# NPN transistor

Discussion in 'Homework Help' started by Nathan Hale, Apr 30, 2014.

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1. ### Nathan Hale Thread Starter Member

Oct 28, 2011
125
2
Hello guys! hope all is well. I have a question regarding a basic NPN transistor. ( just started studying about transistors).
In the pic below the lamp is supposed to glow when the switch is closed because the base current flows and allows current to flow from the emitter to the collector.

My question is why would the electrons wait and bother themselves to go through the collector? i would think they would go through the base and take the "easy" quick way back to the positive terminal through the base itself with the slightest base current! ( i know it doesnt happen that way ) . I would like to know why arent they taking the easy way out ( we all know current takes the path of least resistance )

thank u

2. ### shteii01 AAC Fanatic!

Feb 19, 2010
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There is a couple of flaws with what you said.

First. The current will flow from collector to emitter. You said from emitter to collector.

Second. Bast to Emitter is a diode. Where Base is P region (lots of positive holes), Emitter is N region (lots of free electrons). However, in order for diode to be ON a certain amount of voltage must be applied to it. Typically for NPN transistors we say that BE diode is ON when .65 to .7 volts is applied. So Base must be about .7 volts higher than Emitter in order to turn the BE diode ON and for current to flow from base to emitter.

3. ### Nathan Hale Thread Starter Member

Oct 28, 2011
125
2
the semi conductors book on this website says on page 175 , top of the page says ......"The main current that is controlled goes from collector to emitter, or from emitter to collector, depending on the type of transistor it is (PNP or NPN, respectively)."

4. ### WBahn Moderator

Mar 31, 2012
17,461
4,701
The OP is using carrier current (i.e., electron current) and not charge current (i.e., conventional current). While unfortunate, that's the way it is.

The notion that "current takes the path of least resistance" is one of the more heavily abused sayings in electronics. If you have a 12V car battery and you connect a light that has a resistance of 12Ω across it and also connect a light that has a resistance of 6Ω across it, does ALL of the current go through the 6Ω light since it is the "path of least resistance"? Of course not. Both lights work just fine. There is more current going through the lower resistance path, but there is still current going through the higher resistance path as well. In fact, in this case, the current in one path is unaffected by whether the other path is even available (e.g., disconnect one light and the other will still have the same current flowing in it).

As for the transistor, it might help for you to think of it along these lines. Imagine I build a water valve that is, itself, operated by water. So I have three ports. The supply port (think of the emitter of an NPN) is connected to the water supply (the faucet, let's say). Then there is a small port (think of the base) that has a needle valve on it that you can turn to adjust how much water flows out of it. Even if the needle valve is full open, though, the port is so small that very little water will flow out of it. But, inside the unit is a bigger valve that lets water flow from the supply port to the output port (think of the collector) but this valve is controlled by the water flowing out the small port control. The more water that is flowing out the small port, the wider open the bigger valve gets. Conceptually, this description is actually pretty close to how many hydraulic servovalves work.

5. ### t_n_k AAC Fanatic!

Mar 6, 2009
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The AAC book uses electron flow rather than conventional current flow. Hence the discrepancy.

6. ### WBahn Moderator

Mar 31, 2012
17,461
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The owner of this site has declared that the E-book will use electron current. That is unfortunate, but it is his website and his right.

The names "collector" and "emitter" are in reference to whatever the "majority carrier" is. In an n-type material it is negatively-charged electrons and in a p-type material it is positively-charged "holes". Majority carriers are "emitted" by the emitter (into the device) and "collected" at the collector after traveling through the device.

7. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,297
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In NPN, the main current goes from Collector to Emitter.

In PNP, the main current goes from Emitter to Collector.

From my textbook. The main current is from Collector to Emitter.

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8. ### WBahn Moderator

Mar 31, 2012
17,461
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Haven't you been reading what has been said about the E-book and the OP using electron current. You are insisting that the OP must be meaning conventional current when it is obvious he doesn't, especially since his original post specifically talked not only about the current, but about the electrons themselves.

9. ### vk6zgo Active Member

Jul 21, 2012
677
85
Some people get their "knickers in a twist" about Electron Flow & Conventional Current Flow----I have no idea why!

It is simple to work in either convention.
It took a few minutes to explain their existence in "Elementary Ekectrical Theory" back in 1959,& I & all the others from that time have been happily switching back & forth as required,ever since.

Are later generations more obtuse?

10. ### endolith Member

Jun 21, 2010
27
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Because they are not opposites. Conventional current includes both electron currents and proton currents. Electron current only includes electron currents.

As soon as you put a battery, electrolytic capacitor, neon lamp, or anything else with moving protons in your circuit, electron current becomes wrong, which is why real electronics engineers don't use it.

11. ### crutschow Expert

Mar 14, 2008
12,577
3,082
Positive "holes" do appear to move in a doped semiconductor as part of an electric current but protons never move. They are buried in the nucleus of the atoms. The only place you see moving protons (ionized hydrogen) is in cosmic rays or particle accelerators.

12. ### endolith Member

Jun 21, 2010
27
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Incorrect. Protons can move any time they are in a fluid, like positive ions in the electrolytes of batteries, electrolytic capacitors, the gas in neon lamps, mercury switches, spark gaps, etc. Conventional current accounts for this, electron current does not.

http://amasci.com/amateur/elecdir.html

13. ### vk6zgo Active Member

Jul 21, 2012
677
85
The guy in your quoted text is really stretching to make the real world fit his ideas

Ions are not Protons,they are Atoms with an imbalance of Protons & Electrons,so they possess a net Electrical charge,which may be either positive or negative.

Ions move in Electrolytes,etc,but the Protons only move in the same sense that passengers on a bus move along the road.

Depending on whether their charge is positive or negative,Ions may flow either way.

In older TV Picture tubes,there was a problem with"Ion burn".
Negative Ions which are much more massive than Electrons,were not deflected by the scanning system & would impact the phosphor,damaging it.

The first answer to that was the "Ion trap" where the so-called 'Electron gun" was angled so that it did not directly face the screen.
The Electron beam was readily deflected to point correctly using magnetic deflection .
The negative Ions were not deflected ,& did not reach the screen.

Later designs used a thin layer of Aluminium,which was easily penetrated by Electrons,but was relatively opaque to Ions.

.

14. ### crutschow Expert

Mar 14, 2008
12,577
3,082
The only ions that are protons is ionized hydrogen.

15. ### endolith Member

Jun 21, 2010
27
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Where, specifically?

So?
So you agree.

16. ### vk6zgo Active Member

Jul 21, 2012
677
85
WHY do I bother?

17. ### PRS Well-Known Member

Aug 24, 2008
989
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Let's just agree that the flow of electrons is electrical current but the flow of ions is something else. Can we call it ionic current? Yes, let's do that!

18. ### MrChips Moderator

Oct 2, 2009
12,252
3,283
This topic has been debated already on AAC and for this reason will cause this thread to be closed.