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Transistors explained ?

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3v0

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A while back a member posted a link that explained how transistors worked. It used the notion of electrons being balls that totally filled a copper conductor but were less dense in a semiconductor.

I saved it but do not know where :)

Could I have that link again.
Please
 
3v0, which link are you referring to as the best? Just curious. I've exchanged emails w/ Bill Beatty, and I've told him that his understanding of e-m fields and semiconductor physics is lacking. I recommend courses in physics and/or EE with peer-reviewed text books. Too many self-appointed gurus are propogating their views which oppose the semiconductor industry.

The following can be confirmed on the web sites of acual transistor makers.

References from semiconductor OEMs describing FET as voltage controlled and bjt as current controlled.

On Semiconductor AN-913: under the heading "Comparing and Contrasting Bipolars and Power MOSFETs", top of page 2 - "The most marked difference is that the gate of the MOSFET is voltage driven whereas the base of the bipolar is current driven."

National Semiconductor AN 558: introduction on page 1 - "The high voltage power MOSFETs that are available today are N-channel, enhancement-mode, double diffused, Metal- Oxide-Silicon, Field Effect Transistors. They perform the same function as NPN, bipolar junction transistors except the former are voltage controlled in contrast to the current controlled bi-polar devices. Today MOSFETs owe their ever-increasing popularity to their high input impedance and to the fact that being a majority carrier device, they do not suffer from minority carrier storage time effects, thermal runaway, or second breakdown."

Fairchild Semiconductor AN-7500: under "general characteristics" on page 1 - "A conventional n-p-n bipolar power transistor is a current-driven device whose three terminals (base, emitter, and collector) are connected to the silicon by alloyed metal contacts. Bipolar transistors are described as minority-carrier devices in which injected minority carriers recombine with majority carriers. A drawback of recombination is that it limits the device's operating speed. And because of its current-driven base-emitter input, a bipolar transistor presents a low-impedance load to its driving circuit. In most power circuits, this low-impedance input requires somewhat complex drive circuitry.

By contrast, a power MOSFET is a voltage-driven device whose gate terminal, Figure 1(a), is electrically isolated from its silicon body by a thin layer of silicon dioxide (SiO2). As a majority-carrier semiconductor, the MOSFET operates at much higher speed than its bipolar counterpart because there is no charge-storage mechanism. A positive voltage applied to the gate of an n-type MOSFET creates an electric field in the channel region beneath the gate; that is, the electric charge on the gate causes the p-region beneath the gate to convert to an n-type region, as shown in Figure 1(b). This conversion, called the surface-inversion phenomenon, allows current to flow between the drain and source through an n-type material. In effect, the MOSFET ceases to be an n-p-n device when in this state. The region between the drain and source can be represented as a resistor, although it does not behave linearly, as a conventional resistor would. Because of this surface-inversion phenomenon, then, the operation of a MOSFET is entirely different from that of a bipolar transistor, which always retain its n-p-n characteristic."

Fairchild Semiconductor AN-9010: under "advantages of a MOSFET" on page 7 - "1. High input impedance - voltage controlled device - easy to drive. To maintain the on-state, a base drive current which is 1/5th or 1/10th of collector current is required for the current controlled device (BJT). And also a larger reverse base drive current is needed for the high speed turn-off of the current controlled device (BJT). Due to these characteristics base drive circuit design becomes complicated and expensive. On the other hand, a voltage controlled MOSFET is a switching device which is driven by a channel at the semiconductor’s surface due to the field effect produced by the voltage applied to the gate electrode, which is isolated from the semiconductor surface. As the required gate current during switching transient as well as the on and off states is small, the drive circuit design is simple and less expensive."

These are just a few off the top of my head. You can download any of the above app notes and confirm the above. I've been an EE for 30 years, and every OEM, FAE, app note, and data sheet describes FETs as voltage controlled and bjt's as current controlled. Of course, this is a model based on viewing the device as a 3-terminal black box. This macro view point does not consider internal fields, quantum mechanics, energy bands, doping, traps, crystal bonds and defects, stored charges, etc. It is a "big picture / external" view of the device. At the micro level, both are classified as "charge controlled". At the micro level the fundamental difference between the two is that FET's are "majority carrier" devices, whereas bjt's are "minority carrier devices". The terms "majority" and "minority" refer to *charge* carriers, not currents or voltages. At the micro level, for all of my 30 years as an EE, they are both considered "charge controlled".

The most common misconception in electrical science in general is that currents are "caused" by voltages. It's not true. The light bulb that illuminates your room can be used for example. Does the voltage across the filament cause the current? Or does the current through the filament cause the voltage? The only rational answer is that neither can exist without the other. It is a circular relation. For the bulb to output heat and light, which is power, power must be inputted. The input power is the product of current and voltage. Both must be non-zero to light up the bulb. The light requires both current and voltage. Because the power source is constant voltage, people can rush to judgement that voltage comes first, then current. But the constant voltage source is that way because the power company forces it to be that way. They could provide constant current, but the losses in the lines would increase.

