p-n Junction Diodes Under Bias
Biasing the Junction
Forward and reverse bias
Forward bias lowers the barrier potential and narrows the depletion region, allowing significant current. Reverse bias raises the barrier, widens the depletion region, and ideally allows only a small saturation current.
Diode equation
A common model is , where is saturation current and is the ideality factor. This captures the strong exponential increase in forward current.
Practical non-idealities
Real diodes show series resistance, breakdown, leakage, and temperature dependence, so the ideal equation is only a starting point.
Bias behavior
| Condition | Barrier | Current |
|---|---|---|
| Forward bias | Reduced | Large exponential forward current |
| Reverse bias | Increased | Small leakage current |
| Breakdown | Excessive field | Rapid increase in reverse current |
What happens to the depletion width under forward bias?
Forward bias lowers the barrier and reduces the depletion width.
Correct answer: It decreases
Write the ideal diode equation.
This is the standard current-voltage relation for an ideal p-n junction diode.
Correct answer: I = I_S(e^{qV/(nkT)} - 1)
Reading a diode I-V curve
-
1
Identify the forward region and note exponential growth.
-
2
Locate the reverse-bias leakage region.
-
3
Check for breakdown at large negative voltage.
-
4
Consider series resistance at high forward current.
Forward bias vs reverse bias
Forward bias
- Lower barrier
- Majority-carrier injection dominates
Reverse bias
- Higher barrier
- Only small minority-carrier current flows
Which parameter in the diode equation often captures non-ideal recombination behavior?
The ideality factor reflects departures from ideal diffusion-dominated transport.
Correct answer: Ideality factor
Name one source of reverse-bias current in a real diode.
Reverse current often comes from thermally generated minority carriers and surface leakage.
Correct answer: Minority-carrier leakage