Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Can someone give me a satisfying answer to how Bipolar Junction Transistors work?

Status
Not open for further replies.

Wall-ED

New Member
First, I read so many different explanations from so many different sources.
I can't understand how it amplifies current and at the same time follow Kirchhoff's Law (Ie=Ib+Ic). I can't understand why would a device that requires 2 batteries to work
be efficient? we could just use these batteries (Vcc and Vbb) for common emmitter model instead of going through much trouble. What does Base do? Does it control current or voltage?
And if Collector's voltage is amplified, why does the law say Ie=.... why not Ic=..... that is what we're after anyway?.
Ok here is what I tried to understand.
Electrons enter Emmitter, forward biased. then meet Base, and some electrons recombine with holes in Base, the rest get sucked by the Vcc, however it is reverse bias between Base and Collector?
I just give up. It doesn't sound as logical to me as FETs do.
Please please I need thorough detailed explanation to this. It is driving me crazy.
 
You supply the base with a low current then the collector current is much more so it amplifies the current.
Since it amplifies the current then Ohm's Law says that it also amplifies the voltage.
It is that easy.

But each transistor is different, even if they have the same part number. Their characteristics are affected by changes in temperature so you need to know how to bias them so they are stable. It is easy.
 
I'm just wondering, why not use a diode with a large battery connected forwards? It is better than having an obstacle between the two N's that electrons fall into. You know what I mean?
 
We appear to have a "little" problem here and that is "conventional current" and "electron flow". Electrons go from negative to positive and conventional current goes from positive to negative. Hence the "hole" concept was invented to help understand transistors. The arrows seems to better mimic conventional current or hole flow.

The biasing of a semiconductor device changes the "depletion region" which makes it harder or easier or harder for the "holes" to get across. N material has an excess of electrons, P and excess of holes and I (Intrinsic) has a balance.
 
I'm just wondering, why not use a diode with a large battery connected forwards? It is better than having an obstacle between the two N's that electrons fall into. You know what I mean?
A transistor has a current gain as high as 900 times. Your idea has no current gain.
 
Wall-Ed,

I'm just wondering, why not use a diode with a large battery connected forwards? It is better than having an obstacle between the two N's that electrons fall into. You know what I mean?

No, I don't. Why use a diode or any part of the semiconductor at all? Just connect the battery with the correct polarity, and a current will ensue. What you also want is control, which is what the base and the rest of the transistor is all about.

Ratch
 
KeepItSimpleStupid,

"... Hence the "hole" concept was invented to help understand transistors. ..."

Holes and electrons are physical entities, not concepts. They are both physical charge carriers. Conventional charge flow is a concept invented to facilitate circuit calculations, not to explain holes.

Ratch
 
As Ratchit said, conventional current flow and electron flow have noting to do with holes in semiconductors.

Holes in P-doped semiconductors act like positive charge carriers, but they are actually the absence of an electron in the material. Because of the positively charge protons trapped in nucleus of the atoms, this absence of an electron leaves an effective positive charge in the area of the "hole". These holes bubble through the semiconductor under the force of an applied electric field, appearing as a moving positive charge. Because these holes move more slowly then an electron does, P-Channel MOSFETs and PNP transistors have poorer performance then their oppositely doped counterparts.
 
Wall-ED,
You might find this helpful in understanding the behavior. The base is very thin and when you forward bias the base emitter junction, electron entering the emitter, get moving so well that most of them just can’t make the sharp left turn so they go on through the reverse biased base collector junction and out the collector. Since you have the current flowing through a high resistance, you have amplification. The result is a small emitter/base current is controlling a much larger emitter/collector current. The bipolar transistor is a current operated device so if you want a voltage gain, you send that current through a large resister and look at the changes in voltage across that collector resister (Vcc – Vc= voltage gain of the stage).

Does that make it any easier to understand what is going on? If so, you can go back to your original reference material and start to see the details of how different effects can be achieved by applying this principle to different configurations of input and output signals.
 
What does Base do? Does it control current or voltage?

