# Thread: Electron flow / understanding NPN transistors

1. in this walkthrough:

http://www.explainthatstuff.com/howtransistorswork.html

it says:

"the n-type has a surplus of electrons, the p-type has holes where electrons should be. Normally, the holes in the base act like a barrier, preventing any significant current flow flowing from the emitter to the collector"

I don't understand why/how electrons flow/stop-flowing in this scenario.

A - The language 'the holes.. act like a barrier'. if 'holes' are (lack of electrons/positive charge), doesn't that mean they are LOOKING FOR electrons (i.e. attracting them). How can they both 'attract' electrons but then also 'act like a barrier' / repel (those same electrons)?

B - what stops electrons from just going straight from the ntype plates to the Base as soon as the 'sandwhich' is glued together? Sort of seems like a transistor is a naturally closed circuit. i.e. The base wants electrons, and the ntype plates have them so why don't the electrons just head over? if the answer is 'they need energy to overcome (silicon's) natural resistance to electron flow', wouldn't applying energy to the Base cause both ntype plates to send their electrons straight to the Base until all three plates are balanced?

C - finally, how is there amplification in this? i.e. if I had only one 9V battery for input and the output was to, say, a lightbulb, how would those components connect with a transistor? Would I connect the battery positive to the Emitter and the battery negative to the Base and the output would come out the Collector? And then would that output be > 9V? ('amplified') or increased amps?

So I think electron-flow is the main puzzle piece I am trying to get. For example, regarding say, 100 electrons:

Battery: negative terminal sends 100 electrons to Base, Base has, say, 99 holes, so 1 extra electron wants to go somewhere, but it is surrounded by (ntype silicon) that already has too many electrons (repelling it). So how does it go anywhere?

or how does that one electron 'amplify' the electron flowing form the emitter to the collector? If the electron from the base just adds on the electron coming from the emitter, then '2' electrons would arrive at the collector, but isn't that just balanced? (no net gain of electrons?)

-al

2.

3. Originally Posted by Allan011
in this walkthrough:

http://www.explainthatstuff.com/howtransistorswork.html

it says:

"the n-type has a surplus of electrons, the p-type has holes where electrons should be. Normally, the holes in the base act like a barrier, preventing any significant current flow flowing from the emitter to the collector"

I don't understand why/how electrons flow/stop-flowing in this scenario.

A - The language 'the holes.. act like a barrier'. if 'holes' are (lack of electrons/positive charge), doesn't that mean they are LOOKING FOR electrons (i.e. attracting them). How can they both 'attract' electrons but then also 'act like a barrier' / repel (those same electrons)?

B - what stops electrons from just going straight from the ntype plates to the Base as soon as the 'sandwhich' is glued together? Sort of seems like a transistor is a naturally closed circuit. i.e. The base wants electrons, and the ntype plates have them so why don't the electrons just head over? if the answer is 'they need energy to overcome (silicon's) natural resistance to electron flow', wouldn't applying energy to the Base cause both ntype plates to send their electrons straight to the Base until all three plates are balanced?
I think this article explains what happens at the p-n junction a little better than the other one.
http://electronics.howstuffworks.com/diode1.htm
In order for the current to flow, the battery voltage has to be applied such that the n-type is given a negative polarity with respect to the p-type. That way there is a supply of electrons from the battery to the n-type which can flow across the junction into the holes of the p-type. When you connect the battery to the transistor with the positive terminal on the collector and the negative on the emitter, there are two junctions. The emitter to base junction has the right polarity for conduction but the base to collector junction doesn't. The base to collector junction is blocking the current. If we then supply some current into the base it turns on the transistor and allows the current to flow.

C - finally, how is there amplification in this? i.e. if I had only one 9V battery for input and the output was to, say, a lightbulb, how would those components connect with a transistor? Would I connect the battery positive to the Emitter and the battery negative to the Base and the output would come out the Collector? And then would that output be > 9V? ('amplified') or increased amps?
No, you will not get more than 9 volts. What is amplified is the current. You can put a very small current into the base and get lots of current, relatively, flowing from the collector to emitter. This means you can turn your light bulb on and off with a signal that has very little energy.

