What Are Carriers?

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In this video, we will talk about carriers in semiconductors and their properties.

So what are carriers? Carriers are the entities in a semiconductor that can carry electric current.

Let’s refer back to the bonding model. If there are no broken bonds, all valence electrons are locked to their position and could not carry electric current. Equivalently, in the energy band model, if the valence band is completely filled with electrons and the conduction band is completely empty, then there are no carriers that can carry electric current.

Only when a covalent bond is broken, the released electron is then free to move around in the crystal lattice and becomes a carrier. In the energy band model, this means that this same electron gets extra energy and jumps into the conduction band. Thus, electrons in the conduction band are carriers.

The bonding model and the energy band model are actually equivalent. The energy required to break a bond and the band gap energy Eg are one and the same thing. The freed bonding-model electrons and the conduction band electrons are just different names for the same electrons. The bonding model describes the relationships in space and the energy band model describes the energy relationships.

The other carrier is called the hole. In the bonding model, when a valence electron breaks free it leaves behind a missing bond. Nearby bound electrons could jump into this missing bond and in turn leaves behind a missing bond in its original place. Thus it appears that the missing bond is moving around. Since the electron has negative charge, this hole can be thought as having positive charge.

We can also think of it in the energy band model. The missing bond is an empty electronic energy state, and other nearby electrons can move into this energy state and in turn left an empty state of its own. Thus, the empty state is like a bubble in a liquid that can move freely in the crystal lattice.

An electron has a charge of negative 1.6 x 10-19 coulomb. A hole has a charge of positive 1.6 x 10-19 coulomb. An electron’s charge is denoted as –q, and a hole’s charge is denoted as +q.

Now let’s talk about intrinsic semiconductors. So what are intrinsic semiconductors?

Intrinsic semiconductors are pure semiconductor materials with extremely low impurities. These impurities have no significant effect on its properties.

We are interested in the number of carriers in an intrinsic semiconductor, also called carrier concentrations and are defined as number of carriers per unit volume, usually per cubic centimeter. Carrier concentration is a native property of the material, but it also depends on the temperature. This table lists carrier concentration numbers at room temperature (300°K) for several intrinsic semiconductors.

The electron and hole numbers in an intrinsic semiconductor are equal. This is because that carriers in a very pure material can only be created in pairs. In the bonding model, if a covalent bond is broken, a free electron and a broken bond are created simultaneously. Equivalently in the energy band model, when an electron gets excited into the conduction band, a hole is left behind in the valence band.

At absolute zero temperature, the carrier concentration is zero since all electrons are in a covalent bond. At elevated temperatures, some covalent bond electrons get thermal energy and escape from their bond, thus become carriers. However, in an intrinsic semiconductor, the carriers are in very low numbers. It looks big in absolute value, but compared with the total covalent bond number in a crystal, that’s just one broken bond in every 1013 bonds.

Thus, at room temperature, intrinsic semiconductors have very low conductivity.

Now let’s discuss another important property of carriers : the effective mass. So where does the concept of effective mass come from? This comes from the need to properly describe the motion of carriers in a crystal.

First let’s look at a comparison.

The left picture shows an electron moving in an electric field E in vacuum. The electron moves freely and obeys Newton’s law – Force F = the electron’s rest mass m0 times the acceleration a.

Видео What Are Carriers? канала Fiber Optics For Sale Co.