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Course: Modern Physics (Essentials) - Class 12th > Unit 5
Lesson 1: What makes some materials conduct electricity while others resist it?Conductors insulators and semiconductors
Why do certain things behave like conductors while others don't? This may seem like a pretty simple question, but it's not. To really answer this question we need to dig deeper into the electron energy levels of a solid. In this video, we will use the band theory to figure out what makes certain things behave like conductors, insulators and semiconductors. Created by Mahesh Shenoy.
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- Let's say we heat up a chunk of copper. Now more electrons can go higher up in the outermost big energy band and move around more easily, right? If so then why does the resistance of conductors increase with increase in temperature?(15 votes)
- You're absolutely right. As you heat the chunk, more electrons will move freely. But then the number of free electrons becomes so high, that they start colliding with each other and thus disturb the flow. Hence the resistance also increases(32 votes)
- Question 1: At7:02, he mentions that valence bands contain valence electrons (ie, the valence shell). This is obviously the highest possible energy level for a given atom. Supposing that an electron is excited to the conduction band, where does it go in the actual atom? There is no energy level beyond the valence shell, so is it hypothetical?
Question 2: How can energy bands overlap without violating Pauli's Exclusion Principle?(6 votes)- Hopefully someone corrects me if I'm wrong, but as far as I understand, in the valence band, the electrons are localised to individual atoms, whereas when they get "excited" to conduction band, the electrons are shared by the entirety of the solid, ie, they're free to move.
As far as i understand, and mind you, all this might be wrong, the conduction band is possibly a way to describe the activity of the electrons, and hence Pauli's rule is not violated.(4 votes)
- can you by any chance excite electrons in insulators so that they become semiconductors?(4 votes)
- I think it'd acquire too much energy to do that, so I guess it is nearly impossible.
hope this answer is still of use even though I'm 5 months late ;)(4 votes)
- Can you please explain Fermi level in a little detail . Ideally there doesn't exist and energy state in the forbidden energy gap . Still we say that the Fermi energy is between the VB and CB . Need some clarity on the topic .(4 votes)
- Is this a classical physics concept?(2 votes)
- What will be the band gap of metal when we increase the temperature?(2 votes)
- At6:11, why do you say when the electrons are excited to high energy levels, they are free to move?(1 vote)
- what is the fermi level?(1 vote)
- I have learnt that 0 Kelvin is the temperature at which motion ceases. So can it be attained?
Secondly, why are electrons of insulators forbidden to acquire the any amount of energy of forbidden gap? Is it because of the huge energy difference that they have to encounter.(1 vote) - I got the point that, higher energy band (valence band) is responsible for conducting electrons.
But,at01:25, why are we concerned with the next higher energy band?
What does conduction band means?(1 vote)
Video transcript
- [Instructor] The key to understanding the electrical properties of material, is to look at the energy diagram, the energy band structure of the solids and focus on the highest energy band which contains electrons in them. I mean, think about it,
any material you take, any solid you take, there must be some highest energy band
with electrons in it, right? Because there are a finite
number of electrons. So you pick that highest energy band. It could be anything I don't know, depending on which material we choose, the band can either have
completely filled electrons or maybe partially
filled electrons, right? Any of them is possible. So let me just shade this to show filled electrons over here. Let's assume it has
partially filled electrons. This is even one of them. And since electrons tend to get excited at higher temperatures and we don't want to
look at that right now, let's look at the lowest
energy state possible. That's at the lowest temperature possible zero kelvin. So let's assume this is at zero kelvin, zero kelvin. O K. All right. Now, if you take materials like materials like sodium or magnesium or copper or iron, you see I'm talking
really about conductors. If you take materials like these, which are conductors, it
turns out that if you look at the next available energy band, what you see is that the
next higher energy band ends up overlapping with this energy band. As we saw before energy bands can overlap and in good conductors or in metals, they stay overlapped. And so strictly speaking, these are not two different energy bands because once they overlap they become one single giant energy band so I really should get rid of these divisions over here, I'm just gonna decrease their opacity. There you go. So what we have now is one giant energy band with electrons
not completely filled there are so many vacant
spaces at zero Kelvin. Now think about what will happen if we increase the
temperature just a little bit. If we increase the
temperature just a little bit, the thermal energy is going to try and excite the electrons,
