KAHIBARO
Table of Contents
Donor Doping and Extra Electrons
An n-type material is a semiconductor that has been intentionally modified so that electrons become the main mobile charge carriers. The letter n stands for negative, referring to the negative charge of electrons.
Pure semiconductors such as silicon have relatively few free charge carriers at room temperature. To increase conductivity, small amounts of impurity atoms are added. This process is called doping. In n-type materials, the dopant atoms provide extra electrons that can move through the crystal.
In silicon, each silicon atom normally forms four covalent bonds with neighboring atoms. If a small number of silicon atoms are replaced by atoms from group V of the periodic table, such as phosphorus, arsenic, or antimony, each dopant atom brings five valence electrons instead of four. Four of these electrons participate in bonding, and the fifth is only weakly bound. This extra electron can easily become a free conduction electron.
How Donor Atoms Work
The impurity atoms used to create n-type material are called donor atoms because they donate electrons to the semiconductor. After donating an electron, the donor atom becomes a fixed positive ion inside the crystal lattice. This ion does not move, but the released electron can move and contribute to electric current.
The energy needed to free the extra donor electron is small compared with the band gap of the semiconductor. Because of this, at ordinary temperatures many donor electrons are thermally excited into the conduction band.
In an n-type semiconductor, donor atoms supply electrons to the conduction band, and electrons are the majority carriers.
A simple picture is that doping adds energy levels very close to the conduction band. Electrons from these donor levels can move into the conduction band with little thermal energy.
Majority and Minority Carriers
In n-type material, electrons are the majority carriers, meaning they are far more numerous than holes. Holes still exist, but only in small numbers, so they are called minority carriers.
This distinction is very important for understanding current flow in semiconductors. The conductivity increases mainly because of the larger number of free electrons.
If $n$ is the electron concentration and $p$ is the hole concentration, then in n-type material we have
$$
n > p
$$
and usually
$$
n \gg p
$$
For a doped semiconductor in thermal equilibrium, the product of electron and hole concentrations satisfies
$$
np = n_i^2
$$
where $n_i$ is the intrinsic carrier concentration.
For n-type material, electrons are majority carriers and holes are minority carriers.
Electrical Conductivity
Because n-type materials contain many mobile electrons, they conduct electricity better than intrinsic semiconductors. The electrical conductivity depends on the number of charge carriers and how easily they move.
A simplified expression for conductivity is
$$
\sigma = q(n\mu_n + p\mu_p)
$$
where $q$ is the magnitude of the electron charge, $\mu_n$ is electron mobility, and $\mu_p$ is hole mobility.
In n-type material, since $n$ is much larger than $p$, the conductivity is often dominated by the electron term:
$$
\sigma \approx q n \mu_n
$$
This is why adding donor impurities greatly increases conductivity.
Common Donor Elements
The most common donor impurities for silicon are elements with five valence electrons.
| Dopant element | Group | Role in silicon |
|---|---|---|
| Phosphorus | V | Donor |
| Arsenic | V | Donor |
| Antimony | V | Donor |
These atoms fit into the silicon lattice by replacing silicon atoms, but they contribute one extra electron beyond what is needed for bonding.
Charge Neutrality
Even though n-type material has many extra free electrons, the solid as a whole remains electrically neutral. This is because the free electrons are balanced by the positively charged donor ions left behind after donation.
So n-type does not mean the material has a net negative charge. It means that negative charge carriers dominate the conduction process.
An n-type semiconductor is electrically neutral overall. It contains more mobile electrons, but these are balanced by fixed positive donor ions.
Fermi Level Shift
Doping also changes the position of the Fermi level. In an n-type semiconductor, the Fermi level moves closer to the conduction band than in an intrinsic semiconductor. This reflects the greater probability that electron states near the conduction band are occupied.
This shift is one of the clearest signatures of n-type behavior in band theory.
Comparison with Intrinsic Material
An intrinsic semiconductor is pure and has equal numbers of electrons and holes created thermally. An n-type semiconductor is doped so that electrons strongly outnumber holes.
| Property | Intrinsic semiconductor | n-type semiconductor |
|---|---|---|
| Main source of carriers | Thermal generation | Donor doping |
| Majority carrier | None, equal electrons and holes | Electrons |
| Minority carrier | None, equal electrons and holes | Holes |
| Conductivity | Lower | Higher |
| Fermi level | Near middle of band gap | Closer to conduction band |
Physical Picture
A useful mental image is that the crystal lattice remains mostly the same, but a few special atoms are inserted that contribute nearly free electrons. These electrons can drift when an electric field is applied, making current flow more easily.
This is the essential feature of n-type materials. They are semiconductors engineered so that conduction is mainly carried by electrons supplied by donor impurities.