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I'm taking a solid state course, and is currently on the subject of dielectrics. In one of the sections, concerning "Impurities in Dielectrics" the books says:

"Impurities can also be used to make insulators conductive, exactly like doped semiconductors. Donor or acceptor levels have to lie close to the conduction band and valence band, respectively, in order to give rise to a finite density of electrons and holes and thereby conductivity. The important advantage over a usual semiconductor material is that high-temperature applications are possible. In a narrow-gap semiconductor, high temperatures are a problem because of the exponentially increasing number of intrinsic carriers. In a doped insulator, this is not an issue for practically relevant temperatures"

Now, I'm not getting why more carriers are a bad thing ? I see, that if the gap is too small (Semiconductor), and the temperature is high, the you should be able to excite electrons into the conduction band, instead of just the doped bands, right ? But why is that a bad thing ? I see that high-temperature applications can be bad, since you have to get high temperatures, which is not always easy and great, but if you already are at that temperature, what is the problem ?

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A big carrier concentration can be a bad thing. The advantage of semiconductors is that they change their properties under external actions (generally, the carrier concentration $\Rightarrow$ conductivity). If you already have a lot of carriers than it is difficult to make a considerable change. For example, narrow-gap semiconductors are used in IR-detectors, and the thermally activated carriers can produce a big undesirable dark current (e.g. see Fundamentals of Infrared Detector Materials, chapter 3.1).

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