How Transistor works - verifying

  1. Hi All,
    I'm trying to understand how transistor works. I have drew some pictures for NPN transistor to show my current understanding. Can you please verify this for me.

    npn.PNG

    On the picture:
    "-" and "+" are movable charges;
    circled "-" and circled "+" are immovable ions.

    1. No voltage applied. Two depletion layers are formed.

    2. We apply voltage that we want to control - "-" to Emitter and "+" to Collector. Electrons from Emitter fill the Base forming negative ions. Electrons from Collector are gone; positive ions are formed in there. Because Base is filled with negative ions, electrons from Emitter can't go through - channel jammed.
    Here I'm not sure about Base-Emitter depletion layer - will there be any interaction between Base and Collector..?

    3. We apply controlling voltage to Base. If we have BJT than we will have current between Base and Emitter, if we have FET, electrons inside of Base will be just pulled up, leaving positive charges behind. Electrons from Emitter will flow thought Base to Collector and will not be jammed again because Base is very thin.

    So, is my understanding correct? Do I do any mistakes..?
    Thank you!
     
    Last edited: May 23, 2011
  2. jcsd
  3. Guys please, I spent a lot of time drawing this picture!
     
  4. you can look at transistor as two diodes facing eachother

    --l>l-*-l<l---


    this also forms NPN junction and will let thru if voltage is applied to the between.

    Your assumption is basically correct ~ Applying + to emmiter will draw free electrons from there leaving + ions, applying - to collector will result in moving electrons to base and neutralizing some free 'holes'.
    And when applying + to base most of electrons will go from collector to emmiter, but some will go to + in base and coming back to collector, so the smaller is base the better.
    In ideal transistor Electron flow input in collector should = electron output in emmiter.
     
  5. that's hardly the case. it doesn't account at all for the mechanism of current amplification due to the thin base.

    indeed...

    you won't get a thin base layer by soldering two diodes facing each other.
     
  6. it will work out transistor mechanism just fine.



    never said so.
     
  7. It's difficult to find a good explanation about transistors, but here a link

    << link deleted by berkeman >>

    It's a big article, but it leaves most questions behind once you read it.
     
    Last edited by a moderator: Jul 6, 2011
  8. dlgoff

    dlgoff 3,047
    Science Advisor
    Gold Member
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  9. This article is heresy. Peer-reviewed texts have the best info. I've covered this issue many times as well as others. A search for my post history will show these threads. Then I can address any questions. Stay away from the above web site. It contains just enough truth to look credible, but makes assumptions contrary to known science.

    Claude
     
    Last edited by a moderator: Jul 6, 2011
  10. Hi All,
    Sorry for the delay - I had a lot of thing to do these days. But I'm still full of enthusiasm :)
    I tried to read this explanation before (<< link deleted by berkeman >>) And I agree with author that a lot of other authors does not really explain how transistors work. But since cabraham says that this is heresy, I stopped reading it :)

    Actually I already read a lot of explanations about how transistor works, and I have my understanding. And I want to make sure of it. So this is why I posted this thread.

    Guys, please, can you not just give me another explanations, but read my own and tell me what is wrong there on my picture. So I could use my own understanding as a reference point to move from.
     
    Last edited by a moderator: Jul 6, 2011
  11. Among other explanations this one gives you an understanding of principle of work. Having read this, you can correct mistakes made in the article by reading other more scientific articles and thus come up with the whole and right picture.
     
  12. Sure, but why not just read more scientific articles in the 1st place that don't have mistakes. Then there is no need to correct them. Peer-reviewed books from semiconductor producers & universities are the best source of info. These contrarian web sites just confuse the issue to the point where one does not know who to believe.

    Claude
     
  13. In your pic, 3rd view at bottom, the base region is shown w/ an excess of electrons, herein called "e-", & an absence of holes herein "h+". Also, the collector has many e-, but few h+.

    For the 3rd pic, where Vce > 0, & Vbe > 0, the device is in its active region. We will not cover the saturated region yet. In the collector, there are a small number of mobile h+, as they are minority carriers, & e- are majority carriers w/ high mobility. W/o Vbe biased > 0, the e- are crowded near the collector lead, & the h+ are near the base-collector boundary. The c-b jcn is rev biased & little current exists from base to collector.

    But the b-e jcn is now fwd biased from an external source, & Ib/Ie > 0, & Vbe > 0. The emitter emits many e- toward the base, & the base emits not as many h+ towards the emitter. The h+ from the base enter the emitter & recombine w/ e- near the e-b boundary. The e- from the emitter transit into the base region. Some recombine w/ a h+ in the base, some transit through the base region out the base lead, & nearly all transit through the base into the collector region, where they again are majority carriers.

    The number of e- which do not reach the collector are around 1 in 2000 to 1 in 5000. The number of h+ emitted from the base are about 100 to 200 times less than the number of e- emitted from the emitter. Thus the base current consists chiefly of holes emitted from base to emitter. This is the injection component of base current. The e- emitted from the emitter which recombine in base or exit base lead constitute a very small fraction of the base current & is called the transport component.

    Another component of base current is the charging or displacement component. As frequency increases more h+ & e- get displaced but do not contribute to collector current. So the current gain decreases as freq increases. Eventually there is a frequency where the beta value equals one. This is the transition frequency of the bjt, called "ft" on data sheets.

    The injection & transport components of base current determine the dc & low freq current gain, "hFE" on spec sheets. Adding the displacement component of base current decreases the current gain (beta value) at high freq. The ac & high freq current gain is called "hfe" on spec sheets.

    It's all about the c-b jcn being reverse biased so that very little collector current exists. By forward biasing the b-e jcn, many e- flow from emitter to base. Nearly all are yanked into the collector by the electric field in the b-c boundary region. The base is made thin so as to minimize the number of e- recombining in base region. The base is doped lightly & the emitter heavy so as to minimize the injection component of base current.

    The current gain, bets, is determined by Ic/Ib, where Ib consists of the 3 parts, injection, transport, & displacement.

    Regarding heresy, I've addressed Mr. Beaty on another forum. We had a debate. I'll provide a link if you wish to see the points he & I presented.

    Best regards.

    Claude
     
    Last edited by a moderator: Jul 6, 2011
  14. Hi Claude,

    I'm very interested in the debate you had with Mr. Beaty. Can you post the link? :)
     
  15. Man, I can't believe you guys had the patience to duke it out like that! And it ended with some level of agreement? Truly epic.

    I actually enjoyed reading Mr. Beaty's site, but there should be a rebuttal to it, or a summary of the epic thread published on the web. I think a lot of confused people would appreciate it. Thanks for sharing.
     
  16. One of these days I should copy & paste the whole thread into a text document. Then I can post the text. It gives all the different viewpoints from many different parties. That way, nobody is quoted out of context. I'll do that soon.

    Claude
     
  17. I'm interested too, since I've never had any such debate.
     
  18. Refer to the link I gave a few posts back. The poster called himself Bill Beaty. Once again, here it is:

    http://www.electro-tech-online.com/g...voltage-4.html

    If that isn't you, then someone used your name. Maybe the moderators on that forum can help you. Best regards.

    Claude
     
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