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Equilibrium Carrier Concentrations Lesson

by Greg Lush

Equilibrium Carrier Concentrations

Once we know how to determine the carrier distribution, we can find the carrier concentration by integrating over all energies:

Electron concentration:image33.gif

Hole concentration: image34.gif

After a lot of words and math we derive simple equations we can understand and use:

Electron concentration:image35.gif

Hole concentration:image36.gif

and finally the nopo product relationship:image37.gif

These equations are only valid when the semiconductor is in equilibrium and nondegenerate image48.gif. Another way to say a semiconductor is nondegenerate is that the Fermi level, EF , is more than 3kT from any of the states for which we are counting electrons. The nopo product relationship is one of the most useful equations because once you know one of the carrier concentrations (using the equations for no or po), the other can be easily calculated.

We typically deal with uniformly doped semiconductors and if they are at room temperature, we also assume total ionization of the dopant atoms. With these assumptions we can use the charge neutrality relationship and the nopo product relationship from above to derive equations for no and po that take into account the doping concentrations:

Charge neutrality relationship: po – no + ND – NA = 0

Electron concentration:image38.gif

Hole concentration:image39.gif

These equations can be simplified under a number of situations. Below are the most common:

1. When a semiconductor is not doped, NA = 0 and ND = 0, the semiconductor is intrinsic and no = po = ni. This also occurs when NA and ND are approximately equal, or ni >> |ND – NA|.
2. The equations for the carrier concentrations for a p-type semiconductor, NA >> ni and ND = 0, can be simplified. Since NA >> ni , we can neglect ni in the equation for po and obtain the carrier concentrations using the following equations:
image84.gif
3. The equations for the carrier concentrations for an n-type semiconductor, ND >> ni and NA = 0, can be simplified. Since ND >> ni , we can neglect ni in the equation for no and obtain the carrier concentrations using the following equations:
image85.gif
4. When a p-type semiconductor is compensated, doped with both acceptors and donors (NA – ND >> ni and ND is nonzero), the equations may be simplified similarly to Case 2 because we can still neglect ni in the equation for po. The nopo product relationship can then be used to solve for the electron concentration:
image83.gif
5. When an n-type semiconductor is compensated, doped with both acceptors and donors (ND – NA >> ni and NA is nonzero), the equations may be simplified similarly to Case 3 because we can still neglect ni in the equation for no. The nopo product relationship can then be used to solve for the hole concentration:
image86.gif
6. If the doping concentration, or the difference in doping concentrations if the semiconductor is compensated, is comparable to ni , we cannot simplify the equations. The full expression must be used.

Created on , Last modified on

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