Extrinsic Semiconductor. ...

Recently i came across a theory while studying EDC (electronic device circuit) about doping ... it was : " At very high temperature all extrinsic S.C. devices become intrinsic bcoz Band-Band transition dominates and overthrows impurity and at this temperature usefulness of all electronic devices gets terminated" ... can anyone help me understand this concept???

Generally , if we increase temperature of a metal, the thermal motion of ions also get increased, so it reduces the mean free path of free electrons.This effects ,mobility of electrons(reduces)hence conductivity also reduces. At high temperature ,the semiconductor acquires the properties of metal(because of excessive electrons), so the conductivity of semiconductor also reduces. so the usefulness of device gets terminated.

Temperature causes electrons to be promoted to the conduction band and from donor levels, or holes to acceptor levels. The dependence of conductivity on temperature is like other thermally activated processes:

s = A exp(–Eg/2kT)

where A is a constant (the mobility varies much more slowly with temperature). Plotting ln s vs. 1/T produces a straight line of slope Eg/2k from which the band gap energy can be determined. Extrinsic semiconductors have, in addition to this dependence, one due to the thermal promotion of electrons from donor levels or holes from acceptor levels. The dependence on temperature is also exponential but it eventually saturates at high temperatures where all the donors are emptied or all the acceptors are filled.

This means that at low temperatures, extrinsic semiconductors have larger conductivity than intrinsic semiconductors. At high temperatures, both the impurity levels and valence electrons are ionized, but since the impurities are very low in number and they are exhausted, eventually the behavior is dominated by the intrinsic type of conductivity i.e. transition from valance band to conduction band.

jeet_asic said:

Recently i came across a theory while studying EDC (electronic device circuit) about doping ... it was : " At very high temperature all extrinsic S.C. devices become intrinsic bcoz Band-Band transition dominates and overthrows impurity and at this temperature usefulness of all electronic devices gets terminated" ... can anyone help me understand this concept???

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The activation energy for electrons to be donated by or accepted to impurity states is usually so low that at room temperature the concentration of majority charge carriers is similar to the concentration of impurities. It should be remembered that in an extrinsic semiconductor there is an contribution to the total number of charge carriers from intrinsic electrons and holes, but at room temperature this contribution is often very small in comparison with the number of charge carriers introduced by the controlled impurity doping of the semiconductor.