Why do semiconductors and metals exhibit opposite resistivity when heated?

[Analog]

When a semiconductor is heated, it forms more electron and hole combinations which decrease resistance. That condition in transistors is known as thermal runaway. But why are conductors like copper and aluminum the opposite? When they are heated, their conductivity decreases, and resistance increases.

 

[f95toli]

Because they work in a sligthly different way.
There are no "electrons and holes" in metals, at least not in the way you mean when you discuss semiconductors.
The basic mechanism of conduction in metals is this: A perfect piece of metal (no defects or impurties) is really a crystal, the atoms arrange themselves in a highly organized pattern, in the simplest materials the atoms will sitt in the "corners" of a cube (so-called single-cubic symmetry).

The difference between a metallic element and an insulating element is simply the elecronic structure; in a metal some of the electrons in the atom can travel relatively far from the core (in an insulator they always stay close to the core). In some cases a electron in a crystal can travel so far from their "mother core" that they come very close to a neighboring atom, now they will be under the influence of the new core etc. The end result of this process is that you will have an "electron gas" with electrons moving around more or less freely in a crystal made of ions.
Since the electrons are so weakly coupled to the crystal they are easy to move around, hence metals are good conductors!
However, this is only true at zero temperature (0 K) , if you heat a piece of metal the atoms in the crystal will start to "shake" and the crystal is no longer perfect, hence the electrons that used to be free will start to interact (you could say collide) with the atoms again and they are not free to move as they want anymore, this means that the conductance decreases.

This is a simplified explanation, if you are really interested I suggest you look it up in a book about solid state physics.

 

[pm]

In highly doped semiconductors - like those used in the source, channel and drain of the transistors in a typical modern CPU - resistivity increases with increasing temperature. In these materials, as temperature increases the mean free path of the majority carriers (either holes or electrons depending on the device) is reduced due to thermal vibration and so resistance increases.

 

[Mday]

wait... how do you have a BSEE and MSEE without knowing this?

 

[Rainsford]

pm and f95toli have it, but I would like to ask, what kind of semiconductors are better conductors at higher temperatures? I have never heard of this. As far as I know, the only good superconductors we have need very low temps to work, so better conduction with increasing temps seems illogical. But, I'm only a Sophomore in CprE, so I might just not have heard of it.

 

[Peacekeeper100]

Originally posted by: Mday
wait... how do you have a BSEE and MSEE without knowing this?

You have a good point there. I learned about stuff like this in my BSEE.

I think it was already stated that when it is highly doped it reacts differently than an intrinsic semiconductor. Since the intrinsic is actually a insulator the resistance is high, then as it is heated the created electron hole pairs allow conductivity. But in a doped semiconductor then it is already a conductor so it doesn't react the same.

 

[calbear2000]

The easiest way for me to remember this phenomenon in semiconductors is to remember that mobility of electrons and holes decreases with increased temperature (dominance of photon collisions). This means decreased current, and thus an increase in resistance

 

[Fencer128]

Hi,

Don't you mean "phonon" not "photon" collisions?

Cheers,

Andy