||Creating Carriers with Dopants|
|Density of States||An Analogy||The Fermi Function||Where are they?|
On the other hand, a free electron and a hole give rise to the generic term carriers. When carriers are present there is a current flow, this only occurs if a bond is broken causing an electron to be in the conduction band and/or a hole being created in the valence band of a band diagram.
It is fairly simple to visualize an electron "jumping" to the conduction band and moving in every direction, since we know it is a physical object. However, visualizing a hole is something new to almost, if not everyone, in the class. Since a hole is the absence of an electron, one way to look at it is by picturing a soft drink. When it is being poured, bubbles rise from the very bottom of the glass to the top and are released. As they rise to the top, the liquid is taking their place at the bottom. We can picture the glass to be the valence band and the atmosphere being the conduction band. The bubbles are the holes and the liquid is the electron. Since electrons like to be at the lowest energy state possible, they take the place of the hole in a lower state, making it rise to a higher state carrying the absence of an electron, a positive charge, with it; therefore, we get a positive current due to holes. When the soft drink goes flat, there aren't any bubbles moving around and the glass is full of liquid. Likewise, we can say the valence band is full and the conduction band is empty since positive charges do not occur in the conduction band.
The number of carriers in an intrinsic semiconductor is one of the identifiable
properties of a material. These are denoted n, for the number of electrons
per cubic centimeter, and p, for the number of holes per cubic centimeter.
In an intrinsic semiconductor and under equilibrium conditions, these two
quantities are equal, since they occur in pairs. If a bond is broken, creating
an electron, it also creates a hole in the process. However, this does
not make for an efficient semiconductor. The charge created by the free
electron will be neutralized by the charge created by the hole; therefore,
dopants are added to the semiconductor material to increase either the
electron or hole concentration in the semiconductor.