Semiconductors are materials whose electrical conductivity can be controlled, making them the foundation of modern electronics. Pure silicon conducts only weakly at room temperature, but adding tiny amounts of impurity atoms can dramatically change how charge moves through it. This process, called doping, lets engineers create regions with extra mobile electrons or extra mobile holes.
The ability to shape these regions is what makes diodes, transistors, solar cells, and integrated circuits possible.
In n-type material, donor atoms add electrons that can move through the crystal, while in p-type material, acceptor atoms create holes that act like positive charge carriers. When p-type and n-type regions touch, electrons and holes diffuse across the boundary and recombine, leaving behind fixed charged ions. This forms a depletion region with an internal electric field that opposes further diffusion.
By applying voltage, engineers can shrink or widen this region, controlling current flow in devices such as PN junction diodes and transistor junctions.
Key Facts
- Intrinsic silicon has equal electron and hole concentrations: n = p = ni.
- n-type doping adds donor atoms, increasing electron concentration so n >> p.
- p-type doping adds acceptor atoms, increasing hole concentration so p >> n.
- Mass action law at thermal equilibrium: np = ni^2.
- A PN junction forms a depletion region with fixed ions and an internal electric field.
- Diode current is modeled by I = Is(e^(qV/kT) - 1), where forward bias increases current strongly.
Vocabulary
- Semiconductor
- A material with electrical conductivity between that of a conductor and an insulator, whose conductivity can be controlled by doping, temperature, or light.
- Doping
- The process of adding small amounts of impurity atoms to a semiconductor to change its charge carrier concentration.
- Electron
- A negatively charged mobile particle that can carry current through the conduction band of a semiconductor.
- Hole
- A mobile absence of an electron in the valence band that behaves like a positive charge carrier.
- Depletion region
- The region near a PN junction where mobile electrons and holes have recombined, leaving fixed ions and an internal electric field.
Common Mistakes to Avoid
- Thinking n-type material is negatively charged overall, which is wrong because donor ions and mobile electrons balance so the bulk material is usually neutral.
- Thinking p-type material contains no electrons, which is wrong because it still has electrons, but holes are the majority carriers.
- Forgetting that the depletion region contains fixed ions rather than many mobile carriers, which is wrong because electrons and holes have mostly recombined near the junction.
- Reversing forward and reverse bias, which is wrong because forward bias reduces the junction barrier and allows current, while reverse bias increases the barrier and mostly blocks current.
Practice Questions
- 1 An intrinsic silicon sample has ni = 1.0 x 10^10 cm^-3 at room temperature. If it is doped n-type so that n = 1.0 x 10^16 cm^-3, use np = ni^2 to find the hole concentration p.
- 2 A diode has Is = 1.0 x 10^-12 A and is forward biased at V = 0.60 V. Using kT/q = 0.026 V and I = Is(e^(V/0.026) - 1), estimate the diode current.
- 3 Explain why a PN junction conducts much more easily under forward bias than under reverse bias, using the ideas of the depletion region and internal electric field.