The charge carrier density denotes the number of charge carrier

Charge carrier

In physics, a charge carrier is a free particle carrying an electric charge, especially the particles that carry electric currents in electrical conductors. Examples are electrons and ions...

s per volume

Volume

Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....

. It is measured in m⁻³. As any density

Density

The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 it can depend on position.
It should not be confused with the charge density

Charge density

The linear, surface, or volume charge density is the amount of electric charge in a line, surface, or volume, respectively. It is measured in coulombs per meter , square meter , or cubic meter , respectively, and represented by the lowercase Greek letter Rho . Since there are positive as well as...

, which is the number of charges per volume at a given energy.

The carrier density is obtained by integrating

Integral

Integration is an important concept in mathematics and, together with its inverse, differentiation, is one of the two main operations in calculus...

 the charge density over the energy that the charges are allowed to have.

The charge carrier density is a particle density

Particle density

The particle density or true density of a particulate solid or powder, is the density of the particles that make up the powder, in contrast to the bulk density, which measures the average density of a large volume of the powder in a specific medium .The particle density is a relatively well-defined...

, so integrating

Integral

Integration is an important concept in mathematics and, together with its inverse, differentiation, is one of the two main operations in calculus...

 it over a volume gives the number of charge carriers in that volume

.

where is the position-dependent charge carrier density.

If the density does not depend on position and is instead equal to a constant this equation simplifies to

.

The charge carrier densities enters equations concerning the electrical conductivity and related phenomena like the thermal conductivity

Thermal conductivity

In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....

.

Measurement

The density of charge carriers can be determined in many cases using the Hall Effect

Hall effect

The Hall effect is the production of a voltage difference across an electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current...

, the voltage of which depends inversely on the density http://www.as.utexas.edu/~lindner/random/The%20Hall%20Effect.pdf.

Such measurements show that the density for silver is around 1 x 10^28 m^-3, or around 1.8 per atom.