HSC Physics Question Thread

[pikachu975]

Hey there,

We've just gone through p and n-type semiconductors in class and I have read a few textbooks in addition to consolidate my study. BUT i'm a bit confused as to which explanation to trust; is it one which explains doping in terms of the band structure of the semi-conductor or how the atoms interact in a lattice as a result? In one book it refers to 'dopant' and acceptor levels from which conduction occurs as this is where the electrons jump to from the valence shell (where they leave 'positive' holes). However, most explanations just discuss the complete atom arrangement where electrons from neighbouring Si/Ge atoms try and fill the positive whole gap formed by the doped element. This then constitutes a moving charge and so, current.

Obviously both explanations make sense, but since there's a focus on band theory in the initial content leading explaining valence and conduction bands, should I stick with the band structure explanation? Which one is expected in the HSC?

Please let me know if this question doesn't make any sense!

Any help would be greatly appreciated

We went through acceptor levels and donator levels (Depending on group 3 or group 5 dopant) so I guess that

[jamonwindeyer]

Both explanations would definitely be permitted there - As long as they have enough detail. It's good to have a picture of what happens in terms of both energy levels and atomic level interaction (you could be asked to draw from either)

[beau77bro]

can someone please explain this and or conduction bands? like i can't quite visualise it, its just a means to represent how much energy is needed for an electron to flow basically right? then how does this diagram even work?

thankyou

[katnisschung]

what the heck is a phonon?
i'm trying to self teach myself.... my current understanding is that its a unit of vibrational energy
whereas others are saying its a particle like an electron...confused?
thanks

[jakesilove]

can someone please explain this and or conduction bands? like i can't quite visualise it, its just a means to represent how much energy is needed for an electron to flow basically right? then how does this diagram even work?

thankyou

Hey! This is one of those things you just need to memorise; be able to draw the diagram, and describe the bands (ie. When electrons are in the conduction band, they can conduct!). In my HSC, the way I thought about it was that the additional bands (donor/acceptor due to doping) allowed electrons to 'jump' more easily from the Valence to the Conduction band. In essence, the band structure is a diagrammatically way of representing this complex idea; however, the diagram is actually deeply routed in Quantum Physics concepts far beyond the curriculum.

[jakesilove]

what the heck is a phonon?
i'm trying to self teach myself.... my current understanding is that its a unit of vibrational energy
whereas others are saying its a particle like an electron...confused?
thanks

Hey! In the HSC, you really don't need to know anything about a phonon. In reality, you just need to be able to say something like 'A phonon (vibrational energy) is produced and transmitted from one cooper pair to another' etc. A phonon is exactly that; a unit of vibrational energy. However, recall that waves/particles are very much the same thing; it's all a scale, not a one-or-the-other type situation. Thus, we say that phonons (like photons) come in a 'packet' of energy, similar in form to an electron (in that there is a defined position etc.). However, it is really just energy. All this is beyond the curriculum!

[beau77bro]

Hey! This is one of those things you just need to memorise; be able to draw the diagram, and describe the bands (ie. When electrons are in the conduction band, they can conduct!). In my HSC, the way I thought about it was that the additional bands (donor/acceptor due to doping) allowed electrons to 'jump' more easily from the Valence to the Conduction band. In essence, the band structure is a diagrammatically way of representing this complex idea; however, the diagram is actually deeply routed in Quantum Physics concepts far beyond the curriculum.

omg thankyou jake it just clicked, so basically the levels represent the effect of introduced charge carriers? so for the first one, it's showing that the n-type doped conductor has electrons added that can move more easily into the conduction band - so the donor level just means the extra electrons can move easier into the conduction band.
and the acceptor band means the electrons can move into a hole, they do not require as much energy to move into the conduction band but can instead move to adjacent holes?
is that kinda the jist of it? they have to jump lower energy due to the addition of charge carriers?

[beau77bro]

also, i was wondering about ur research into quantum tunneling and how that effects processing chips when they start getting super small. we have a research task on transistors and electronics development and effects on society, and one of the parts is about current research and unanswered questions relating to transistors. so i was just wondering if you could give me some insight into the very very basic concepts of your research and some of the issues. i recognise that by making the devices smaller in the integrated circuit, you can fit more making them more powerful processors, and that by making them smaller you reduce energy loss and time between devices, making it faster as well as cheaper to produce as the size decreases. so basically the aim is to make them as small as possible.

