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How does a transistor amplify current or voltage?

Discussion in 'General Electronics Chat' started by jac4b, Jul 23, 2010.

  1. Ratchit

    Ratchit Well-Known Member

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    Claude,

    Well, a FET is considered a VC device, whether current is involved or not. But suitability is not the question. We both agree on what is suitable. The question is internal control.

    Yes, I understand what you are saying. There is an external stimulus that provides energy to generate Vbe. No doubt about that. What I am saying is that Vbe is at the location in the internal transistor where is controls the Ic or Ie if you will. I think this is a philosophical argument about what defines the control mechanism. You say it is the external stimulus, and I say it is the voltage that the base-emitter junction sees.

    Yes, I understand and agree perfectly what you are saying about needing energy to control a BJT. To use another analogy to illustrate our differences, You consider the driver as controlling the steering of a car, whereas I consider the rack and pinion mechanism near the wheels as the controlling mechanism. Who is right? Philosophical question. This seems to be a good place to end this discussion.

    I like discussing problems like this with you. Your points are cogent and pertinent to the discussion. And you are explanatory instead of denigrating. If you don't bother to answer this, it has been a pleasure to communicate with you. I hope to do so in the future.

    With respect,
    Ratch
     
  2. Ratchit

    Ratchit Well-Known Member

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    tvtech,

    Yes, it is called a typo. I wonder why the spell checker did not pick it up.

    And good hearing too, I bet.

    I know folks say they sound different, but no one can tell me why. Even when the outputs are put through equalizers to mimic the output spectrum of each other, some folks say they can still tell the difference. I am going to stay out of the crossfire of that discussion.

    Ratch
     
  3. tvtech

    tvtech Well-Known Member Most Helpful Member

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    Now that is the very best decision you have made for the day.

    Peace Brother

    Cheers
     
  4. dave

    Dave New Member

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  5. Ratchit

    Ratchit Well-Known Member

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    Brownout,

    As I said before, I have not demonstrated anything about engineering, good or bad. I acknowledge that many veteran engineers do not give up their beliefs easily. No one suggested that Wiki was a engineering school. I believe that I did have my ducks in a row, and your opinion of my presentation is subjective. I have heard bad things about Wiki before, but if one evaluates what is said, I know of no faster or concise way of getting a first acquaintance with a subject. The Wiki reference I quoted about Vbe influencing the depletion region was not wrong, but the way. Wiki did not say that Vbe controlled Ic. I did. Yet you jumped on Wiki like a cat chasing a fast moving object. That is what I mean when I said previously that you evaluate the message by who delivers it instead of what it contains.

    Ratch
     
  6. BrownOut

    BrownOut Banned

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    You've demonstrated enough to me about your lack of engineering experience and knowledge. You've made many comments about engineers are this and engineers are that. It doesn’t take very long to sniff out when someone is commenting about things he knows little about. If you want to convince knowledgeable, educated and experienced people of device physics or device operation, you have to do much better. Your wiki is incorrect and misleading, and shouldn't even be considered in an intelligent discussion about devices. If you want to reference information, then use factual sources that backs up their conclusions with data, analysis and verifiable conclusions, particularly if you're going to be contradicting the information from learned, experienced people who know what they are talking about.
     
  7. Claude Abraham

    Claude Abraham Member

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    Likewise. But if this just about personal viewpoint/semantics, why so much effort to disprove semicon OEM teachings? We universally agree that current control, CC, is a most fitting external mAcroscopic model, when only terminal characteristics are needed, not internal physics. Fair enough? So the CC model espoused by every semicon OEM & most universities I'm aware of is perfectly valid for a user who wished to employ a bjt w/o regard to internal atomic phenomena. You agree, I agree, as does nearly the whole world.

    But, you & some have argued that CC is inadequate to describe the internal device physics at the atomic level. I've always concurred. If I flat out state that the CC model is limited to external drive, & is inadequate at the internal/atomic level, then what's the beef? I never offered CC as an internal behavior model, only external. Thus CC is not a "myth, misconception, marketing ploy" etc. he CC model is fairly & correctly presented as an external view not considering internal physics.

