P-N Junction, Depletion Layer, and Barrier
A p-n junction reaches equilibrium when diffusion current is balanced by drift current in the built-in electric field. The depletion width extends farther into the more lightly doped side.
Why this shows up in the exam
Explaining junction formation · Comparing depletion widths on unequally doped sides · Interpreting the built-in electric field
Learn the idea
Carrier diffusion creates a depletion region whose electric field opposes further diffusion. After p-type and n-type regions meet, majority carriers diffuse and recombine near the boundary. Fixed ions left behind form an internal field and a carrier-poor depletion layer.
🧠 Memory hook: Diffusion leaves ions; ions build the barrier.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- I_diffusion + I_drift = 0 at equilibrium — zero net junction current with no external bias
- N_A x_p = N_D x_n — depletion charge balance per unit area in the abrupt-junction approximation
How to approach it
- 1Start from the carrier concentration gradient
- 2Mark fixed-ion signs on both sides
- 3Put the larger depletion width on the lower-doped side
Common slip-ups that cost marks
- •Calling the depletion layer completely free of charge
- •Saying the depletion layer is wider on the more heavily doped side
- •Attributing equilibrium diffusion only to electrostatic attraction
🌟 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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