Doping and Semiconductor Types
Pure (intrinsic) semiconductors like silicon don't conduct electricity very well on their own. Doping is the process of intentionally adding tiny amounts of impurity atoms to change that. Even a small amount of dopant can increase conductivity by several orders of magnitude, which is what makes semiconductor devices possible.
Doping Process and Effects
When you dope a semiconductor, you replace a few of the host atoms in the crystal lattice with atoms that have a different number of valence electrons. This does two things:
- It creates additional energy levels inside the bandgap, making it much easier for charge carriers to move around.
- It dramatically increases the number of free charge carriers available to conduct current.
The type of dopant you use determines whether you get an n-type or p-type semiconductor.
N-type Semiconductors and Donors
N-type semiconductors are made by doping with donor impurities. These are pentavalent atoms (five valence electrons), such as phosphorus or arsenic.
Here's what happens in the crystal lattice: silicon has four valence electrons, and each one forms a covalent bond with a neighboring silicon atom. When a phosphorus atom (five valence electrons) takes the place of a silicon atom, four of its electrons bond normally with the surrounding silicon. The fifth electron is only loosely bound to the phosphorus nucleus.
That extra electron needs very little energy to break free and jump into the conduction band. This is because the donor atom creates an energy level just below the conduction band edge. The result: lots of free electrons available to carry current.
N-type summary: Donor atoms (e.g., phosphorus in silicon) provide extra electrons. Electrons are the dominant charge carriers.
P-type Semiconductors and Acceptors
P-type semiconductors are made by doping with acceptor impurities. These are trivalent atoms (three valence electrons), such as boron or gallium.
Boron has only three valence electrons, so when it sits in the silicon lattice, one of the four covalent bonds is incomplete. That missing bond is a hole, which behaves like a positive charge carrier. A neighboring electron can jump over to fill the hole, but that just creates a new hole where the electron came from. The hole effectively moves through the lattice.
Acceptor atoms create an energy level just above the valence band, making it easy for electrons to hop up into that level and leave behind mobile holes.
P-type summary: Acceptor atoms (e.g., boron in silicon) create holes. Holes are the dominant charge carriers.
Charge Carriers
Electrons and Holes
Current in a semiconductor is carried by two types of charge carriers:
- Electrons are negative charge carriers. In an intrinsic (undoped) semiconductor, electrons reach the conduction band when they gain enough energy from heat or light to jump across the bandgap.
- Holes are positive charge carriers. A hole is simply the absence of an electron in the valence band. When an electron leaves its position, the empty spot it leaves behind can be filled by another electron, so the hole appears to move in the opposite direction.
Both electrons and holes contribute to current flow, but doping controls which type dominates.
Majority and Minority Carriers
In any doped semiconductor, one type of carrier far outnumbers the other:
| Semiconductor Type | Majority Carriers | Minority Carriers |
|---|---|---|
| N-type (e.g., phosphorus-doped Si) | Electrons | Holes |
| P-type (e.g., boron-doped Si) | Holes | Electrons |
| The concentration of majority carriers is orders of magnitude higher than minority carriers. This imbalance is what gives doped semiconductors their useful electrical properties. Minority carriers still exist (generated thermally), and they play a surprisingly important role in devices like p-n junction diodes, but majority carriers dominate the overall conductivity. |
Energy Levels
Fermi Level and its Significance
The Fermi level () represents the energy at which there's a 50% probability of finding an electron at thermal equilibrium. It's a statistical concept, not a physical energy band that electrons sit in.
How doping shifts the Fermi level:
- In an intrinsic semiconductor, sits roughly in the middle of the bandgap, halfway between the valence and conduction bands.
- In an n-type semiconductor, the abundance of donor electrons pushes upward, closer to the conduction band.
- In a p-type semiconductor, the abundance of holes pushes downward, closer to the valence band.
The position of relative to the band edges tells you the concentration of electrons and holes in the material. The closer is to the conduction band, the more free electrons you have, and vice versa for holes.
This matters for devices because when two differently doped regions come into contact (like in a p-n junction), the difference in their Fermi levels creates a built-in potential. That built-in potential is the foundation of how diodes, transistors, and solar cells work.