    At this point there should be no debate that a bjt when used to control other devices such as motors, lighting, audio, video, process data, etc. & only external characteristics are pertainent, internal physics not needed, then it is a CC device. Nobody chasllenges that.

    The difference of opinion seems to be how to model the internal physics of a bjt device at the atomic level. Again I discarded the CC model a long time ago when it comes to the bjt "innards". I've used the bjt as a switch many many times. Instead of active region, amplifying a small signal, I drive it between 2 states, cutoff, & saturation. I often need to know the time involved in making the state transition. The data sheets give the parameters needed. These are rise time, fall time, storage time, & delay time. Interested persons can look these up.

    Anyway, the device model used for switching is charge control, QC. Neither VC nor CC is adequate to describe time delays & speed limitations. Ditto for a FET. The equation number 2) I stated earlier as follows:

    2) Ic = alpha*Ies*exp((Vbe/Vt)-1) features no info involving time or frequency. Likewise, neither do eqns 1) & 3);

    1) Ic = beta*Ib
    3) Ic = alpha*Ie.

    When forced with speed & timing performance optimization, neither equation, voltage or current based, gives us any clue at all. With simple 2 terminal diodes, the same is true. We need to obtain the time delay data to understand how fast the device can change states. Two diodes, bjt, FET devices can have identical voltage parameters & terminal values with completely different switching speeds. In a nutshell, as soon as you approach frequencies where the internal time delays of the diode/bjt/FET become significant, the current & voltage equations become quite useless.

    But the QC model provides results which are usually quite reliable. I design a lot of switch mode power supplies, SMPS, as well as motor drive inverters, MDI. I use the QC model for the bjt, FET, IGBT (FET/bjt combo) as well as p-n junction diode. It provides great results.

    Finally if you wish to fabricate semicon devices in discrete or IC form, you must use QM models, i.e. Schroedinger's equation &the Kronig-Penney model. Those curious can look it up. Again, the VC model at this level is worthless, as is the CC model.

    So, let's summarize. When using bjt & FET devices, concerned with only external properties not internal physics, & operating at frequencies well below the inherent speed limitations due to internal delays, the CC model works great for the bjt & the VC model is ideal for the FET. Neither accurately describes internal physics, but in these cases, it is adequate.

    If, however, we are using FET/bjt devices at speeds approaching the inherent limitations due to internal time delay, only the QC, charge control, model suffices. Both the CC & VC models do not address speed/time/frequency. For SMPS & MDI, I make extensive use of QC models including diodes.

    At the atomic level needed for device fabrication, QM takes over. QM is over my head, & I freely admit it. I took QM courses in undergrad & grad school. I cannot challenge it because I don't know enough. When the prof explains something using QM, I just nod my head in agreement. I do not question the science community regarding QM.

    I present the CC model for bjt as a good external model, & I get the 3rd degree because it does not cover internal physics. I acknowledge that, but internal is not always necessary to include in the model. So the contrarians present VC model, per Ebers-Moll equation as the correct internal physics explanation. But VC is completely inadequate for internal analysis. FWIW, the authors of the Ebers-Moll eqn, Drs. Ebers & Moll, published their equations in Dec 1954. I have it, so email me for a copy.

    In their model, they use 2 current controlled current sources for a bjt. One CCCS is conventional. Ic = alph_n*Ie, where alpha_n is the normal alpha value, & another involves alpha_i*Ic. Alpha_i is the inverted current gain, where collector & emitter are swapped. In the saturated region, both currents come into play. In active region, only alpha_n (normal) is needed.

    Again, for a bjt model, CC is fine for external. For a closer view, QC works better. For atomic level, QM works best.

    Finally, Vbe is not the engine. A change in Vbe happens after Sue sings. Ie starts increasing before Vbe does. As Sue sings, charges enter the b-e jcn with energy, provided by Sue. They diffuse through the base region. When does Vbe change? After the increase in Ib/Ie. More charges are flowing. More holes exit the base towards the emitter. They arrive & encounter recombination which takes a finite time (lifetime). Likewise with e- emitted from emitter to base. An increased flow of charges results in more accumulation at the edges of the depletion region. A change in E field & Vbe takes place. Vbe increases after Ib/Ie increase after Sue increases vocal energy. Ib/Ie change ahead of Vbe, but Sue is the ultimate driver. Next in the event sequence is Ib/Ie in unison.