Enough for now. BR.

Claude
 
The most common misconception in electrical science in general is that currents are "caused" by voltages. It's not true. The light bulb that illuminates your room can be used for example. Does the voltage across the filament cause the current? Or does the current through the filament cause the voltage? The only rational answer is that neither can exist without the other. It is a circular relation. For the bulb to output heat and light, which is power, power must be inputted. The input power is the product of current and voltage. Both must be non-zero to light up the bulb. The light requires both current and voltage. Because the power source is constant voltage, people can rush to judgement that voltage comes first, then current. But the constant voltage source is that way because the power company forces it to be that way. They could provide constant current, but the losses in the lines would increase.

Sorry, but a load of rubbish!.

You can have voltage and no current, but you can't have current and no voltage - of course the voltage causes the current.
 
Hi there Claude,

I have to strongly disagree with the app note quoted in your post:

START QUOTE
Fairchild Semiconductor AN-9010: under "advantages of a MOSFET" on page 7 - "1. High input impedance - voltage controlled device - easy to drive. To maintain the on-state, a base drive current which is 1/5th or 1/10th of collector current is required for the current controlled device (BJT). And also a larger reverse base drive current is needed for the high speed turn-off of the current controlled device (BJT). Due to these characteristics base drive circuit design becomes complicated and expensive. On the other hand, a voltage controlled MOSFET is a switching device which is driven by a channel at the semiconductor’s surface due to the field effect produced by the voltage applied to the gate electrode, which is isolated from the semiconductor surface. As the required gate current during switching transient as well as the on and off states is small, the drive circuit design is simple and less expensive."
END QUOTE

The last sentence is totally erroneous. The gate can sometimes draw very significant current and most knowledgeable EE's know this and know how to
deal with it.
It sounds almost like a sales pitch where they are trying to push sales of their
MOSFET's or something :)


Also, you can have voltage with no current in an inductor and you
can have current with no voltage in a capacitor. Being an EE, you
know that there is always some inductance just because of space
itself, and that inductance between the two leads of even a carbon
resistor means that the voltage across the device appears before
the current can start to flow.
 
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EDIT: The class has sophmores, juniors, and seniors (think 14-16 year old) students with no technical background. The goal is to get them hooked.

Maybe it would help if I explained where I am comming from. I was looking at Bill Beatty's explanation of transistors because it was easy to follow. But if it is wrong...

I would like to cover transistors prior to using them to drive LEDs.
In the near term we will only be looking at transistors as switches. If anyone have a good link to text along that line I would like to see it. We can only spend a day or two on the subject max. If I have to I can drop back to teaching what will happen instead of why.

Our first term is just half over. Most of what we covered:
binary, hex, 2's compliment, base conversions, BCD, ASCII
logic gates, but not simplification
Ohm Law and Power equations
resistor series and parallel, as pullups and current limiting
LED's and how to calculate a limit resistor for one.
A brief view of how a computer works including ALU, buses, memory types & config mem
PIC port registers, PORT and TRIS​

I want to spend the next few weeks on simple programs using the Junebug. Next term I want to move on to the fun stuff.
 
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And in a ideal short circuit.
BTW,
"Nullators are strange in the sense that they simultaneously have properties of both a short (zero voltage) and an open circuit (zero current). They are neither current nor voltages sources, yet both at the same time."

Hi Willbe,

He he, that's funny. I have to also think about superconductors here.
 
Sorry, but a load of rubbish!.

You can have voltage and no current, but you can't have current and no voltage - of course the voltage causes the current.

But you can't show scientific reasons to support your position.

You can have V w/ no I only under *static* conditions, i.e. a charged cap with a steady dc voltage. This is all V no I. But a lossless inductor is the counterpart of the cap scenario. A superconducting inductor has I w/ no V. Again, these are *static* conditions. Thus I exists w/o V, just as V exists w/o I. Of course caps have physical inductance, as inductors also have physical capacitance, but at dc, these have no influence.

In actual practice, there is no such thing as static. The cap could not acquire V unless I was inputted originally. The inductor could not acquire I w/o V. There is no such thing as "true dc" since all caps and inductors had to be energized initially.

Under *dynamic* or time-changing conditions, I cannot exist w/o V, and V cannot exist w/o I. In a cap, I leads V per Eli the ice man. Any change in V takes place AFTER a change in I. Thus the ac current cannot be "caused" by the ac voltage. The effect cannot precede the cause, as it is illogical. For an inductor, a change in current takes place after the voltage change. Thus I cannot cause V.

Two quantities, I & V, cannot exist alone, can only exist together, and depending on conditions, either can change before or after the other. This relationship is clearly not one of causality. In the static condition, either one can exist w/o the other. Clearly, neither can be the cause of the other.

The notion that "V causes I" is a common misconception easily obtained since all physical power sources are designed and optimized for constant voltage operation. The wall outlet has V but negligible I until you plug something into it. But the power company could just as well generate constant current. A short across the current source keeps the voltage at zero. Placing a load across and removing the short develops a voltage across the load. Here the current is always present, then a voltage develops when loaded.

