Doping, Ionization, and Compensation
Engineering Carriers with Dopants
Donors and acceptors
Donor impurities such as phosphorus in silicon contribute extra electrons, creating n-type material. Acceptor impurities such as boron create p-type material by introducing available hole states. Dopants shift the Fermi level and strongly increase conductivity.
Compensation
If both donor and acceptor dopants are present, the effective carrier type depends on the difference between their concentrations, not just the presence of one dopant species.
Ionization and temperature
At low temperature, some dopants may remain neutral if thermal energy is insufficient to ionize them. As temperature increases, dopant ionization improves, but very high temperature can also increase intrinsic carrier generation and reduce the dominance of doping.
Dopant roles in silicon
| Dopant type | Example | Carrier produced |
|---|---|---|
| Donor | Phosphorus | Electron |
| Donor | Arsenic | Electron |
| Acceptor | Boron | Hole |
| Acceptor | Gallium | Hole |
A silicon sample doped with boron is most likely to be what type of semiconductor?
Boron acts as an acceptor in silicon, producing p-type material.
Correct answer: p-type
What is compensation doping?
Compensation reduces the net free-carrier effect of either dopant species alone.
Correct answer: The presence of both donor and acceptor dopants that partially cancel each other
Estimating majority-carrier concentration
-
1
Identify donor and acceptor concentrations.
-
2
Determine the net doping level using the difference between donors and acceptors.
-
3
Assume charge neutrality when fully ionized if justified.
-
4
Use the net majority-carrier concentration as the first approximation.
n-type vs p-type
n-type
- Electrons are majority carriers
- Donors shift the Fermi level upward
p-type
- Holes are majority carriers
- Acceptors shift the Fermi level downward
Why can heavily doped semiconductors show stronger conductivity even before considering mobility changes?
Dopants introduce carriers, increasing the concentration term in conductivity.
Correct answer: Because dopants directly create more free carriers
Name one common donor dopant used in silicon.
Phosphorus is a standard donor in silicon technology.
Correct answer: Phosphorus