    The theory that Vbe is the "driver" or "engine", with Ib/Ie just being consequential holds water like a net. On a scale of 10 that theory is a 0. Great chat, feel free to question/comment/criticize, whatever. BR.
     
  8. Ratchit

    Ratchit Well-Known Member

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    Brownout,

    I have demonstrated nothing about engineering to you, good or bad. You seem to have a hard time understanding that.

    And basically I said they complemented the theorists, which is true. I never said anything bad about engineers or engineering.

    I just put out a viewpoint. Everyone evaluates and believes what they want.

    In this case, the part of Wiki I referenced was correct. You can argue as to whether my conclusions were correct, but the sentence or two that I referenced was correct.

    It is well known that the voltage across a diode changes the depletion layer. No need to verify that again.

    Ratch
     
  9. tvtech

    tvtech Well-Known Member Most Helpful Member

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    It all started like this, an innocent question from a newcomer to this site:

    How does a transistor amplify current or voltage?

    Isn't that like creating something from nothing?


    And it turned out into a 10 page Physic's Analysis......

    Common guys....really.
     
  10. BrownOut

    BrownOut Banned

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    I understand perfectly that you've demonstrated a lack of knowledge and experience in engineering. The discussion was not if voltage across a diode changes the depletion layer ( actually the width, you never get that part right ) It was if voltage controls collector current. I could give a hoot less what wiki thinks about depletion layers and effects of voltage, because that is irrelevant. And because you drew incorrect conclusions, the article is at best misleading, just as I said in the first place.

    This is not my viewpoint. You're not going to pull out crap and make cookies with knowledgeable and experienced people. You're going to need to be better than wiki or he said/he said baloney. They will see right through an attempt to tap dance.

    You've convinced nobody with your babble. This is not my viewpoint, but I'm sure of it. If you had, someone would have said so by now.
     
    Last edited: Jul 27, 2010
  11. Ratchit

    Ratchit Well-Known Member

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    Brownout,

    So you think.

    And don't you think that changing the boundary layer would control the Ic?

    The depletion layer is important in transistor physics.

    If I drew the wrong conclusions, which I don't think I did, that is not Wiki's fault.

    I am sure you subscribe to that viewpoint. I am not a baker.

    I don't think anyone or any publication will change your mind.

    Who knows?

    I am sure you subscribe to that viewpoint.

    Not everyone wants to get involved is discussions like this one.

    Ratch
     
  12. BrownOut

    BrownOut Banned

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    You can't even make up your mind what you want to talk about. Boundary layer? Depletion layer? Know the difference?

    It isn't about changing my mind, it's about learning from those who know. You know very little, so it's best to listen to those who do. When speaking of such things to knowledgeable people, you need to have material or knowledge that shows them something. Just saying "because someone says so" doesn’t cut the mustard. You might have changed my mind if you had anything worthy, but you don't.
     
  13. Ratchit

    Ratchit Well-Known Member

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    Claude,

    I am not trying to disprove the OEM CC model. For their purposes, it works as a model. The OEM's are mute about a non-model explanation of what the internal mechanism is that controls the Ic.

    There is no beef with the CC model used to describe the output of the BJT. I said so many times already. If fact, I agree that as a external model, CC must be used. I also agree that it is "inadquate" to describe internal BJT physics.

    That's correct.

    I have no disagreement with the above as long as you are talking about models. As I said before, models tell you what a device will do, not how and why it works. That is why models are so useful for circuit analysis programs like Spice, where results are desired, not explanations. So that is why I reject models to explain how and why something works. I am sorry I did not make that point more emphatically before. CV in a BJT should not be used to determine what it is going to do, but instead why and how it does it.

    I did not say Vbe is the engine. I believe I said that Vbe is at the location in the internal transistor where is controls the Ic or Ie if you will. I would call the external stimulus or Sue the engine. I gave a analogy in my previous message where either the driver or the rack and pinion controlled the steering, and it was a philosophical choice as to which is which.

    No, I do not remember saying Vbe is the "engine". I believe I said Vbe was at the spot where control occurs.

