为什么bjt是电流控制型器件
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Why is FET voltage controlled and BJT current controlled?
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来源:互联网 发布:知盈医学课堂是真的吗? 编辑:程序博客网 时间:2024/05/16 00:34
http://www.physicsforums.com/showthread.php?t=335368
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Bipolar transistors are so named because they conduct by using both majority and minority carriers.
The field-effect transistor (FET), sometimes called a unipolar transistor, uses either electrons (in N-channel FET) or holes (in P-channel FET) for conduction.
So it is just our preference as to which model we choose?
A bit awkward question but still it confuses me......
A BJT is a current controlled device because its output characteristics are determined by the input current.
A FET is voltage controlled device because its output characteristics are determined by the Field which depends on Voltage applied.
Now the question is that current is also generated due to Voltage and still BJT is current controlled and FET voltage controlled.
Any Ideas?
The b-e and c-b jcns have differing doping densities, so the "Is" in a diode, scaling current, is denoted by "Ies" for the b-e jcn, and "Ics" for the c-b jcn. Hence,
Ic = alpha_n*Ies*(exp(Vbe/Vt) - 1) - Ics*(exp(Vbc/Vt) - 1).
Ie = Ies*(exp(Vbe/Vt) - 1) - alpha_i*Ics*(exp(Vbc/Vt) - 1).
Someting I don't understand is, since Ies is such a small number,how does the Ebers Moll model accurately describe the emitter current? I mean,it's possible to have transistors with many milliamperes or amps flowing through them, and I don't see how you get those numbers when you're multiplying everything by 10^-16 or some such.
Edit: Also, if you have a chance - how would the above equations be different when the transistor is not in the active region, but in saturation with both the base to emitter and base to collector diode forward biased?
So it is just our preference as to which model we choose?
The reason a bjt is called a current controlled device goes back to the Ebers-Moll 1954 paper.
As a device physicist, most naturally I have strong doubts on what you are saying here.
Could you please point out that paper you are mentioning showing the point where "current-controlled device" is defined for the first time?
This is most probably incorrect, because the reason BJT is called a current controlled device has nothing to do with a "back-to-back" diode model created for circuit applications at a time where COMPUTERs were not as powerful as they are today to do the EXACT simulations.
Nowadays, commercial tools like MEDICI or TAURUS know NOTHING about Ebers-Moll, because it is much stronger (and accurate) to solve the problem from a charge-control point of view... Where drift-difussion equations are solved self-consistently with poisson equation.
So Ebers-Moll is NOT AT ALL fundamental and it was a mere convenience at the time.
The real reason why BJT is called a current controlled device is related to the DEVICE PHYSICS as I explained in my previous posts.
There's so much techno-babble in this thread, and none of it really addresses the OP's quesiton.
Simply put, a FET is a "voltage-controlled" current source, because the output current is modulated by a the GATE VOLTAGE which changes the electro-static FIELDs inside the channel, hence the name FIELD-EFFECT transistor.
On the other hand, a BJT is a "current-controlled" source, because the output current is modulated by the amount ofBASE CURRENT that is injected from the base terminal. The characteristic "amplification" of current in a BJT is directly related to the amount of electrons that enter from the base...
This is essentially what he needs to know. What does it even have to do with the Ebers-Moll model?? It's just a way of modelling the device from a circuit perspective...
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Techno-babble? That was rude! He asked why the bjt & FET are classified as CCCS & VCCS resp. So, I gave him an historical account of the math and modeling of these devices. I did go deep into details, but one cannot explain device physics in 1 paragraph. My treatise exactly addresses the OP question, how can you suggest otherwise?
It has everything to do with E-M! E-M was the 1st model attempt at the bjt, and still stands today. It has been modified and improved per Gummel-Poon. Also, for large signal operation and/or switching mode, the charge control model is extensively used. Also. the collector current is modulated by the emitter current, NOT BASE current. Base current is necessary for bjt operation, as is base-emitter voltage, but Ic is controlled by Ie. Every semi maker affirms the same. "Current-controlled" refers to emitter, not base current. You don't know semi phy.
I was just giving him perspective. I studied EE at the BE & MS level in the 70's. In 2007 I returned to grad school for the Ph.D. I have this semester and next to complete my course work. I've been a practicing EE for 32 yrs. in R & D. I have developed many many pieces of equipment using discrete bjt, diodes, FETs, SCR, triac, IGBT, LED, photodiode, etc. I took semiconductor physics from the physics dept. in the 70's as a senior. As a grad student (MSEE) I took semi phy from the EE dept in '79. As a Ph.D. student, I took semi phy 2 in 2008, fabrication in 2008, and advanced sensors in 2007.
