KAHIBARO
Table of Contents
Creating a p-Type Semiconductor
A p-type material is a semiconductor that has been modified so that positive charge carriers, called holes, are the dominant carriers of electric current. This does not mean the material has an overall positive electric charge. The crystal as a whole remains electrically neutral. The term p-type means that conduction is mainly associated with holes.
Pure semiconductors such as silicon have four valence electrons per atom and form four covalent bonds with neighboring atoms. In a pure crystal, the number of free electrons and holes is small. To make the material more useful in electronics, small amounts of impurity atoms are added. This process is called doping.
Acceptor Impurities
To produce p-type material, the semiconductor is doped with atoms that have one fewer valence electron than the host semiconductor. In silicon, which has four valence electrons, common dopants are boron, aluminum, gallium, and indium, each with three valence electrons.
When one of these atoms replaces a silicon atom in the crystal, it can form only three complete covalent bonds with neighboring silicon atoms. One bond is left short of an electron. This missing electron is described as a hole.
The impurity atom can accept an electron from a nearby bond, which is why it is called an acceptor impurity. After accepting an electron, the dopant becomes negatively charged as an ion, while the missing electron appears as a mobile hole in the crystal.
In a p-type semiconductor, acceptor dopants create holes as the majority carriers.
The crystal remains overall electrically neutral, even though it contains mobile holes and negatively ionized acceptor atoms.
How Holes Move
A hole is not a physical particle like an electron. It is the absence of an electron in a bond. Still, it behaves in many ways like a positively charged mobile carrier.
If a neighboring electron moves to fill the hole, it leaves behind a new hole where it came from. Repeating this process makes the hole appear to move through the crystal. Under an applied electric field, holes drift in the direction of the field, opposite to the drift of electrons.
Majority and Minority Carriers
In p-type material, holes are the majority carriers and electrons are the minority carriers. Both kinds of carriers still exist, but holes are much more numerous because of the acceptor doping.
This difference is very important because it determines how the material responds to electric fields and how current flows in semiconductor devices.
For p-type materials:
Majority carriers are holes.
Minority carriers are electrons.
Energy Picture
Acceptor atoms introduce energy levels very close to the valence band. Because this acceptor level lies only slightly above the valence band, an electron from the valence band can easily move into the acceptor level. When this happens, a hole is left behind in the valence band.
So, in p-type material, conduction is mainly associated with holes in the valence band.
Examples of p-Type Dopants
For silicon and germanium, p-type doping is achieved by using elements from group III of the periodic table.
| Host semiconductor | Common p-type dopants |
|---|---|
| Silicon | Boron, Aluminum, Gallium, Indium |
| Germanium | Boron, Gallium, Indium |
Because these dopants have three valence electrons, they are often called trivalent impurities.
Charge Balance and Ionization
When an acceptor atom captures an electron, it becomes a fixed negative ion in the lattice. The hole that is created can move, but the ionized dopant usually cannot. So current in a p-type semiconductor is due mainly to moving holes, not moving impurity atoms.
At ordinary temperatures, many acceptor atoms are ionized, meaning they have accepted electrons and produced holes. This is why doped semiconductors can conduct much better than intrinsic semiconductors.
Acceptor dopants in p-type material are typically fixed in the crystal lattice.
The mobile charge carrier is the hole created in the valence band.
Simple Carrier Relation
If the acceptor concentration is much larger than the intrinsic carrier concentration, then the hole concentration is approximately equal to the acceptor concentration:
$$
p \approx N_A
$$
where $p$ is the hole concentration and $N_A$ is the acceptor concentration.
The electron concentration becomes much smaller than the hole concentration.
This chapter does not go further into full carrier equations or junction behavior, because those belong to later topics.
Physical Picture Summary
A p-type semiconductor is made by adding acceptor impurities to a pure semiconductor crystal. These impurities have one fewer valence electron than the host atoms. As a result, holes are created in the valence band. These holes act as positive mobile carriers and dominate electrical conduction.
| Feature | p-type material |
|---|---|
| Dopant type | Acceptor |
| Typical dopant valence | 3 |
| Majority carrier | Hole |
| Minority carrier | Electron |
| Fixed ion charge after ionization | Negative |
| Main conduction band involved | Valence band |
Key idea of p-type materials:
Doping with acceptor atoms creates many holes, and these holes carry most of the current.