electrons over here a little bit higher. The question is, are the energies allowed a little bit higher? The answer is yes, there
are energies allowed. There are so much energy allowed it's just the whole thing is a continuum. Remember they don't have to jump anywhere over here, it's allowed and so as a result, all these electrons, all of them end up becoming free electrons because they can freely move over here. You can sort of think of this like a big classroom with only partially occupied students, the classroom is empty and the kids are going to move around. This is the situation for conductors. So what we're dealing with
over here are conductors. On the other hand, if you
look at some other material and check their energy
bands, we'll see that their highest energy band, again, this is the highest energy band will be completely filled at zero Kelvin, will be completely, completely filled at zero Kelvin. Again, remember, this is O K, zero Kelvin. And now if you look at the
next available energy band, you will see it's not only
not, it's not overlapping, but there's a huge gap between them. There's a huge energy gap between them and this energy gap is
called as the band gap. It's also called as the
forbidden energy levels because electrons are forbidden to be anywhere over here. Usually call it as Eg. And if this band gap,
Eg, if it is somewhat more than four electron
volt, now it's not, again, this is not a very strict thing sometimes you could
write five electron volts or four but if it's sort of like more than four electron volts, then we'll call this as an insulator. We'll call this as insulators, all right? So this will be the example for say glass or diamond, they're excellent insulators. Can you see why they end
up becoming insulators? Because now if you increase
the temperature a little bit and the electrons try to get excited, well now the electrons
can't get excited so easily because if you try to excite the electron from here to here, well
they can't accept that because remember these
energy levels are forbidden. So if you want to excite these electrons, you have to excite them
all the way till here and the probability of an electron being excited all the way over there is extremely low even
at room temperatures. And so in such materials
even at room temperatures, you will find extremely tiny amount of electrons found in this empty band and so only little bit of, or negligible amount of
free electrons are found and that's the reason insulators behave like insulators. And if you look at silicon or germanium and focus on their highest energy bands, again, it's found that at zero Kelvin, this is completely filled, zero Kelvin this whole thing is completely filled just like an insulator. All right. But now if you look at it's next available energy band, what you find is situation is very similar to insulator but you
can see the difference is that that same band gap, that forbidden energy gap, that band gap is extremely tiny. Usually it's, for Silicon I remember it's for silicon it's about 1.1 electron volt, for germanium about 0.7 electron volt so it's usually found to be less than about two electronic volt we can say somewhere like that, again
it's not very rigorous we don't have to worry too much all that matters is very low. And now as a result if you increase the temperature say to room temperature and you try to excite the electrons well, compared to insulators, electrons will get more readily excited over here. So at room temperature, you will find a lot more electrons in
this higher energy band ready to conduct compared
to that of insulators. But of course they're
not as good as conductors because in conductors, you don't have a gap at all there is no band gap and so all the electrons
become free over here. Here, a very few amount of electrons can become free, here almost no electrons, almost, I say almost because some electrons will get free and that's the reason these guys end up becoming semiconductors, semiconductors. Now the only one detail we need is to know the names of these two
energy bands, all right, this highest field energy
band at zero Kelvin, we call that as the valence event. So these bands, the highest field bands are called callers valence bands. So whenever someone says valence bands, what usually comes to my head is it's the highest field band at zero Kelvin and it's most of the cases
it's completely filled at least when you take
insulator or semiconductor, it is completely filled. And by the way, it's called valance because even in the atoms we have something called valance shells, the final shell which
has electrons, right? This, the word comes from there itself. And the next higher
band in which conduction takes place like I mean the electrons have to go there to get conduction, we call that as the conduction band. So these bands are
called conduction bands, conduction bands. And notice at zero Kelvin, the conduction band is always empty, completely empty at least for insulators and semiconductors. And now you might ask, what
do you do for conductors? Well simple, don't define valance bands and conduction bands because
you see in conductors, you actually just have one giant band you could call it as a valance band, this whole thing because it is the band with the highest energy electrons. We can also call it as a conduction band because that's the band in which conduction can take please. So they lose their meanings for conductors but they have very specific meanings for insulators or semiconductors.