    Ratch
     
  14. Ratchit

    Ratchit Well-Known Member

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    Brownout,

    Yes, for our discussion they can be considered the same. It doesn't change the argument.

    So, go forth and learn.

    How do you know how much I know?

    Yes, I know that.

    It depends on who says it.

    I don't think I could.

    Ratch
     
  15. Claude Abraham

    Claude Abraham Member

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    Once again, Ratchit, we agree that CC does not describe internal/atomic phenomena in a bjt. But you still insist that CV describes how & why internally. Every OEM & semicon physics reference uses QC, then for a really close look, QM. VC is never used at the atomic level. The late Dr. William Shockley stated in the early bjt days, that only QM can fully explain bjt inner action. You earlier asked what Dr. S would say about bjt innards, & if you search, you can find his early papers, where he employs QM. I don't have any of those papers.

    So you acknowledge that singing Sue imparts the needed energy. But you state that once the energy arrives at the bjt device, then Vbe "determines" Ic. But how can that be? In a SMPS, input power is taken from the mains, then stored in an inductor when the power switch is on. Then it turns off, so the inductor transfers its energy to the output cap & load. Although the inductor needs constant replenishment from the mains supply, it is an intermediate storage device.

    But Vbe is not an intermediate variable. When Sue sings, her energy is imparted to the mic element & converted to electrical energy, I & V. The current in the mic cable enters the bjt in the mic preamp. The electrons have energy already. When they enter the base & emitter terminals, Vbe is still unchanged. Thus Ib/Ie have increased before Vbe. When the number of electrons entering the emitter lead increase, then the number collected at the collector also increases as a result. Hence Ic increases due to an increase in Ie. Ib increased as well, but Ib does not contribute to Ic.

    In the meantime, these extra electrons entering the emitter, as well as the extra holes injected from base to emitter, cross the depletion zone & recombine. The statistical average time for this to occur is called "mean carrier lifetime". A result of this charge increase is change in the depletion width, an increase in Vbe, & an increase in the stored excess minority charge carries. Thus the change in depletion width, change in Vbe, are merely responses consequential to Sue cranking up her volume.

    Ie & eventually Ic have already begun to increase before the depletion width has even had time to change. But let me make this clear. Just as Ib/Ie/Ic are not consequences of Vbe, it is equally true that neither Ib nor Ie is the sole driver of bjt action. Like I stated earlier, the E field due to the reverse biased c-b jcn with its associated voltage Vcb, & displacement current, is what "collects" the electrons emitted from the emitter.

    So a bjt relies on Vcb, Icdisp, Ib, Vbe, & Ie, as well as an external power source as the stimulus. The people telling you that Vbe controls depletion width, which then controls Ie/Ic, w/ Ib as a residue, cannot explain in solid state physics terms the sequential order of events. If you do not believe me, just set up a bjt, or even a FET. Use a rather high freq, say 1 to 10 MHz. Carefully place voltage & current probes at Ib, Ie, Vbe, & Ic. Measure the timing. You will be surprised as the bjt does not behave as the contrarians describe.

    This constant lecture about "CC is only valid externally, not internally" is preaching to the choir. Everybody knows the CC model is only external, & not internal. Your beef is between VC vs. QC & QM. I have studied this for 3 decades at the BS, MS & now Ph.D. level. I've developed electronic hardware in industry for 32 yrs. I've designed hundreds of bjt & FET networks, operating them as a linear amplifier, as well as a cutoff/saturated switch. The OEM data is amazingly reliable.

    For 32 yrs., they've been telling me that the bjt is CC device at low speeds, & a QC device when used as a high-speed switch. Their CC & QC equations have been "dead-right-spot-on" for as long as I remember. My 1965 reference text "Pulse, Digital,& Switching" covers CC & QC very thoroughly. Nowhere in any of my uni-approved peer-reviewed reference texts in EE or semicon phy will we find that Vbe controls depletion width which controls Ic, with Ib a residue. I've never seen it.

    The contrarians simply cannot accept anything being controlled by a current. They firmly insist that every current has a voltage driving it, making voltage the ultimate controlling quantity. Attempts to illustrate the reason for CC modeling are met with hostility & dogma. Every contrarian has firmly stated that Vbe is dog wagging the tail, but conservation of energy, charge, & close examination of sequence of events refutes their position.

