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