help in transistors

Discussion in 'Electronics Resources' started by zeerum, Nov 15, 2006.

  1. zeerum

    Thread Starter New Member

    Nov 15, 2006
    4
    0
    hi
    m an undergrad level student. i have used this website n found it very good
    the author explained every thing with reasons n concepts in beginning but when it came to BJT, he refrained from going into the semiconductor physics of it
    i actually searched this site for understanding whats actually going on inside a transistor .....how base current can actually control collector current
    u know books explain these things on conventional flow grounds...n i was hoping that i would find answers to my confusions n questions in this website as it was doing it on electronic flow basis n was very conceptual in beginning....but prob still is unresolved
    could u please refer any article or website where i can find answers to my conceptual querries
     
  2. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
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  3. Nirvana

    Well-Known Member

    Jan 18, 2005
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    0
    Hi there, I will attempt to explain what actually takes place within a BJT. As you know there are two types of Bipolar Junction Transistor, NPN & PNP. I am assuming that you are familiar with PN junction behaviour, if not just let me know. I will begin to describe the NPN transistor although the discussion applies equally well to that of a PNP transistor except for the current & voltage polarities.
    The NPN transistor as its name suggests is a three layered device where the N type material has an excess of negative charge (electrons) due to the controlled doping of the material during manufacture. The P type material has a lack of negative charge, this is where electrons have been removed from the atom leaving the material with 'holes' leaving spaces for any electrons to 'pop' into. The overall charge therefore of a P Type material is said to be positive due to the lack of negative charges (more holes than electrons within the material). The NPN BJT as indicated, has two layers of N type material which are either side of a P Type material. The top layer (N) is called the Collector, the middle layer (P) is called the base and the bottom layer (N) is called the Emitter. A terminal extends from each of these layers where connections are made. Many people compare the device to two diodes back to back since a diode has a single PN junction and with a BJT we have 3 layers which resemble two diodes. Simply connecting two diodes back to back would not produce transistor action, actually very little would happen. Transistor action described shortly only takes place because the base is extremely thin which is very important. So in your minds eye imagine this NPN layered device (it may be useful to draw a quick sketch of three squares NPN). The Collector is packed with electrons just sitting there, the emitter is also packed with electrons and the base is full of holes just waiting for electrons to come in and pop into these holes. Before I proceed I would like to point out that although there are no external connections made yet in this discussion, electrons from the collector and emitter will pop into some of the holes in the base because the free electrons in the N type are right next to these holes. This 'drift' of electons into the holes at this stage is known as a drift current. But soon after the electrons have popped into these free holes there is no longer a charge associated with the hole or electron since they have combined and are now neutral. This sets up a barrier between either side of the base, that is a barrier between the collector and base and one between the base and emitter (hence the likeness to a diode). This barrier or neutral zone is where the charges (holes and electrons) have combined, therefore electrons can no longer get to the holes because they are seperated by a neutral gap. I.E. the only reason that the electrons went into the holes in the first place was due to their opposite charges (opposites attract). But now that a neutral barrier has been set up there is a gap between these two charges caused by the neutral region. This neutral region is called the depletion region. Apologies for going on a bit but just to expand on this behaviour for a moment; As you may already know from PN junctions, when you have a negative voltage terminal on the P type side and positive terminal on the N type the width of the depletion layer will increase. This is because the negative terminal attracts all the holes (in reality it repels the electrons which make it look as if it were attracting the holes) similarly the positive terminal will attract the electrons towards it, therefore the distance now between the electrons and holes has been increased, since the electrons and holes are pulled away from each other consequently increasing the the depletion region. Clearly no electrons can flow through this material they are just sitting at either end, when I say electrons flowing I mean that they can't hop to the P type side by travelling through the material by popping into the holes; this is because the electrons and holes are too far away from each other. This non conducting state is what is called reversed biased. On the other hand, if we apply a positive terminal to the P type and a negative terminal to the N type the following takes place: The negative terminal repels (pushes) the electrons away from it so they are sitting very close to the junction (joint) of the two types of N & P Type material. Similarly the positive terminal repels the holes (actually attracts the electrons towards it which makes it look as if it were repelling the holes because the electrons leave holes behind when moving to the positive terminal) therefore the holes approach the junction (joint) of the two materials. They can almost 'see' each other they are that close, as you can imagine these holes and electrons at either side of the junction are pulling towards each other, therefore the depletion layer is getting smaller. As the voltage is increased these holed and electrons are so close that an effect called 'tunneling' is experienced where the electrons have enough energy that they can burrow through the junction and populate/swarm the holes. These electrons make their way toward the positive terminal on the P type material which they are now in. (can think of like a force such as a pen going through a piece of paper when enough force or energy is used). The material is now said to be forward biased (conducting). The amount of voltage needed to cause a constant flow of current through the material differs between materials, it is approximately 0.7V for a silicon device or 0.2 for germanium. A current will flow when reversed biased but this is negligible (too small to consider) it's approximately 1nA (nano amp 0.000000001A) this is called the leakage current. Note that these holes and free electrons were created manually by injecting impurities into some pure silicon. By impurities I mean not the substance/chemical which it is being injected into (here Silicon).
    To get back to the NPN BJT then, well we have two of these described junctions.
    The collector of the transistor is usually (most commonly) made more positive than the emitter for reasons that will become apparent shortly. If we left the base unconnected at this point 'open circuit' no current will flow because we have a reversed biased junction, the positive terminal is on an N type material (Collector) this won't produce transistor action, the electrons in the collector will flow down and into the emitter through our voltage source (may be a battery) but nothing more, this small currnet is a leakage current where some electrons have leaked away, not a lot though, its value is again negligible. Now lets have a voltage between the base and the emitter, the base being more positive than the emitter. As you know we have a connection between a P and N type material with the positive terminal on the base and the negative terminal on the emitter therefore assuming its a silicon device and we have at least 0.7 volts across this junction it will be forward biased. Now as described above the electrons tunnel through to the base. The emitter remember was teaming with electrons it is huge compared to the size of the P type region which has a few holes. The electrons tunnel through and swarm the base, electrons hoover up the hole but they can't find enough. Now look at the bigger picture, the holes have been hoovered up, therefore we can think of the BJT as having just two parts, one being the collector and the other being the combined base and emitter. Meaning that the base and emitter can now be thought of as being one because they both have electrons in them i.e. one big excess of negative charges. Also remember that this negative base-emitter charge has the negative terminal of the emitter pushing the electrons towards the collector. Even though there is a positive connection to the base , the base emitter voltage will only be 0.7V whilst the collector voltage will be much bigger since the collector and the emitter don't have a junction. They are not related by this 0.7V relationship. Therefore the emitter negative terminal forces the electrons to tunnel through to the collector, we now have the electrons from the base and emitter entering the collector. Thereofre it can be seen that a small base current causes a much larger current to flow to the collector. The emitter is the sum of the base current + collector current.
    I hope that describes it well enough for you, if you have any problems just let me know.
    Nirvana.
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Much of the information that Nirvana has painstakingly written in his reply can be found in the AllAboutCircuits Tutorial on BJT devices.

    Once you have read nirvana's writeup you can review the material at the link provided for any additional information it may contain on the subject.

    While you are visiting the BJT tutorial section at www.allaboutcircuits.com I would encourage you to take the opportunity to review the wealth of additional material it contains on the interesting world of electronics.

    Enjoy,
    hgmjr
     
  5. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    144
    Much of the information that would be of use to you is in Volume III - Chapter 2, which I am currently proofreading and will shortly be added to AAC.

    Dave
     
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