[jakesilove]

omg thankyou jake it just clicked, so basically the levels represent the effect of introduced charge carriers? so for the first one, it's showing that the n-type doped conductor has electrons added that can move more easily into the conduction band - so the donor level just means the extra electrons can move easier into the conduction band.
and the acceptor band means the electrons can move into a hole, they do not require as much energy to move into the conduction band but can instead move to adjacent holes?
is that kinda the jist of it? they have to jump lower energy due to the addition of charge carriers?

You're 100% correct, looks like you have a solid grasp!

also, i was wondering about ur research into quantum tunneling and how that effects processing chips when they start getting super small. we have a research task on transistors and electronics development and effects on society, and one of the parts is about current research and unanswered questions relating to transistors. so i was just wondering if you could give me some insight into the very very basic concepts of your research and some of the issues. i recognise that by making the devices smaller in the integrated circuit, you can fit more making them more powerful processors, and that by making them smaller you reduce energy loss and time between devices, making it faster as well as cheaper to produce as the size decreases. so basically the aim is to make them as small as possible.

You're totally right re making them smaller. However, I think the gist of your question is something along the lines of 'well what does quantum tunnelling have to do with it?'

Since it's way beyond the curriculum, but not too difficult to understand, I'll give you a brief overview. Let's look at an electron, in a classical sense.

We look at an 'infinite square well'.


Why? Well, we imagine that the electrons are 'bound' within a certain location (eg. a part of the transistor), and are allowed to bumble around in that region. A transistor only works if we can control where the electrons are (eg. are they in a p-type semiconductor? An n-type?). Basically, we need to know and be able to affect their movement!

So, in classic physics, we put our electron in an area of the transistor (let's call this our electron well). There are 'walls' stopping the electron from escaping. The 'walls' need to be stronger than the electron, so the electron can't jump over it (I'm talking about energy here, but you can also think of it as physical walls that are taller than an electron can jump). So, we can say if the walls are high enough, our electron stays in the right place, and our transistor works.

However, we make our transistor smaller and smaller. All of a sudden, we need to control EVERY electron, not just 'most' electrons. When we get too small, crazy quantum shit starts to happen.

Our electron is still in this well. However, instead of being 'trapped' by the walls, they have a certain probability of just going straight through! This is insane; it's like me saying that I'll throw a pingpong ball at a wall, and it passes straight through. Or, rather, I try to bounce the ping pong ball OVER the wall, but it's maximum height is half of the height of the wall, and it STILL goes through!!

Now, this messes with our transistor. All of a sudden, we have electrons flowing where we don't want them to flow. This is less of an 'unanswered question' as it is an 'unsolved problem'; we know what's happening, we just don't know the best way to stop it! Dammit, Quantum Physics!

[Bubbly_bluey]

Hi! How has Planck's theory of Black Body radiation added to existing theories about electromagnetic radiation?
Thanks

[jakesilove]

Hi! How has Planck's theory of Black Body radiation added to existing theories about electromagnetic radiation?
Thanks

Hey! Essentially, it comes down to the fact that energy can be emitted/absorbed as discrete packets, rather than as waves. Planck solved the ultraviolet catastrophe by proposing that light (and thus energy) could come in phonons. This completely revolutionised our understanding of electromagnetic radiation, and a few years later Einstein would come and formalise the theory (think all-or-nothing principle, etc.). Additionally, Planck introduced the formula E=hf, from which we could accurately determine the energy/frequency of electromagnetic radiation. However, his calculation of the constant, h, was pretty far off (this would be refined later).

This is a broad overview of the topic area; basically, look at how Planck solved the Black Body problem. If you have any more specific questions, or want any clarification, let me know!

[beau77bro]

You're 100% correct, looks like you have a solid grasp!

You're totally right re making them smaller. However, I think the gist of your question is something along the lines of 'well what does quantum tunnelling have to do with it?'

Since it's way beyond the curriculum, but not too difficult to understand, I'll give you a brief overview. Let's look at an electron, in a classical sense.

We look at an 'infinite square well'.
(Image removed from quote.)

Why? Well, we imagine that the electrons are 'bound' within a certain location (eg. a part of the transistor), and are allowed to bumble around in that region. A transistor only works if we can control where the electrons are (eg. are they in a p-type semiconductor? An n-type?). Basically, we need to know and be able to affect their movement!

So, in classic physics, we put our electron in an area of the transistor (let's call this our electron well). There are 'walls' stopping the electron from escaping. The 'walls' need to be stronger than the electron, so the electron can't jump over it (I'm talking about energy here, but you can also think of it as physical walls that are taller than an electron can jump). So, we can say if the walls are high enough, our electron stays in the right place, and our transistor works.

