In tutorial 2, we briefly looked at how current flows and the difference between Alternating Current and Direct Current; in this tutorial we will look closely at current flow in a semiconductor and at the definition of electrical conductivity and resistivity.

AC vs DC Current

AC vs DC Current

Electrical current is the flow of charge: it is essentially how many charged particles pass a certain point over a certain time. It is dependent on several things:

  •  How fast the particles are moving
  • The cross sectional area of the conducting material
  • How much charge is carried by the particle
  • The density of these mobile charges

With this in mind we can define current as:

  •  is the current
  •  is total charge
  • is time

Or when thinking about the number of charger carriers:

  •  is the density of charge carriers
  •  is the charge per charge carrier
  •  is the velocity of the charge carriers
  •  is the cross sectional area of the material

Note the dot product ().

Current Density

Current Flow is the same

Current Flow is the same

In the diagram above, we can see 2 types of current flow: uniform and non-uniform. In both cases the current is the same and this is due to Current Density ().

In Uniform Current Flow, the current density threading a cross sectional area is defined by:

In Non-Uniform Current Flow, the current density must be integrated with respect to the cross sectional area:

Drift Current Density

The drift current density is the current density within an electric field. We calculate this differently depending on negative (electrons) or positive (holes) charges:

  • For electrons:
  • For holes:

The total current density in a semiconductor can then be calculated as:

Electrical Conductivity

Electrical conductivity () is a measure of a materials ability to conduct a current, it measured in Siemens per metre () and can be defined as:

Similarly the Electrical Resistivity () is a measure of how a material opposes current. It is calculated by examining the resistance of a material along with its volume and it is measured in Ohm metres (). It is defined as:

  •  is the resistance of the material
  •  is the cross sectional area of the material
  •  is the length of the material

That’s it! You have reached the end of the physics tutorial. As we mentioned in tutorial 1, we can apply all of this knowledge to a variety of Semiconductor devices; and all of those tutorials can be found in the Semiconductor Devices category.