    If I'm a fool for accepting the OEM/university/engineering profession's teachings, so be it. That is the type of fool I prefer to be. There is, however, an even bigger type of fool. One who trusts his own judgement, intuition, ability to reason, to the point of rejecting the whole science community because his intuition cannot be wrong. Believe me, that is the biggest type of fool one can be. Such a person is both the leader & the follower of a 1 person cult. The notion that current is not driven by voltage, that neither is the dog nor the tail, that they are mutually inclusive, & are both driven by the energy conversion process in the power source, is a tough pill for some to swallow. They have to define everything in terms of voltage. They acknowledge the role of current, but only as secondary, & consequential, with voltage as the main driver.

    I've argued this issue since I was an undergrad in the mid-70's. A small minority always says I'm wrong. But every OEM still publishes the same result since the bjt was introduced.
     
    Last edited: Jul 27, 2010
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  16. BrownOut

    BrownOut Banned

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    No they can't be considered the same. There is a depletion layer or space-charge layer. There is no boundary layer. You have to care enough to at least get the vocabulary correct. I know how much you know by reading your misinformation. Better educated members have tried in vain to educate you with factual, correct information. By contrast, you've written meaningless tripe ( like boundary layer or waste current ) If you had correct information, it doesn't make any sense that you wouldn't want to share it. As others have already pointed out, your information has zero usefulness.

    I was actually prepared to discuss the physics of CC and VC, but when I read you repeating the same wrong statements each time someone wrote anything useful, I though it just wouldn't be worth the effort. Why should I take the time to get it right, just so you can summarily and categorically contradict, never with any useful alternative data or information, but with cheesy, uniformed lazy comments. I wish the mods watched for trolling more closely and cleaned house of those who troll. Had I not been involved in this thread, I would have reported you for trolling. Next time, I will. Not out of retribution, but because this site is better without trolls.

    What I would have discussed is pretty much what Claude A. has already said. Every knowledgeable person has this same position. Although, Mr Claude A., I do have a couple issues and would have loved to discuss them with you, had the thread been more intelligent and less trollish. Maybe in the future, we might have an intelligent, knowledge based discussion on this topic. I look forward to that day.

    Basically, the argument of every non-knowledgeable person I've read involves VBE being in the terminal or internal equations. To the uninformed, uneducated or just intellectually lazy, this looks like voltage control. Those who know better know that VBE isn't required as a controlling variable and can be a result of current. And for many, very good, well thought out reasons already given, BJT's, though can be controlled by voltage or current, are considered current controlled devices. Nothing in this discussion has reasonably confirmed otherwise.
     
    Last edited: Jul 27, 2010
  17. Claude Abraham

    Claude Abraham Member

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    One more thought. All 3 eqns are useful & needed. When using a bjt as a linear amplifier, we have 3 optional topologies, common emitter, common base, & common collector aka emitter follower. With EF we have voltage gain just under unity, current gain of beta+1. With CB we voltage gain much greater than unity, or Av = Rc/Re, where Rc/Re are the collwctor/emitter resistors resp., & with CE we get current gain of beta, w/ voltage gain of -Rc/Re.

    The reason that all 3 are important is that no amp stage can ever have a gain larger than the raw bjt device, current or voltage. Eqn 1:

    Ic = beta*Ib, conveys the effectiveness of the bjt device at amplifying current. The bias resistors on the bjt input side draw some current, so that the current gain of the stage, Ai, is always less than that of the device, which is beta.

    The 2nd eqn, 2) Ic = alpha*Ies*exp((Vbe/Vt)-1), conveys the effectiveness of the bjt device at amplifying voltage. THe transconductance of the overall amp stage can never exceed that of the raw bjt device. Some voltage is dropped across elements in the biasing network, including emitter degenerating resistance, as well as bjt internal resistances. If we examine the ic vs. vbe ratio by differentiating eqn 2), we get a small signal transconductance parameter "gm" as gm = Ic/Vt, so that ic = gm*vbe, where the lower case "i & v" indicate ac small signal quantities.

