What makes a semiconducting material a semiconductor?

[Ravian]

what really makes a semiconductor? Is it just the presence of a band gap but then insulators and metals also have band gaps or carrier concentration? Or we need to perform Hall measurements and also need to study I-V characteristics? Or do we need all of these parameters to establish a material as semiconductor?

 

[Pete99]

In metals you have half-filled bands, so that the fermi level is within the band. In semiconductors and insulators the fermi level is in the band gap, so at 0 K the bands are either completely filled (valence band) or completely empty (conduction band).
I think that the difference between semiconductor and insulator is that in insulators the gap is much larger than in semiconductors.

 

[Ravian]

Thanks! So what sort of experimental measurements we should make in order establish a material as a semiconductor, metal or insulator? Apparently having a band gap is not enough.

 

[Dadface]

Thanks! So what sort of experimental measurements we should make in order establish a material as a semiconductor, metal or insulator? Apparently having a band gap is not enough.

One indicator is to measure how resistance changes with temperature.With intrinsic(pure) semiconductors resistance reduces with temperature.

 

[ZapperZ]

Thanks! So what sort of experimental measurements we should make in order establish a material as a semiconductor, metal or insulator? Apparently having a band gap is not enough.

Do a resistivity versus temperature measurement. Metals will have a decreasing resistivity with decreasing temperature, whereas a semiconductor/insulator will have an increasing resistivity with decreasing temperature.

Zz.

 

[Edgardo]

On a side note this property, namely decreasing resistance when the temperature is increased is used in http://en.wikipedia.org/wiki/Thermistor" .

You can test this yourself by connecting the thermistor to a multimeter:
http://www.hk-phy.org/energy/commercial/print/act_experiment_e.html
http://www.practicalphysics.org/go/Experiment_701.html
http://www.brighthub.com/engineering/mechanical/articles/27687.aspx

 

[LostConjugate]

The way I understand it is.

Perfect Insulator: No energy band is partially filled. Relatively large amount of energy required for a particle to reach the next energy level.

Conductor: Partially filled energy band, very little energy required to move to the next highest energy level.

Semi: Highest energy band is mostly empty, band before that is almost but not full. Some particles start to gain energy and quickly reach a gap, others are free as if they were in a conductor.