4. So electrons can leave their (proton partners) in an atom, but in many cases it seems the 'entire atom' is drawn toward the (opposing force). For example static electricity - a charged balloon sticks to a wall. If the wall were small/light enough and the balloon held steady, the wall would (come to) the balloon.. i.e. the wall's electrons wouldn't break off (the wall), but they would drag the whole wall towards the (positively charged) balloon.

e.g. when a postive force is presented to a negative one, what determines whether:

a)electrons break off that atom and move one by one to the positive force (holes)
or
b)they drag (their entire atom) with them towards the positive force instead of breaking off/out of their orbital shell

5. I always wonder how and why electricity is not transmitted same way sound is. Not even with the aid of the step-up and step-down transfromers. I've been thinking along this line of lately. I think I have an Idea about that. TRANSMITTING ELECTRICITY WIRELESS IS POSSIBLE!

6. hello...you///

Transistors amplify current, for example they can be used to amplify the small output current from a logic IC so that it can operate a lamp, relay or other high current device. In many circuits a resistor is used to convert the changing current to a changing voltage, so the transistor is being used to amplify voltages.
The operation of a transistor is difficult to explain and understand in terms of its internal structure. It is more helpful to use this functional model:

* The base-emitter junction behaves like a diode.
* A base current IB flows only when the voltage VBE across the base-emitter junction is 0.7V or more.
* The small base current IB controls the large collector current Ic.
* Ic = hFE × IB (unless the transistor is full on and saturated)
hFE is the current gain (strictly the DC current gain), a typical value for hFE is 100 (it has no units because it is a ratio)
* The collector-emitter resistance RCE is controlled by the base current IB:
o IB = 0 RCE = infinity transistor off
o IB small RCE reduced transistor partly on
o IB increased RCE = 0 transistor full on ('saturated')

Transistors cannot switch AC or high voltages (such as mains electricity) and they are not usually a good choice for switching large currents (> 5A). In these cases a relay will be needed, but note that a low power transistor may still be needed to switch the current for the relay's coil!

* Relays can switch AC and DC, transistors can only switch DC.
* Relays can switch high voltages, transistors cannot.
* Relays are a better choice for switching large currents (> 5A).
* Relays can switch many contacts at once.
* Relays are bulkier than transistors for switching small currents.
* Relays cannot switch rapidly, transistors can switch many times per second.
* Relays use more power due to the current flowing through their coil.
* Relays require more current than many ICs can provide, so a low power

7. Originally Posted by Malo Solomon Saleh Badejo
I always wonder how and why electricity is not transmitted same way sound is. Not even with the aid of the step-up and step-down transfromers. I've been thinking along this line of lately. I think I have an Idea about that. TRANSMITTING ELECTRICITY WIRELESS IS POSSIBLE!
of course it is possible and currently being done - radio waves work in the same way as they ionize the receiver aerial which is then amplified, all you need is more of it.

In my ignorance I don't like wireless electricity though:

less efficient to send through air than a wire,
more dangerous if you cross the transmission,
more EM pollution in the air.............
etc.............

