BeginnerTopic 3 of 104Electronics & Embedded
PN junction
Contact of p and n regions forms a depletion region that allows current mostly one way.
In depth
Joining p and n regions makes electrons and holes diffuse across, leaving a depletion region with a built-in potential about 0.7 V in silicon. Forward bias narrows it and allows current; reverse bias widens it and blocks current except a small leakage.
Key points
- Depletion capacitance varies with voltage
- Breakdown at high reverse voltage
- Basis of diodes and transistors
ExampleA varactor diode uses the junction capacitance for tuning radios.
What is PN junction?
Contact of p and n regions forms a depletion region that allows current mostly one way.
Why is it important to understand PN junction?
The junction between p and n material is the basis of nearly every semiconductor device, from diodes to solar cells and LEDs.
Why does the depletion region form?
Electrons diffuse from n to p and holes diffuse the other way, leaving fixed charged ions that create an electric field and a depletion region with few free carriers.
Why does a junction conduct in one direction only?
Forward bias lowers the barrier so carriers flow easily, while reverse bias raises it so only a tiny leakage current flows.
How is PN junction used in practice?
Typical applications include making a diode that protects a circuit from reverse polarity, building a solar cell that converts light into a voltage and explaining the working of a photodiode in a light sensor.
What should I learn before PN junction?
Study "Intrinsic and extrinsic semiconductors" first (topic 2 of 104 in Electronics & Embedded). It covers the groundwork this topic builds on.
What are the key points of PN junction?
Depletion capacitance varies with voltage. Breakdown at high reverse voltage. Basis of diodes and transistors.
Can you give an example of PN junction?
A varactor diode uses the junction capacitance for tuning radios.
What comes after PN junction?
The next topic is "Diode characteristics", which builds directly on this one.
How difficult is PN junction?
It is classed as Beginner in Electronics & Embedded (topic 3 of 104). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.