I was mentored in the 80's by a boss who became an EE just as the bjt hit the market in the 50's. In his drawer of sample parts, he had germanium diodes & bjt parts from the 50's. The part nos. were 1N3, 1N4, 2N3, etc.! He knew transistor physics quite well and I learned much from him.
If you don't see how my historical treatise based on years of post-graduate studies and mentoring from sages who have been there done that is relevant, and comes across as "techno babble", then maybe you should examine your own background. Maybe you are the one lacking in knowledge. What credentials have you got? Where do you get off rudely rebuking me? How many semi phy courses have you taken and passed? Do you have work experience in the semi industry.
The problem is that electronics is a field where everybody sees themself as an expert. Anybody that's heard of Ohm's law thinks they are the equal to a professor. What folly.
Claude
Also. the collector current is modulated by theemitter current, NOT BASE current. Base current is necessary for bjt operation, as is base-emitter voltage, but Ic is controlled by Ie. Every semi maker affirms the same. "Current-controlled" refers to emitter, not base current. You don't know semi phy.
Current-controlled" refers to emitter, not base current. You don't know semi phy.
I'll get the E-M paper at the university where I study. I'm sure they have it. Meanwhile here is a Univ of Berkely lecture note on E-M. They employ a CCCS, with Ic = alpha*Ie.
As far as Ib being the "controlling current" vs. Ie, Ie is usually employed.
With a classic bias network, a resistive divider whose midpoint sets the base terminal voltage wrt ground, an emitter resistor, and a collector resistor, the emitter current is controlled via the resistors. The Vbe value is very close to 0.65V. As temp varies and hfe varies, the Vbe remains near 0.65V. The Ie value stays constant over the span of temp and device variations. When Ie is established at some quiescent value, then
Ic = alpha*Ie.
Alpha is 0.99 +/- 0.01 for all devices at all temps.
If however, we set Ib at some value, then
Ic = beta*Ib. This is not a good thing as beta is device dependent, temp dependent, and current level dependent. A circuit which employs the base current to control Ic is called "beta dependent". Except for switching applications, controlling Ib is usually not done.
Amplifiers, including monolithic op amps do, however, employ base current control of Ie in the 2nd stage. The 1st stage is emitter coupled differential stage. The emitters of the diff pair have their emitter current controlled via a current source/sink, or large resistor. The 2nd stage is a common emitter stage. The 1st stage outputs a current source which feeds the base terminal of the 2nd stage bjt. But, the open loop gain of the op amp depends on beta. The higher the beta, the higher the open loop gain. Global feedback corrects this problem.
Thus, for the 2nd stage, Ic = beta*Ib. Here, the collector current is indeed controlled by the base current, not emitter current.
We can control Ic with either Ib or Ie. Ie is usually the preferred way of doing it, but Ib is often used. It depends on the application.
From a device physics view, however, Ie determines Ic as Ie supplies the carriers, not Ib. But Ib is absolutely necessary or the device is non-functional. Likewise, Vbe is necessary but is not the controlling quantity. ithout Ib and Vbe, there is NO Ie at all.
FWIW, I agree that beta is indeed a very good figure of merit for a bjt. It describes how effective it is at amplifying current. Beta is very very important, especially the worst case minimum value. A good design will function consistenly well for any beta value above the wcm (worst case minimum). But the overall performance of the network is improved when using a device with a high wcm beta value. No argument there. Beta is all important especially when there is only 1 stage of amplification. Beta is all important here. With several stages, the beta values multiply, and a high bWeta for each stage may not be needed. The fewer the stages, the more important is beta.
Make sense?
Claude
Also, when used as a switch, the base current is usually controlled in a manner that forces a base current greater than Ic/beta_minimum. The collector saturates, and beta dependency is not an issue.
Is it this paper?
Large-Signal Behavior of Junction Transistors
Ebers, J.J. Moll, J.L.
Bell Telephone Labs. Inc., Murray Hill, N.J
Proceedings of the IRE, 1954
If it is, you don't have to send it to me,
just point out the relevant portion in the paper.
If it's not, post the name of the paper so others can see it as well.
I have access to the journals, so it's no problem