V & I are mutually inclusive. Asking or stating which is the cause or effect is a chicken and egg viscous circle. They cannot exist separately.

MrAl

Of course MOSFET gates can need substantial current. I design power electronics as well as small signal circuitry. I use MOSFETs and IGBTs to convert power up to 800 volts, and 30 amps, with 10 hp motors. I even have rolled my own FET gate drivers that deliver substantial peak current to drive through the "Miller plateau" to switch the device as rapidly as possible. I know FET gate drives. The fact that FET gates need current is not under discussion. What the app note was referring to was under low frequency conditions. A FET used to switch on and off at a very slow rate needs very small gate drive. Using FETs in PWM mode in the 100's or even 10's of kHz requires high peak currents for the gate. But the AVERAGE gate current is MUCH LESS than the peak. A bjt, OTOH, needs high average base current even at dc or low frequencies. That is a marked difference.

Bill Beatty's site refutes all OEM semiconductor teachings. Bill claims to know more than Fairchild, Texas Instruments, On Semiconductor etc. That is quite a claim! His theory is filled with false assumptions, half truths, omissions, rush to judgment etc., and in fairness, some limited measure of truth. Bill and I exchanged emails a while ago. If you or anyone is interested, I could forward that email.

If Bill is right, the semi companies would have discovered what he claims a half century ago. They have billions in funds, research labs with state of the art equipment. Do you seriously believe that these PhD semiconductor physicists ALL got it wrong for over 50 years in many countries and companies? And, that Bill "discovered" what all of them couldn't grasp?

Get serious!
 
Hi Willbe,

He he, that's funny. I have to also think about superconductors here.

Hi, Mr. Al;

Yeah, ideal components do weird stuff.

I hope you have more fun in NJ [I lived a few miles from the GW Bridge]
than I did!
:)
 
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Although I don't agree with Claude on his view of V/I relationship, most of what I read in that article by Mr. Beaty was utter nonsense. I believe this as the same article I read 2 years ago, or someone with a similar line of thought. This guy reminds me of a religious zealot who breaks from the church to start his own sect. He's way off base and I believe he thought himself out of the theory of operation (probably because it didn't make any sense to him) and came up with his own theory. WOW!!! Everything I have been taught or teach as been wrong? Is this a "new" movement?

Claude,

You may not get anyone here to disagree with you on this point, and so far I don't think there has been. But your giving them something to argue with your V/I, chicken-before the-egg scenario. Your view on this is almost as radical as the crap I just got through reading (15 minutes of my life I'll never get back).
 
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You can have voltage and no current,
Only between two charged bodies in a vacuum, if there's matter involved there'll always be a small leakage current - capacitors slowly dissipate thier charge, a small current leaks through the gate of a MOSFET.

but you can't have current and no voltage
Rubbish, what do you think superconductors are?
 
But you can't show scientific reasons to support your position.

You can have V w/ no I only under *static* conditions, i.e. a charged cap with a steady dc voltage. This is all V no I.

You've just totally contradicted your previous post - and given the 'proof' for mine.

Are you perhaps using the "tree in a forest falling down" excuse - if you're drawing no current from a battery, how do you know there's any voltage there :D
 
sigh

Is there a short but reasonable explaination suitable for teenagers?

hi 3v0,
Cant offer anything at the moment.

I would suggest you get the virtual dustpan and brush, sweep up the rubbish from most of these posts and tip it in the trash can.:)
PS dont forget to include the link document.!

Members keep giving examples, quoting relationships to capacitors, inductors and the like.
Why is it we all quote examples when asked how does it work, instead of explaining how it works.
If you notice, whatever the question, we all start using examples and analogs to explain it rather than answer the question.

Is that because we dont really know.?:rolleyes:

EDIT: hi 3v0,
This pdf is one version that looks suitable for students.

Have been trawling the net for 'transistor semiconductor theory'
 

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I was a bit amazed by the squabble.

The PDF is in a presentation format and needs some explanitory text.

I have at least 2 weeks of programming to teach prior to going back to electronics and teaching transistors. If you (or anyone) comes up with more info please post it. I am out of my element so a bit of digging and study will be required. Like they say, if you want to learn a subject teach it.

It is hard to imagine that info at the level I need is not on the web. I will look further and if needed start writting.
 
I was a bit amazed by the squabble.

The PDF is in a presentation format and needs some explanitory text.

I have at least 2 weeks of programming to teach prior to going back to electronics and teaching transistors. If you (or anyone) comes up with more info please post it. I am out of my element so a bit of digging and study will be required. Like they say, if you want to learn a subject teach it.

It is hard to imagine that info at the level I need is not on the web. I will look further and if needed start writting.

"if you want to learn a subject teach it"

How very very true. Plus one gets subjected to questions that never seem to amaze ;)

Lefty
 
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