    In the CB mode, the current gain is limited to alpha, per eqn 3) Ic = alpha*Ie.

    All 3 eqns provide useful info as to what a bjt can offer in terms of signal gain, both current & voltage. Neither is the dog or the tail. Both are needed. What makes an active device useful is its ability to amplify current & voltage BOTH with gain values >> 1. To output a specific value of current & voltage requires minimum input values of current & voltage. How much is needed? The 3 eqns provide the answer.

    None of the 3 eqns involve "causality", consequential, etc. They convey useful info to the user detailing device limitations. The QC equations detail the speed limitations of the device.

    I have millions of products whose hardware I designed out in the field worldwide. My products don't come back for repair. They work first time & indefinitely after that. Every engr colleague I ever worked with views bjt as CC, FET as VC. I worked for an aerospace/defense firm in Baltimore with thousands of engrs, tech, Ph.D. etc. I never once heard of bjt being VC. I've never met anyone who actually develops hardware saying otherwise. These contrarians love to stir up trouble, provide no useful info, & develop nothing in the real world that is of benefit to anyone. I swear they get a rush just putting down actual practitioners. In industry, I've met people who were hostile to me on my 1st day on the job for no good reason. Being an EE seems to annoy a handful of people for reasons I wish I knew.
     
  18. Sceadwian

    Sceadwian Banned

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    If you take a simple semi conductor junction and take an instantaneous moment of it's existence there can be no current flow. Current flow requires a period of time to have elapsed to measure. However in that instantaneous moment of time voltage tensors are easily calculated. In the macro world the current flow is an easy way to model it mathematically; that doesn't means it's the current that make it work. The voltage field at ANY instantaneous moment of time is what determines the state of a semi conductor material.

    Is that any clearer than other posts here? If anyone can prove to me that a current is able to flow during an instantaneous moment of time then I'm wrong.

    This does not mean that charge does not have to flow to change the state of a semi conductor material, but the actual operating force on the atomic level is the voltage field. This can even be easily understood looking at the terms charge and current. Charge is just that, a certain electron density, current is the flow, the movement of these charges, in an instantaneous moment charge can not move, it can ONLY have potentials, IE voltage fields. If current (the flow of charge) is required for a semi conductor to function then on a quantum level it would fail because for any given instantaneous moment there can be no current flow to support the possibility of the conduction state of that material for that instant.
     
    Last edited: Jul 27, 2010
  19. BrownOut

    BrownOut Banned

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    You're wrong is anyone can prove current flows in instantaneous time or not. There is no requirement that control be evaluated in zero time. The theory works for continuous time, as I've already told you. Show me any text that requires zero time to evaluate physical control. There isn't any.

    Friends, please read Claude's comments. Those who dogmatically insist it must be voltage control will resort to any contortions to promote their misunderstandings. Instantaneous time? Time to get real.
     
    Last edited: Jul 27, 2010
  20. Sceadwian

    Sceadwian Banned

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    If there is no control in an instantaneous moment of time then there is no control. The conglomerate effect of the voltage fields over time can be modeled with the modern current theories, which are fine and dandy, but do not explain the quantum level reason for the semi conductor effect to exist, it is voltage field dependent ONLY. This doesn't mean BJT's or real world semi conductor devices don't require currents to function, it however does show that the voltage field itself is what causes the effect to occur. If you can't agree with this you're throwing all of known quantum theory out the window as unimportant.
     
    Last edited: Jul 27, 2010
  21. BrownOut

    BrownOut Banned

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    What? There is indeed control over a continuous time and at any instantaneous time. No semi phy theory requires current to be flowing instantaneous time. In fact, current is always flowing, in continuous time, instantaneous time, any time. What you're proposing that charge can't move in zero time, and that's a trivial, meaningless idea. Current is always flowing, and the theory explains semiconductors on any level you want. The theory has already been explained on here by those who know what they are talking about. It is correct, and doesn't throw anything out of the window. If you think otherwise, then explain exactly what principles of quantum physics that is being violated, and exactly how semi phy theory violates it. Just saying it’s all thrown out of the window is to do nothing more than to perpetuate the meaningless blather from the VC cult.

    The theory works in the real world. That's all that matters.
     
    Last edited: Jul 27, 2010

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