Someone once told me, when I was first learning about bipolar transistors, that a transistor was similar to a faucet. The tap itself was likened to the collector (on an NPN) and the sink (not talking about FETs here) is like the emitter. The base is like the knob that "turns the flow of current on and off." That is to say that if you have a little bit of voltage going into the base of the transistor, the collector and emitter are connected electrically. This was the simple explanation I got when I was a kid. Though it is actually a little more complicated than that, it did help me to understand basically what a transistor did.
As for the p-section between the two n-sections, it is actually relatively easy. I don't know how much you know about chemistry, so I'll try to make this simple:
If you look at a periodic table of the elements, you will see silicon (#14) in the group 4A, meaning it has four free electrons that are not bonded to anything. Therefore, when several silicon atoms are bonded together, they form a lattice design, as in the picture below:
**broken link removed**
There are no free electrons to move around, and therefore, current cannot flow. To make the semiconductor work, the silicon must be "doped" with another element that has a different number of electrons. To make the n-section, phosphorus or arsenic are often used. These elements, as you can see in the periodic table, are in group 5A, meaning they have 5 non-bonded electrons. When silicon is doped with phosphorus or arsenic, four of the electrons bond, but the 5th ones remain free:
**broken link removed**
*Note: sorry about the huge picture
Because of the free electrons, electricity is able to flow.
For the p-type section, silicon is doped with an element from group 3A (three "extra" electrons) such as boron or gallium, so when bonded together, there is a "hole" (where an electron could be):
**broken link removed**

As you can see, there is one electron less (next to the boron atom). In an NPN transistor, the p-doped silicon is used as the base. When a little bit of electricity is applied to the p-section, the hole fills up, and the current can flow through between the two n-sections. This link may also help:
https://www.elektropage.com/default.asp?page=cat&cid=2&tid=27
I hope this helps, and let us know if you have any more questions!
Der Strom
 
Last edited:
A quote from "Intiative Ic Electronics", Frederiksen, p19 " The P denotes Positive and represents a surplus of "Places for electrons" (called holes) that now exist.

Any way, the concept of "hole current" really means conventional current flow. The underlying concepts to explain semiconductors might well be different if Ben Franklin got the sign of the electron right.
 
Any way, the concept of "hole current" really means conventional current flow. The underlying concepts to explain semiconductors might well be different if Ben Franklin got the sign of the electron right.
As has been explained, the concept of holes has really nothing to due with conventional current flow. If Ben had gotten it right, it would not change the explanation of how doped semiconductors conduct electricity other then that "holes" would now be negative and electrons would be positive.
 
I always understood that N-type was negative because it had an excess of electrons (negative charge) and P-type was positive because it had a deficiency of electrons. That's how I always remembered it, anyway.
 
Last edited:
@ DerStrom8 - could you please be as kind as to tell me the source of the p-type doped silicon image so i can get it in the same resolution as the n-type one! That would help me a lot thanks very much!!
 
@ DerStrom8 - could you please be as kind as to tell me the source of the p-type doped silicon image so i can get it in the same resolution as the n-type one! That would help me a lot thanks very much!!

Hi, rettol. Welcome to ETO! :)

Generally we try not to resurrect old threads here, but seeing as you're new, I'll help you out ;)
I recently found a better image that shows each situation side-by-side. The site is https://www.filmscanner.info/en/CCDSensoren.html, and I have posted the image below:

**broken link removed**

I hope this helps you out. Good luck! :)
Regards,
Der Strom

P.S. The image is in a different language, but the labels simply mean n-doping and p-doping :D
Let me know if you really need the links to the original images.
 
Last edited:
Thank you very much DerStrom8- I really appreciate your help!

danke, aber ich verstehe die Betitelung auch :) !

Since I would need the illustrations for an english research paper, the original two sources of the previously posted english images might be very helpful (simply cannot find n-type and p-type image in same resolution)!

Just in case you remember the original source, let me know- otherwise I might try to use the german image!

Thanks for the welcome to the superb forum!!
Kind regards,
rettol
 
Thank you very much DerStrom8- I really appreciate your help!

danke, aber ich verstehe die Betitelung auch :) !

Since I would need the illustrations for an english research paper, the original two sources of the previously posted english images might be very helpful (simply cannot find n-type and p-type image in same resolution)!

Just in case you remember the original source, let me know- otherwise I might try to use the german image!

Thanks for the welcome to the superb forum!!
Kind regards,
rettol

I am very glad to help!

Ich wußte nicht, daß du sprechen deutsch ;):D

In case you need the original images, I finally found them here

Good luck! :)
Der Strom
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top