8. We think of electricity as electrons that flow. Thinking about diodes I came up with some alternative theories for how they work. Nobody considers the role the metal to N or metal to P junction might have -- for example. For some reason, thinking seems to center on the P-N junction itself, and the fact the electrons make it across the metal to non-metal most seem to take for granted.
In my research it seemed that silver is a complement of Phosphorus and Silicon: the basis of most chips. Silver is used with tin and lead to make solder -- as you know.
As i recall my work, the N material is doped with boron I think and the P with Phosphorus -- I centered on those two. Boron is the lighter and Phosphorus the heavier.
But I am a christian so perhaps I should not continue? Suffice to say there seems to be a simpler reason for why electrons flow one way but not the other.
I went to worldbook under transistors. It seems to me both the NPN bipolar and Mosfet transistoras there work the same way. The electrons enter the emitter and into the N and into the P but at the PN junction there they have to find a way to lose speed by the radiation of excess "energy". When the base is switshed on (or the gate on the right) the electron flow there provides a way for the electrons to lose their excess energy, slow down and enter the final N material. Have you any comments?
We know that in a high resistance (denser material) electron flow must be slower -- right!? Or in a thin wire the electron flow has to be faster -- like the flow of water in a thin pip from a fat pipe -- true? So we can understand all transistors as a need to absorb or release radiation as the case may be. eg LEDs and solar collectors.

9. Originally Posted by Joshua Stone
But I am a christian so perhaps I should not continue?
What?!!!!!

10. Originally Posted by Joshua Stone
But I am a christian so perhaps I should not continue?
You should not continue, but the reason is that you are not making any sense.
Silver is a complement of phosphorus and silicon? What does that even mean?

Electrons do not need to find a way to lose speed. Their motion is mostly random, with a little bit of drift velocity in the electric field. The momentum of an electron is negligible and does not factor at all into any electrical calculations.
http://en.wikipedia.org/wiki/Drift_velocity

11. Well as I see it the electrons get stuck in the P side of the diode and bounce off the interface like ping pong balls off a wall. They do it to conserve their high velocity.
Then they must radiate to lessen their speed to make it across the boundary. The nearby presence of a absoption interface (in transistors) triggers them to radiate and drop speed. That is they have to gain mass.
But electrons have fixed mass dont they?

12. You didn't read that wikipedia article, did you? The drift speed of the electrons is about like the speed of a minute hand on a watch. Hardly anything.

Electrons have mass, yes. The mass of an electron is very small. If it dropped speed it would not gain mass. Why would it? Why would "nearby presence of a absoption interface" trigger electrons to radiate? You are just throwing stuff out as if it made sense. It doesn't. Try to have some basis for the things you say. Don't just make stuff up.

13. I thought I was making sense. The problem is drift speed is one thing but the books also say the energy of electricity is conveyed at the speed of light. Why do the electrons drift at all if the energy is delivered at light speed? Wouldn't it be more efficient if the electrons just vibrated in position and carried the energy of the passing wave through? So is nature inefficient or efficient? Or is the model inadequate?

14. What books say the energy of electricity is delivered at the speed of light? It is close, but not quite the speed of light. Nature is neither efficient nor inefficient. It just is what it is. The model is not inadequate. Your understanding of the model is.

15. OK, you know about a diode right, only flows one way due to a p-n junction, well a transistor is 2 diodes back to back,, or face to face (PNP NPN) , during saturation the "sandwich meat", middle material is forced in to its conductive state, allowing the flow.

REMEMBER: diodes can only stop voltage before they breakdown, but more importantly in your question the transistor responds to one electron, like a solar cell responds to one photon... the more the merrier

However due to the nature of your question, i'd suggest you look in to how diodes work first

Also: its not about how many electrons are in the materials, it's the type of material, p-type, n-type means the compound signature and its response to holes/electrons

ie;;; npn triggers on a positive voltage, but without the power on base, transistor is off, when hooked up we connect emitter to ground and collector to device&source, but that has a positive voltage on N typr plate, acts as a wall, when the P material is powered at the base it becomes saturated and allows the N plate charged electrons at the EMITTER(GND)(N material - electron source(GND)) to flow through the now saturated P type base and hit the N type collector with electrons allowing flow,,,ie onstate

NPN
CBE

16. The n is over the p which is more dense over less dense, and very thin so that light penetrates the n and goes into the p. Therefore it is the light flow which prevents electrons going back across the p-n boudary because they would be going against the flow of the light?
The n is over the p because the light has to slow and then speed up again since the p is less dense?
So therefore if this is true then thin silver over aluminium should work as well, if no part of the silver is in the shade? or is this idea ridiculous?
Also, since the light has to accelerate into the p after the junction, then that is all the more reason why electrons cannot cross the boundary?

