Ok since theres some confusion i will explain this, an easy way and an indepth way. fornowagain is on the right track
easy way:
Semiconductors when heated have more thermal energy, the extra thermal energy gets the electrons more excited and so they break away from there bond which results in a free electron, as we know current is the flow of electrons through a materaial, so if giving more heat allows the meterial to have more free electrons then it will have a lower resistivity which results in a lower resistance which again allows higher currents, the higher current will produce more heat in turn however there is a limit to which this can happen, the Fermi level (look it up in wiki or sumthin) can explain this.
indepth way:
Ok here goes, semiconductors are made up of insulators and conductors(hence the semi) they are Intrinsic semiconductors (have no doping) there outer electron layer has 4(usually) electrons(take silicon or germnanium), Both silicon and germanium have valency 4, and they crystallise in the diamond structure by covalent bonding of the four valence electrons each one valence electron from its four nearest neighbours.
At room temperature some of the valence electrons acquire enough thermal energy to break away from the valence shell(away from the bond) thus giving a free electron and a hole(a vacancy for an electron in the valence shell).
This hole being a vacancy for an electron consists of a missing negative charge and is equivalent to the presence of a positiver charge. The materia, remains overall neutral since breaking each bond produces one electron and one hole, and therefore the number of conduction electrons per unit volume(ne) and the number of holes per unit volume(nh) are equal(nh=ne only for intrinsic materials))
Current can therefore be carried through the material by the motion of the free conduction lectrons in one direction and the motion of holes in the opposite direction.
The energy required to break a bond and so generate an electron hole pair is different for each material.
silicon=1.12eV germanium=0.66eV
eV is a unit of energy and 1eV= 1.6x10^-19 Joules
So from this at any temperaure (T in Kelvins) the average thermal energy of a particle is given by kT where k is Boltzman constant. if the the given temperature is high enough to allow the thermal enegy to be higher than the energy to break a bond ie 1.12eV for silicon then the increased thermal energy will break more bonds to generate more electron hole pairs and so the resistivity decreases.
Obviously a lower resistivity will create a lower resistance and therfore a higher current but there is a limit, as eventually all the covalent bonds will be brocken and all the electrons will be free so there is an optimal temperature where all the electrons will be free and then applying more heat or thermal energy wont decrease the resistivity any further, this can be shown in the Fermi function.
This is for intrinsic semiconductors however and is only the tip of the iceberg it gets a lot more complicated when using extrinsic semicondutors (p-n junctions etc) eventually after the temperature is so high, in a semiconductor junction (uses 2 types of extrinsic materials) which is what transistors and processors are made of, the junction breaks down and the current flows either way and destroys the gate. Also, higher thermal energy increases gate leakage, dusnt alow the gate to operate correctly at high frequencies beacuse of the MILLER effect which basically is due to capacitance between the gate and drain and not forgetting Thermal noise, shot noise, 1/f noise etc
Extrinsic semiconductors have other metals fused into the structures to alter the properties however i wont go into that unless u want me too lol
hope this clears things up
(sorry for my bad typing and spelling)