However, we make our transistor smaller and smaller. All of a sudden, we need to control EVERY electron, not just 'most' electrons. When we get too small, crazy quantum shit starts to happen.

Our electron is still in this well. However, instead of being 'trapped' by the walls, they have a certain probability of just going straight through! This is insane; it's like me saying that I'll throw a pingpong ball at a wall, and it passes straight through. Or, rather, I try to bounce the ping pong ball OVER the wall, but it's maximum height is half of the height of the wall, and it STILL goes through!!

Now, this messes with our transistor. All of a sudden, we have electrons flowing where we don't want them to flow. This is less of an 'unanswered question' as it is an 'unsolved problem'; we know what's happening, we just don't know the best way to stop it! Dammit, Quantum Physics!

oft. just oft. that is very very interesting, so the walls becoming to small increases the probabilty of electrons escaping and tunneling through - basically due to having a given energy that, when the walls are too small, they can move through? or just pre much teleport hahahaha? and this is a massive issue because the electrons won't do their jobs and can't be directed and controlled efficiently - but not all the electrons are tunnelling through right?

a very, probably silly question, but when we say energy of the wall, is that thickness or is there a required property? like the electrons are escaping because the walls get too thin? and you guys (gods) are basically working a way to decrease the probability of the electrons moving through the walls, but how do u (theoretically) decrease the probablity? basically what im tryna ask is what is the issue with the walls that allows the electrons to pass through, and how can it be fixed without widening the walls (theoretically)? because we say energy, so how do we increase the energy of the wall (sorry if ive missunderstood)

[jakesilove]

oft. just oft. that is very very interesting, so the walls becoming to small increases the probabilty of electrons escaping and tunneling through - basically due to having a given energy that, when the walls are too small, they can move through? or just pre much teleport hahahaha? and this is a massive issue because the electrons won't do their jobs and can't be directed and controlled efficiently - like not all the electrons are leaving the well right?

a very, probably silly question, but when we say energy of the wall, is that thickness or is there a required property? like the electrons are escaping because the walls get too thin? and you guys (gods) are basically working a way to decrease the probability of the electrons moving through the walls, but how do u decrease the probablity?

Yeaaaah so look you're pretty much right, obviously there's heaps of detail and maths that I'm not going to get into. Effectively, though, you're correct. When it comes to the walls themselves, there are two factors; the height of the wall (the higher the wall, the less likely Electrons are to jump through) and the thickness of the wall (the thicker the wall, the less likely Electrons are to jump through). However, smaller devices will have 'thinner' walls (although not necessarily 'less high' walls). So, tunnelling may become more likely! Which is bad, for all the reasons you described.

Yep, they pretty much teleport; they have to 'exist' inside the wall, somehow, but they're not 'physically' present there, they just sort of jump through.

Still, given how hard this is, you've got a pretty solid grasp aha

[beau77bro]

Yeaaaah so look you're pretty much right, obviously there's heaps of detail and maths that I'm not going to get into. Effectively, though, you're correct. When it comes to the walls themselves, there are two factors; the height of the wall (the higher the wall, the less likely Electrons are to jump through) and the thickness of the wall (the thicker the wall, the less likely Electrons are to jump through). However, smaller devices will have 'thinner' walls (although not necessarily 'less high' walls). So, tunnelling may become more likely! Which is bad, for all the reasons you described.

Yep, they pretty much teleport; they have to 'exist' inside the wall, somehow, but they're not 'physically' present there, they just sort of jump through.

Still, given how hard this is, you've got a pretty solid grasp aha

hahah omg that is reassuring. thankyou soo much, i can't imagine how complex this must get and i appreciate the analogies and nice ways in which you explained it, still confused (ofc) but i think i get it. so thanks very much - p.s. i added a mini weird question at the end of my last post.

[jakesilove]

hahah omg that is reassuring. thankyou soo much, i can't imagine how complex this must get and i appreciate the analogies and nice ways in which you explained it, still confused (ofc) but i think i get it. so thanks very much - p.s. i added a mini weird question at the end of my last post.

Hmm, good question. We do it in a number of ways; we can just increase the voltage (energy) between two regions, we can increase the physical distance between two regions, we can put substances/metals that are 'hard' to get through in between the regions... beyond that, it's sort of just a fundamental limit of nature! I haven't done extensive research on the 'application' of quantum theory; that's for Engineers to figure out