When temperature is increased in case of a semiconductor the free electron gets more energy to cross the energy gap to the conduction band from the valence band.so now more electrons can go easily to the conduction band so resistance decreases with temperature.But due to temperature increase,in conductors the atoms of the conductor vibrate which gives a resistance to the free electrons.But why does this happen?In conductor temperature makes atoms vibrate but in semiconductors the electrons vibrate?
[Physics] Why in a conductor increase in temperature makes the atoms vibrate but in a semiconductor the kinetic energy of the free electrons is increased
electrical-resistanceelectricityelectronic-band-theorysemiconductor-physicstemperature
Related Solutions
Semiconductors can be split into two groups. Intrinsic semiconductors have a band gap that is around thermal energies, so a few electrons can be promoted from the valence to conduction band at room temperature. This corresponds to the third picture from the left in your post.
Extrinsic semiconductors have had a dopand added, and this creates new states in the band gap. These extra states can either accept electrons from the valence band or donate electrons to the conduction band. In the former case you get conduction due to holes in the valence band (p type) and in the latter you get conduction due to electrons in the conduction band (n type). This corresponds to the rightmost picture in your post, though whether the dopant states form a band is debatable, though maybe this is just terminology. Note that conduction is movement on holes or electrons in the semiconductor valence or condustion bands, and not due to transport in the dopant states.
Now to your questions:
I suppose all semiconductors have some intrinsic semiconduction, but for an extrinsic semiconductor this is usually negligable. The conductivity is dominated by the doping.
I think this is covered by the into above. An extrinsic semiconductor has either holes in the valance band and an empty conduction band, or electrons in the conduction band and a full valence band, but not both.
In a p type semiconductor I suppose you could excite electrons from lower bands into the holes in the valence band, but the energies required are far greater than thermal energy so this doesn't happen at room temperature. In n type or intrinsic semiconductors you can excite electrons from inner bands into the valence band because the valence band is full.
Extrinsic semiconductivity isn't based on electrons jumping between the valence and conduction bands.
The I-V characteristics of materials and devices should always be measured at the same thermodynamic conditions, i.e. at the same temperature. Mixing the actual isothermal I-V characteristic with the temperature dependence doesn't lead to any useful data for the purposes of physics (but it is occasionally done in electrical engineering and electronics design for certain parts like NTC heaters and breakers).
A pure semiconductor at a constant temperature would be a pretty good Ohmic conductor, i.e. the current will be proportional to the applied voltage. This is a lot harder to measure properly on semiconductors than on metals, though, because of junctions formed with the metal wires that one has to attach for the measurement.
The conduction characteristics of semiconductor devices with one or multiple different materials forming junctions, on the other hand, is highly non-linear and can be made very complex. These devices will also have a temperature dependence, but it can be tuned very finely with appropriate material combinations and geometries.
Pure metals have typically increasing resistance with increasing temperature, but alloys can be made that have almost constant temperature characteristic (i.e. they are both Ohmic and temperature independent). One can also make metal alloys with negative characteristics, if necessary. Both constant and negative temperature characteristic is of enormous importance for the design of electronics, almost none of which would function properly if we couldn't make these near zero-TC metal alloys for resistors and NTC's for temperature measurement and compensation.
Non-metallic materials with very strong negative temperature characteristics often use percolation phenomena, i.e. on grain boundaries in sintered crystal powders, where conduction can only happen in very few narrow points in the material. As the material expands, these points of contact may get lost and the resistance may increase by many orders of magnitude over the technical temperature range of the material. The physics of these systems is very different from that of metals and semiconductors.
I think it would be better to say that power lines are designed to avoid ohmic heating rather than that they make use of it. I am not sure about the potential advantages of the heating for lines that may otherwise be weighed down and damaged or destroyed by snow and ice in cold climates, though. One would have to look at the design requirements for these power systems to understand if their designers make explicit use of these otherwise unwanted losses.
You are correct that one can trade current for voltage and vice versa by adjusting the resistance in circuits. Much of electronics design is a repeated application of that principle.
As for the question of how to design materials that have nearly temperature independent characteristics, that would require a very deep dive into solid state physics and materials research and I will leave that to someone who actually has the necessary detail knowledge. The guiding principle in many of these practical applications is that one tries to offset a positive gradient of one material with the negative gradient of another or one tries to combine multiple materials in such a way that the physical effects (like the formation of defects in the mixed material) offset bulk effects like the increase in the number of conduction band electrons in either of the constituents.
Best Answer
There are two things that affect the resistance. The number of mobile charge carriers (typically electrons or holes) and their mobility (mainly limited due to scattering from atoms).
When you increase the temperature both the electrons and the atoms will obtain more energy. This holds for both a regular metallic conductor and for a semiconductor.
Both materials will see an increase in scattering of electrons from the increasingly vibrating atoms.
The big difference between the the two materials is the band gap of the semiconductor. Electrons in the semiconductor that do not have sufficient energy cannot contribute to the conductivity. Therefore increasing the energy of the electrons, increases the number of electrons or holes (charge carriers) that can contribute to the conductivity, thereby drastically lowering the resistance. This effect does not occur in metals.
Because the latter effect is typically much bigger than the first, even though there is an increase of scattering at higher temperatures, the resistance of semiconductors decreases with increasing temperature.