Semiconductor impurities

March 24, 2016
Semiconductor.impurities
How Does Adding Impurities Affect Conductivity of Semiconductors?
Now let's consider adding impurities to a semiconductor.
When we add impurities to semiconductors we call them dopants and the process is called doping. The result is a dilute (100 -1000 ppm) substitutional solid solution.
There are two kinds of dopants: one will give negative charge carriers (to make an n-type semiconductor) and the other will give positive charge carriers (to make a p-type semiconductor).

N-type Semiconductor
N-type semiconcdutors have dopants from the VA group, such as P+5. These donor impurity atoms are in substitutional solid solution. The extra valance electron not needed for the sp3 tetrahedral bonding is only loosely bound to the P atom in a donor energy level, Ed. The energy of this donor energy level is close to the lowest energy level of the conduction band (in Si it is 0.4 eV) and so it is easy to promote an electron from the donor level to the conduction band. These promoted electrons become charge carriers that contribute to the material's conductivity. Since they are negative, the result is called an n-type semiconductor.
As temperature increases, more and more of these donor electrons will be promoted into the conduction band. Eventually, a temperature will be reached such that there will be none left. The donor electrons will be "exhausted". During this process the relationship of conductivity to temperature will look like this:

sigma = sigma0 e – (Ec-Ed) /kT

This is referred to as extrinsic semiconduction. The conductivity depends on the dopants.
After these electrons from the dopants are all promoted to the conductance band, (i.e. are exhausted, ) there is a range of temperatures before intrinsic semi-conduction kicks in where the conductivity remains essentially constant. After that, as temperature increases, there will be a promotion of electrons from the valance band into the conduction band (intrinsic behavior).
Note that the temperatures needed to promote the dopant electrons into the conduction band are lower than the temperatures required to promote the intrinsic electrons into the conduction band.
Also note that the slope of the exrinsic range is less steep than the intrinsic range. This reflects the fact that the activation energy to promote a dopant electron into the conduction band is less than the activation energy to promote an intrinsic electron into the conduction band.
P-type Semiconductor

P-type semiconductos have dopants from the IIIA group such as B+3 . These donor impurity atoms in substitutional solid solution. The lack of an electron needed for sp3 tetrahedral bonding is easily filled by a neighboring Si atom into an acceptor energy level, Ea of the dopant atom. The energy of this acceptor level is only slightly above the valance band and so it is easy to promote an electron from the valance band into it. For each promotion of an electron into one of these acceptor levels, a hole is left in the valance band. It is these holes that become the charge carriers and contribute to the conductivity of the semiconductor. Since these holes are positive, the result is called a p-type semiconductor.
Note that the temperatures needed to promote the dopant electrons into the conduction band are lower than the temperatures required to promote the intrinsic electrons into the conduction band.
Also note that the slope of the exrinsic range is less steep than the intrinsic range. This reflects the fact that the activation energy to promote a dopant electron into the conduction band is less than the activation energy to promote an intrinsic electron into the conduction band.

As temperature increases, more and more of electrons from the valance band will be promoted into these acceptor energy levels. Eventually, a temperature will be reached such that all the acceptor energy levels will have electrons in them. The donor acceptor levels will be "saturated". During this process the relationship of conductivity to temperature will look like this:

sigma = sigma0 e – (Ea-Ev) /kT

This is referred to as extrinsic semiconduction. The conductivity depends on the dopants.
After the acceptor energy levels have been saturated, there is a range of temperatures before intrinsic semi-conduction kicks in where the conductivity remains essentially constant. After that, as temperature increases, there will be a promotion of electrons from the valance band into the conduction band (intrinsic behavior).
Note that the temperatures needed to promote electrons from the valance band into the acceptor levels (leaving holes in the valance band) are lower than the temperatures required to promote the intrinsic electrons into the conduction band.
Source: fog.ccsf.cc.ca.us
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