor. Its approximate emitted photon energy is set by Eg, while a series resistor limits current and power.
Why this shows up in the exam
Indicator lamps and displays · Estimating LED color from band gap · Choosing a current-limiting resistor
Learn the idea
A forward-biased LED converts electron-hole recombination energy into light whose color depends on band gap. Electrons and holes injected across the junction recombine and release energy as photons. A larger band gap gives a higher-energy, shorter-wavelength color.
🧠 Memory hook: Bigger LED gap means bluer, shorter-wavelength light.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- E_g approximately h c / lambda — approximate emitted photon energy and wavelength relation
- P_LED = V_F I — electrical power used for a stated forward voltage and current
How to approach it
- 1Confirm forward bias
- 2Use consistent eV-nm or SI units
- 3Apply both current and power limits
Common slip-ups that cost marks
- •Operating an LED in reverse breakdown
- •Equating electrical input power with emitted optical power
- •Using h c/lambda as exact for every real LED spectrum
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A transistor has common-emitter current gain beta = 50. If base current is 20 microA, find collector current.
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