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Post by Progenitor A on Feb 21, 2011 14:49:13 GMT 1
Using only QM concepts, explain how an NPN transistor gives cutrrent and/or voltage amplification.
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Post by Progenitor A on Mar 1, 2011 18:40:40 GMT 1
This question is extremely difficult and way beyond the physics capability of anyone on this board (including me!)
Still, never mind, Shockley was a very clever man equally famous for his racial characteristic comparisons which are, of course, rejected out of hand by thoise that know!
Well, never mind, he changed the world in ways that the PC will never achieve
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Post by abacus9900 on Mar 1, 2011 19:53:11 GMT 1
This question is extremely difficult and way beyond the physics capability of anyone on this board (including me!) Still, never mind, Shockley was a very clever man equally famous for his racial characteristic comparisons which are, of course, rejected out of hand by thoise that know! Well, never mind, he changed the world in ways that the PC will never achieve naymissus, this is a pictorial representation of a NPN transistor with forward bias at the emitter-base junction and reverse bias at the base-collector junction.  en.wikipedia.org/wiki/Bipolar_junction_transistor#IntroductionTransistors are composed of three parts, a base (B), a collector (C), and an emitter (E). The base is the 'controller' for the main current that runs through the transisitor, the collector is the region where the larger electrical supply flows through and the emitter is the outlet for that supply. The 'N' regions are 'doped' so that they consist of an excess of electrons while the 'P' region is doped to produce an excess of 'holes' (a hole is a 'vacancy' in the atomic crystal that will accommodate an electron thus encouraging electron flow). By 'biasing' the three regions with a battery the transistor may be made to produce electrical flow from a combination of electron/hole exchanges. The trick is, by sending a varying amount of current from the base region the amount of electrical current through the 'gate' (the central region) from the collector may be varied. In this manner a small amount of current may be used to control a large amount of current so that application can be achieved. So, where you have a very weak sinusoidal signal, as in a radio frequency, you can 'replicate' it and make it stronger by applying it to the base input. What happens is the 'signal' current from a weak source replicates and amplifies itself via the main current already existing in the main body of the transistor. By using a number of transistor stages it is possible to amplify very weak radio signals to drive loudspeakers. With a PNP transistor everything is the other way round. |
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Post by Progenitor A on Mar 2, 2011 8:32:26 GMT 1
Good diagram there Abacus
I am familiar with the normal description of how transistors work. What I am seeking here is an explanation of how they work in QM terms
I had an embarrassment once in my engineering lecturing career when I taught semiconductors and their applications. I was happy with the description of operation as outlined by yourself and was quite succesful in teaching it
Then at microwave frequencies tunnel diodes started to be used because of their negative resistance at extremely high frequencies.
The description of how that negative resitance is used is straightforward, so one day I attempted to explain the operation of the tunnel diode in terms of the 'standard' transistor model.
I found myself in the embarrassing position of contradicting myself and I realised that the semiconductor model I had used for some years was inadequate and I did not understand how it worked!
That embrrassment was somewhat lessened when I found that my colleagues also did not have a clue how the tunel diode works!
In fact the only way (I think) of describing its opertion is using QM, but as those explanations (as we have seen) are unsatisfactory there is no completely satisfactory way of decribing how it works!
So I fell back on the 'black box' approach which describes the operation from its external effects (the same model of using transistot 'h' and 'T' parameters)
My first introduction to QM!
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Post by abacus9900 on Mar 2, 2011 10:02:16 GMT 1
Well, I do know that in the tunnel diode it is the 'uncertainty principle' that applies. Electrons are faced with an 'energy barrier' to overcome in the depletion layer, as you know. Now, in QM it is a standard part of the theory that you cannot always predict where a quantum object is going to be (in this case electrons) so that in a tunnel diode things are designed to allow for this effect thus permitting a proportion of electrons to 'bypass', if you like, the barrier and migrate to the other side. Now, you are going to ask me how this happens, well, as we have said many times in the past nobody really understands QM, however, we know it happens and therefore can utilise it in practical applications like electronics. There is a point where you simply have to accept things happen that you can't really explain but can, nevertheless, use to good effect. I think the trouble is if you visualize electrons as tiny little balls of charge it makes it hard to imagine how they do what they do but actually it's more helpful to look at electrons and other sub-atomic particles as more like 'bits' of energy so that they are not 'solid' as such but dispersed and therefore more flexible.
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Post by Progenitor A on Mar 2, 2011 15:24:02 GMT 1
Well, I do know that in the tunnel diode it is the 'uncertainty principle' that applies. Electrons are faced with an 'energy barrier' to overcome in the depletion layer, as you know. Now, in QM it is a standard part of the theory that you cannot always predict where a quantum object is going to be (in this case electrons) so that in a tunnel diode things are designed to allow for this effect thus permitting a proportion of electrons to 'bypass', if you like, the barrier and migrate to the other side. Now, you are going to ask me how this happens, well, as we have said many times in the past nobody really understands QM, however, we know it happens and therefore can utilise it in practical applications like electronics. There is a point where you simply have to accept things happen that you can't really explain but can, nevertheless, use to good effect. I think the trouble is if you visualize electrons as tiny little balls of charge it makes it hard to imagine how they do what they do but actually it's more helpful to look at electrons and other sub-atomic particles as more like 'bits' of energy so that they are not 'solid' as such but dispersed and therefore more flexible. Yes, I am a great one for 'just accepting things happen' and leaving it to the clever people to really understand. Mind you I have excellent BS antennae that can detect BS at 300 yards! I doubt ther is anyone on this board that can explain how QM describes the current/voltage amplification of a transistor. Indeed I have never ever seen such an explanation! I suppose Schockley knew |
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Post by abacus9900 on Mar 2, 2011 15:43:50 GMT 1
Well, (and I am not trying to BS you, honestly) just because we are able to measure something doesn't necessarily mean we really understand it. I keep going back to what Richard Feynman and indeed others have said about nobody really understanding what QM is. STA keeps falling into this trap; she thinks that because we can precisely measure QM effects we understand it, well, we don't. This is why there are several main interpretations of QM. We still have a lot to learn.
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