Photodiode
A photodiode is normally operated in reverse bias, where incident photons with energy at least comparable to Eg generate carriers and increase reverse current approximately with optical intensity.
Why this shows up in the exam
Light-intensity measurement · Optical communication receivers · Band-gap estimation from cutoff wavelength
Learn the idea
A reverse-biased photodiode converts absorbed light into a measurable change in current. Photons with enough energy create electron-hole pairs near the depletion region. The junction field separates them, so stronger suitable illumination produces a larger reverse photocurrent.
🧠 Memory hook: Photodiode: reverse bias, light creates reverse current.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- h nu >= E_g — idealized photon-energy threshold for carrier generation
- lambda_max approximately h c / E_g — longest detectable wavelength in the simple threshold model
How to approach it
- 1Identify whether the device emits or detects light
- 2Check photon energy against Eg
- 3Treat the source's threshold data conservatively if OCR is incomplete
Common slip-ups that cost marks
- •Confusing a photodiode with a forward-biased LED
- •Reversing the wavelength threshold inequality
- •Claiming any wavelength creates carriers
🌟 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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