Diode Biasing and I-V Characteristic
A p-n diode is forward biased when its p-side is at higher potential than its n-side. It is reverse biased for the opposite polarity; reverse current remains small and nearly voltage-independent before breakdown.
Why this shows up in the exam
Identifying forward and reverse bias · Reading a diode I-V curve · Explaining reverse saturation current and breakdown
Learn the idea
Forward bias lowers the junction barrier, while reverse bias raises it until breakdown. Connecting p to higher potential pushes majority carriers toward the junction and permits a large current. Reversing the battery pulls them away, leaving only a small minority-carrier current before breakdown.
🧠 Memory hook: P higher than N means forward bias.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- I = I_s [exp(V_D/(eta V_T)) - 1] — idealized diode equation, with eta set by device behavior
- V_T = k T / e — thermal voltage used in the diode equation
How to approach it
- 1Label anode p and cathode n
- 2Compare their potentials before replacing the diode
- 3Check whether the stated voltage reaches breakdown
Common slip-ups that cost marks
- •Using terminal signs without comparing actual node potentials
- •Saying majority carriers dominate ordinary reverse current
- •Treating breakdown as safe without current limiting
🌟 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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