Dear Qaisar,
The conductivity of a semiconductor material depends on the mobile charge concentration and the mobility. As the doping increases the carrier concentration increases and the mobility decreases. In relatively low oping the carrier concentration increases linearly with doping concentration while the mobility decreases in a lesser extent with doping hence the conductivity increases with the doping. It can that at heavy doping an increased fraction of the doping atoms will not be ionized and remains inactive If the impurity is increased beyond the solid solubility limit the dopants will be clustered in the material and the electron concentration no longer increases while these doping atoms will cause larger scattering of the electrons of trhe electrons and larger decrease in the mobility. Hence the conductivity may begin to fall at certain specific doping concentration.
In order to invistigate this behavior it advisable to measure the carrier concentration using Hall effect and measure the mobility by the time of flight method.
Dear Qaisar,
The conductivity of a semiconductor material depends on the mobile charge concentration and the mobility. As the doping increases the carrier concentration increases and the mobility decreases. In relatively low oping the carrier concentration increases linearly with doping concentration while the mobility decreases in a lesser extent with doping hence the conductivity increases with the doping. It can that at heavy doping an increased fraction of the doping atoms will not be ionized and remains inactive If the impurity is increased beyond the solid solubility limit the dopants will be clustered in the material and the electron concentration no longer increases while these doping atoms will cause larger scattering of the electrons of trhe electrons and larger decrease in the mobility. Hence the conductivity may begin to fall at certain specific doping concentration.
In order to invistigate this behavior it advisable to measure the carrier concentration using Hall effect and measure the mobility by the time of flight method.
In general the doping of amorphous materials is not well understood. From the theoretical point of view, doping has now effect on the electrical properties of a-Si. But experimentally, an effect is observed.
Another example is glass. An amorphous material with "tons" of contaminations. It must absorb vivisble light, but it is transparent.
With greater doping than a certain limit, dopant atoms are gathered in the form of clusters which cause increase in resistivity while decreasing the value of conductivity. So, it is better to dope under the limit to avoid the drawback over the limit.
Thanks to all for your valuable input but still i am confused how mobility decreases while increasing doping?
When you add boron or phosphorus to your Si lattice, an important fraction thereof is incorporated as dopant atoms (via tetrahedral coordination, i.e., by establishing four hybrid sp3 orbitals and therefore imitating and replacing silicon) but also a fraction incorporates to the lattice without doing so (i.e. keeping the original 2p or 2p3 orbital(s)). These latter atoms do not behave as dopants (do not create any free electron or hole), but rather act as recombination centres, since their presence in the lattice distorts its overal order. The more you dope the silicon, the more boron or phosphorus defects you insert in the lattice. When the concentration of such non-tetrahedrally bonded atoms becomes large enough, the lost in carrier lifetime and mobility overcomes the effect ot the free carriers created by the addition of tetrahedrally-bonded dopant atoms.
The effect is more pronounced when doping with boron because this atom is appreciably smaller than that of silicon, whereas phosphorus atoms are nearly equal to silicon atoms in size. Phosphorus is more sucessful in imitating silicon than boron is.
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