Describe the properties of donor and acceptor dopant atoms and how they affect the densities of charge carriers in a semiconductor material.
What will be an ideal response?
Analysis:
An "intrinsic" semiconductor material is undoped. When dopant atoms are added the material becomes an "extrinsic" semiconductor. Doping results in the replacement of an intrinsic atom with a dopant atom. As few as one out of every million intrinsic atoms may be replaced.
A "donor" dopant atom has 5 valence electrons. Only 4 are required to complete the bonding structure in the semiconductor material. The 5th electron requires very little energy to escape to the conduction band and become a negative charge carrier. This leaves behind a donor atom with one missing electron or a negative atomic ion. The ion is immobile and cannot move through the material; therefore, IT IS NOT A CHARGE CARRIER.
Each donor contributes an additional negative carrier to the material. The increased density of negative carriers results in an increased recombination rate which reduces the density of positive carriers. [The PRODUCT of the two densities remains constant.] Materials doped with donor atoms are N type extrinsic semiconductors. The majority carriers are conduction band electrons, the minority carriers are valence band holes.
An "acceptor" dopant atom has 3 valence electrons; however, 4 are required to complete the bonding structure in the semiconductor material. The "missing" 4th electron causes a vacancy or hole in the bonding structure. Another valence electron may move to and occupy this hole thus eliminating it and generating another hole and a negative atomic ion. The ion is immobile and cannot move through the material; therefore, IT IS NOT A CHARGE CARRIER.
Each acceptor contributes an additional positive carrier to the material. The increased density of positive carriers results in an increased recombination rate which reduces the density of negative carriers. [The PRODUCT of the two densities remains constant.] Materials doped with acceptor atoms are P type extrinsic semiconductors. The majority carriers are valance band holes, the minority carriers are conduction band electrons.
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