The mechanism
Doping deliberately changes a semiconductor by incorporating a small population of other elements. In silicon, a donor such as phosphorus can supply an electron; an acceptor such as boron creates a hole in the carrier description. The crystal remains a material with extended electronic states, rather than isolated dopant atoms acting as individual switches. N-type and p-type describe the dominant mobile carrier, not a permanently charged bulk object. Spatially different doping regions help establish junctions and useful terminal behavior. Modern devices may combine heavily doped source/drain regions with lightly doped channels, so a single uniform sprinkling is only a teaching view, not a production doping map.
- Replace selected silicon host atoms with donor or acceptor species.
- Allow these species to change the available mobile electron or hole populations.
- Arrange different regions spatially to obtain useful junctions and transistor terminal behavior.
Science and assumptions
Mass action and donor control
At equilibrium the electron–hole density product equals intrinsic density squared. In the donor-dominated limit, electron density follows donor density and hole density is intrinsic density squared divided by donor density.
Symbols and units
- n
- Electron density
cm⁻³
- p
- Hole density
cm⁻³
- nᵢ
- Intrinsic carrier density
cm⁻³
- N_D
- Ionized donor density under the stated assumptions
cm⁻³
For donor-dominated silicon, the majority electron density follows the activated donors while the minority hole density decreases. This connects atomic substitution to measurable conductivity.
Assumptions Thermal equilibrium, full donor ionization, negligible compensation, much larger than nᵢ, and nondegenerate statistics; heavily doped materials require a better model.
Engineering example
If is much larger than , the simple estimate gives n≈ and p≈²/; compensation and incomplete ionization can change both.
Distinctions and tradeoffs
N-type does not mean the bulk material carries an uncompensated permanent negative charge; it identifies the majority mobile carrier.
More carriers can reduce resistance. Impurities and statistical variation can also degrade transport and uniformity.
Doping helps make complementary switching networks possible. Those networks become memory, arithmetic, and control circuitry.
Specifications and interfaces
- Donor example
- Phosphorus
Group-V atom in a silicon host.
- Acceptor example
- Boron
Group-III atom in a silicon host.
- Quantity to specify
- Concentration and profile
An actual device requires process-specific characterization.
Interfaces
- Host crystal Input
- Substitutional sites
- Carrier distribution Output
- N-type or p-type regions
Check your understanding
What distinguishes n-type silicon?
- Its majority mobile carriers are electrons
- Every atom is negatively charged
- It contains no holes at any temperature
Answer and reasoning
Its majority mobile carriers are electrons
N-type refers to which mobile carriers dominate. Ionized dopants and the rest of the solid also participate in charge balance.
Evidence and scope
The displayed relation is a nondegenerate, charge-neutrality approximation.
Use the switching investigation to connect physical charge, circuit load and arithmetic; the numerical library has no standalone model of this lesson’s full mechanism.