17. what you said is neither right nor wrong.
its gibberish

18. Hello!
Just curious why Al is not added to PNP transistors? (It should have also 3 free elecrons!)
Thanks!

19. Originally Posted by Allan011
in this walkthrough:

How does a transistor work? A simple introduction

it says:

"the n-type has a surplus of electrons, the p-type has holes where electrons should be. Normally, the holes in the base act like a barrier, preventing any significant current flow flowing from the emitter to the collector"

I don't understand why/how electrons flow/stop-flowing in this scenario.
-al
You've been given many answers in this thread, and unfortunately most are total nonsense. Look at the reference harold pointed you to, and pretty much ignore the rest.

What follows is my attempt to clean up this mess. It is longish, but still vastly shorter than it needs to be for a comprehensive explanation. You'll need to supplement it with further study, to be sure. But here's an outline.

You'll need to understand a standard PN diode first. You have no hope of understanding an NPN transistor without understanding how a diode works.

What is meant by an N-type semiconductor is that it has a surplus of mobile electrons. It is still electrically neutral. That's important.

A P-type semiconductor has a deficit of mobile electrons. It turns out that the behavior is the same as if it had a surplus of mobile positive charges. We call them holes. Shockley (co-Nobelist for inventing the transistor) noted that it's much the same as tracking the bubbles in Coca-Cola, rather than the Coke itself. The bubbles are a deficit of Coke. It's easier to do the accounting by looking at the bubbles, not the Coke.

Ok, so you have two electrically neutral semiconductors, but one has a surplus of mobile negative charges, and the other has a surplus of mobile positive charges. Now abut them. What happens?

Diffusion happens, that's what. The mobile charges start to redistribute themselves. Electrons from the N-type side diffuse to the P-type side, and holes from the P-type side diffuse to the N-type side. If there were no other forces at work, this process would continue until you had a uniform distribution of holes and electrons everywhere. But there are other forces at work, and they are powerful: Electrostatic ones.

Each electron that diffuses over to the P-side leaves behind a positive charge. Similarly, each hole that diffuses away from the P-side leaves behind a negative charge. Note the polarities; they are arranged to oppose diffusion, so diffusion eventually stops. The equilibrium condition is one in which the diffusive flow is exactly countered by the electrostatic attraction; no net flow occurs, so there's no current. The diffusion that does take place leaves a region near the junction of the P and N regions pretty much depleted of mobile charges. That area is consequently known as the depletion region.

Now apply a voltage across this thing, say, with positive voltage applied to the P-type side. Like charges repel, so holes are pushed across to the N-side, and electrons are pushed across to the P-type side. That means current flows. It turns out that the current flow is an exponential function of the applied voltage. A 60mV change in voltage will change the current by a factor of 10 (at room temperature, and for an ideal diode).

Apply the voltage in reverse. Unlike charges attract. Mobile carriers are therefore pulled away, expanding the depletion region. No current flows across the junction. That's why a diode is a one-way valve.

To make a transistor, add another N region. To the first P-N diode, apply a voltage to make current flow. Electrons again flow from the N-type side (we'll call it the emitter) into the P-type side (we'll now call it the base). Once these electrons make it to the base, they will again diffuse across it. If the base is thin enough, they will survive the trip without recombining with all the holes there. We connect the new N-layer (we'll call it the collector) to a positive voltage, so that the surviving diffusing electrons will constitute a current through the collector.

A tiny base-emitter voltage change can command a large collector current change. That's why a transistor is potentially useful as an amplifier. A subtlety is that true power amplification occurs only if more power is delivered than is expended in the base-emitter. I'll leave you to ponder that for now, and we can follow up on that question after you've had a chance to digest the rest. [Teaser: We dope the emitter much more heavily than the base]

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