Formation of a p-n Junction and Depletion Region
A p-n junction is formed when p-type and n-type semiconductor regions are joined. Electrons and holes diffuse across the junction, recombine near it, and leave behind fixed ionised impurities, creating a depletion region and a built-in potential barrier.
In one line: Diffusion starts the junction story; the barrier limits it.
Concept Practice Coming SoonUnderstand it
- When p-type and n-type regions are in contact, electrons from the n-side diffuse into the p-side and holes from the p-side diffuse into the n-side because of carrier concentration differences.
- Near the junction, electrons and holes recombine.
- This leaves uncovered positive donor ions on the n-side and negative acceptor ions on the p-side.
- These fixed ions form an electric field across the junction, opposing further diffusion.
- The region depleted of mobile carriers is called the depletion region, and the corresponding barrier is called the potential barrier.
Example
Formula and rule
Key physics rule: diffusion current is caused by concentration gradient, while drift current is caused by the electric field of the depletion region.
At equilibrium, diffusion tendency is balanced by the built-in electric field.
The potential barrier opposes further majority-carrier diffusion.
See it worked out
Follow the example line by line, then use the same method on new questions.
Worked example
Explain the effect of increasing doping on the depletion width.
- 1Higher doping means more ionised impurity atoms are present near the junction.
- 2A smaller physical width is needed to build the required space-charge region, so the depletion layer becomes narrower compared with a lightly doped junction.
Method you can reuse
- 1Start with carrier concentration difference.
- 2State electron diffusion from n to p and hole diffusion from p to n.
- 3Add recombination near the junction.
- 4Identify fixed ions left behind.
- 5State that these ions create an electric field and potential barrier.
- 6End with equilibrium between diffusion and drift effects.
Sketch it in your answer
Draw p-side on the left and n-side on the right. Label holes, electrons, depletion region, fixed negative acceptor ions on the p-side, fixed positive donor ions on the n-side, electric field direction from n-side positive ions to p-side negative ions, and potential barrier.
Check yourself
Answer the question in your head before opening the answer.
Try it yourself
What exactly is depleted in the depletion region of a p-n junction?
Think first, then show answerHide answer
Common wrong answer
Saying that the depletion layer forms because charges physically disappear is wrong. Mobile electrons and holes recombine near the junction, while fixed ionised impurities remain and create the barrier field.
Don't mix this up
Exam tip
Write it in the exam
Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.
How CBSE asks it
- 1
Enough for 1 mark
A p-n junction is formed when p-type and n-type semiconductor regions are joined. Electrons and holes diffuse across the junction, recombine near it, and leave behind fixed ionised impurities, creating a depletion region and a built-in potential barrier.
- 2
Add for 2 marks
Example: In a silicon p-n junction, electrons initially diffuse from the n-side to the p-side and holes from the p-side to the n-side. After recombination near the junction, a narrow depletion region forms. The built-in electric field then reduces further diffusion, establishing equilibrium.
- 3
Add for 3 marks
When p-type and n-type regions are in contact, electrons from the n-side diffuse into the p-side and holes from the p-side diffuse into the n-side because of carrier concentration differences.
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