Understanding NPN Transistors / electron flow

[Allan01]

in this walkthrough:

 

http://www.explainth...istorswork.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

[darkenlighten]

I was going to try and break this down (I'm kind of tired), but I'll start here. If you have learned anything about quantum mechanics it is very useful when studying semiconductors and how they work. Essentially its due the materials potential. Imagine your three materials lined up, NPN, they each create a potential well corresponding to the material and its length. So these wells placed next to each with also create "walls" if one is higher or lower than the other, this causes the blocking.

 

Another thing to remember is that these are semiconductors, not metals. In a semiconductor you don't have a "sea" of electrons, that is why it is a semiconductor, sometimes it acts as an insulator and sometimes as a conductor, but criteria has be met for this to be done, such as applying a voltage, or current flow.

 

Hopefully this is a start to your understanding.

[Xittenn]

Amplification occurs because a flow of electrons in or out of the base pending your configuration PNP or NPN causes a drop in the counter acting potential across the base-collector thereby lowering the barrier which electrons are required to tunnel through from emitter to collector(configuration) thereby allowing a higher number of electrons to tunnel..... this effect is not one to one, generally speaking!

 

Reverse biasing causes the natural state to become amplified so excess becomes excess and no flow occurs.... Just off the top of my head. :/

Edited August 5, 2010 by buttacup

[cypress]

in this walkthrough:

 

http://www.explainth...istorswork.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"

 

The holes are lack of electrons in the silicon due to the doped material capturing some of them from the silicon. The current will ultimately flow through the silicon since there is not a contiguous path from one doped atom to the next. So with p-type we have silicon that is ready to receive electrons and the silicon has a positive charge even though the material as a whole is neutral.

 

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)?

 

Yes they are looking for electrons and once they get them there will be no additional propensity for current flow unless there is a positive external potential pulling the now excess electrons out of the silicon to open up new holes for more electrons. So a current will flow if you hook the p-type material to an external positive potential and no flow if you hook it to an external negative potential (since the negative potential will flood the silicon with electrons and fill the holes). Remember it is only because there are holes that large currents will flow. Once the holes are filled there will be no more current flow. Think about what you would need to do to keep current flowing ----> make holes. To make holes ----> pull out more 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?

 

The doped material gives up electrons to the silicon so the silicon has an excess of electrons but the n-type plate overall has a neutral charge so just gluing the thing together does nothing. It is only when you hook up external potential that the magic starts. Hopefully you see that they do need energy but it is not because of the silicon's natural resistance to flow.... the doping material eliminates/reduces the resistance by creating electron bridges and holes that allow for unimpeded flow as long as you keep replacing them as they are "used" ..... it is because the external potentials (energies) are needed to replenish the positive holes (electron sink) and negative bridges (electron source). It takes work to keep the fire stoked.

 

If you applied a positive charge to the base and negative charges to both the collector and the emitter would cause both to send electrons to the base and it would be a double diode, but if you apply a negative charge to the emitter and a positive charge to the collector then you would get a current amplifier.

 

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?

 

Keep in mind that the amplification is a current amplifier not a voltage amplifier. and the amplification is not relative to the battery current, it is collector current relative to the base current. The current (energy) has to come from somewhere, there is no free lunch, and this somewhere is the battery. It is easy to take a transistor and simulate a voltage amplifier, just add resistors to the base and collector and take voltage taps off the resistors.

 

So it is increased amps. You connect the negative to the emitter and positive to the base and collector. You feed electrons to the emitter to replenish the electron bridges that collapse into the bases holes and pull electrons from the base to replenish the holes that get filled by the collapsed bridges and the rest of the electron bridges "fling" electrons over to the collector because the potentials created cause most of the electrons to jump the bridges and overshoot the holes. So to speak anyway.

 

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?

 

To see if you got it, I'll let you try to fix the error in your example.

 

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

 

Yes but for most transistors it is not one to one, it is more like one to twenty and it is a function of voltage applied to the base to. A little positive voltage on the base gives you one to two while a lot of voltage gives you one to one hundred. In addition they are influenced by the voltage biasing between the base and the collector so you can get all kinds of complex amplifications until you overdrive it and then the collector current hits a ceiling (and in the case of audio you blow out your speakers due to high frequency overtones).

 

Hope this helps, this is not easy stuff to understand.

[deepu]

CAN ANY WAY EXPLIAN ABOUT JUNCTION THEORY?

[cypress]

Yes, can you be more specific about your issue?

[praveeseo]

Hi to all then Keep in mind that the amplification is a current amplifier not a voltage amplifier. and the amplification is not relative to the battery current, it is collector current relative to the base current. The current (energy) has to come from somewhere, there is no free lunch, and this somewhere is the battery. It is easy to take a transistor and simulate a voltage amplifier, just add resistors to the base and collector and take voltage taps off the resistors.

 

 

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[granpa]

holes and electrons recombine in the forward biased emitter-base junction.

holes and electrons are created in the reverse biased base-collector junction.

 

you might want to look at phototransistors