Semiconductors are materials whose electrical conductivity can be controlled, making them the foundation of diodes, transistors, solar cells, and computer chips. Pure silicon conducts poorly at room temperature, but adding tiny amounts of selected atoms creates many more mobile charge carriers. This process is called doping, and it produces n-type material with extra electrons or p-type material with mobile holes.
When p-type and n-type regions meet, they form a PN junction with special one-way electrical behavior.
At the junction, electrons and holes diffuse across the boundary and recombine, leaving behind fixed ionized dopant atoms. This creates a depletion region with very few mobile carriers and an internal electric field that opposes further diffusion. Applying forward bias lowers the barrier so current can flow easily, while reverse bias widens the depletion region and strongly blocks current.
This rectifying action is why a PN junction is the basic structure of a diode.
Key Facts
- Intrinsic silicon has few free carriers, while doped silicon has many more electrons or holes available for conduction.
- n-type doping adds donor atoms that provide extra electrons as majority carriers.
- p-type doping adds acceptor atoms that create holes as majority carriers.
- The depletion region contains fixed ions but very few mobile electrons or holes.
- Conventional current in a forward-biased diode flows from p-side to n-side when the barrier is reduced.
- Ideal diode model: forward bias conducts, reverse bias blocks; silicon diode approximation: V_D ≈ 0.7 V when conducting.
Vocabulary
- Semiconductor
- A material with electrical conductivity between that of a conductor and an insulator, often controllable by doping, temperature, or light.
- Doping
- The process of adding small amounts of impurity atoms to a semiconductor to change its charge carrier concentration.
- Hole
- A missing electron in a semiconductor crystal that behaves like a mobile positive charge carrier.
- Depletion region
- The region near a PN junction where mobile electrons and holes have recombined, leaving fixed charged ions behind.
- Forward bias
- A voltage connection that makes the p-side positive relative to the n-side, reducing the junction barrier and allowing diode current.
Common Mistakes to Avoid
- Thinking p-type material is positively charged overall is wrong because both p-type and n-type semiconductors are electrically neutral before the junction forms.
- Saying holes are physical particles is wrong because a hole is the absence of an electron that moves through the crystal as neighboring electrons shift.
- Assuming current flows freely both ways through a PN junction is wrong because the depletion region and built-in electric field make the diode strongly direction-dependent.
- Forgetting the diode voltage drop in a circuit is wrong because a conducting silicon diode usually has about 0.7 V across it, which changes the voltage left for other components.
Practice Questions
- 1 A silicon diode is in series with a 9.0 V battery and a 1.0 kΩ resistor. Using V_D = 0.7 V for a forward-biased diode, calculate the current through the resistor.
- 2 A forward-biased diode circuit has a 5.0 V supply, a silicon diode with V_D = 0.7 V, and a resistor carrying 20 mA. What is the resistance of the resistor?
- 3 Explain why the depletion region becomes wider when a PN junction is reverse biased, and describe how this affects the current.