12 - ECE331 Doping of Semiconductors A similar argument to...

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1 Lu ECE331 Doping of Semiconductors A similar argument to that above may also be made for an ACCEPTOR in silicon but now the pseudo hydrogen atom has a POSITIVE HOLE that orbits a NEGATIVE core * This gives a SIMILAR estimate for the binding energy of the HOLE - h A GROUP III DOPANT IN A SILICON LATTICE CAN ALSO BE VIEWED AS A PSEUDO HYDROGEN ATOM IN THIS CASE HOWEVER THE DOPANT CORE APPEARS NEGATIVELY CHARGED AND IS ORBITED BY A HOLE Lu ECE331 Binding Energies of Dopants • The ACTUAL donor and acceptor binding energies measured in experiment agree reasonably WELL with these SIMPLE estimates PA s S b BA lG a I n Si .045 .049 .039 .045 .057 .065 .016 Ge .012 .013 .010 .010 .010 .011 .011 BINDING ENERGY FOR DONORS (eV) BINDING ENERGY FOR ACCEPTORS (eV)
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2 Lu ECE331 Doping of Semiconductors Importantly the donor and acceptor binding energies are significantly SMALLER than the BAND-GAP energy in the host material * Now the electron supplied by each donor enters the CONDUCTION band while the hole provided by each acceptor enters the VALENCE band * The effect of the dopants is therefore to create IMPURITY LEVELS that lie very CLOSE to the conduction or valence band edge E c E v DONOR LEVEL ( Ed ) E c E v ACCEPTOR LEVEL ( Ea ) Lu ECE331 Doping of Semiconductors The conductivity of doped semiconductors is STRONGLY dependent on temperature * At ZERO temperature NONE of the dopants are ionized and the semiconductor is INSULATING * With increasing temperature electrons are excited into the conduction band FROM the donor levels and from the valence band INTO the acceptor levels ZERO TEMPERATURE INTERMEDIATE TEMPERATURES ROOM TEMPERATURE n-TYPE CONDUCTION p-TYPE CONDUCTION E c E v ZERO TEMPERATURE INTERMEDIATE TEMPERATURES ROOM TEMPERATURE
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3 Lu ECE331 Doping of Semiconductors What about the doping of COMPOUND semiconductors such as GALLIUM ARSENIDE ? * The principles are basically the SAME as discussed for silicon but now the dopants may replace EITHER Ga OR As atoms in the crystal lattice GROUP VI elements (S, Se, Te) act as DONORS for the Group V element As GROUP II elements (Be, Mg, Zn) act as ACCEPTORS for the Group III element Ga * However, what about if we dope with a GROUP IV element such as Si This typically acts as a DONOR that replaces gallium atoms but may ALSO act as an ACCEPTOR to replace arsenic under certain conditions Silicon is thus an example of an AMPHOTERIC dopant Lu ECE331 Semiconductor Density of States We would now like to provide a QUANTITATIVE description of the OCCUPATION of electron and hole states in semiconductors * The first quantities we need to introduce are the DENSITIES OF STATES in the valence and the conduction band g c ( E )d E is the number of CONDUCTION BAND states per unit volume lying in the energy range between E and E +d E g v ( E )d E is the number of VALENCE BAND states per unit volume lying in the energy range between E and E +d E 3s 3 p ELECTRON ENERGY
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12 - ECE331 Doping of Semiconductors A similar argument to...

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