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Transistor operation

Discussion in 'General Electronics Discussion' started by mahoutekiyo, Feb 28, 2011.

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  1. mahoutekiyo


    Feb 28, 2011
    I've been trying to do some research on transistors and I understand the bottom line but I'm unclear as to how to get there.

    Firstly, I don't understand the difference between Bipolar transistors and MOSFETs, the only thing I've discovered online is their respective schematic symbols.

    But generally, I assume that a if a voltage potential is applied across a transistor's emitter/collector leads or source/drain leads, that a current will not be conducted because as electrons attempt to pass from one substance to another, they fill holes at the borders creating stable, non-conductive compounds that break the circuit.

    However, when a charge is applied to the base/gate then my options are:
    a) a positive charge applied to an electron rich material will attract the electron surplus toward the base/gate, leaving behind a bridge of conductive holes.

    b) a positive charge applied to an electron lean material will attract (few) electrons toward the gate creating a severe enough electron drought in the sandwiched material that would break-down the depletion zone. (however, then, why wouldn't that happen normally?)

    c) a negative charge applied to an electron lean material will repel (few) electrons from the base/gate, creating a bridge of holes near the base/gate... again, why does this not occur naturally?

    d) a negative charge applied to an electron rich material will repel electrons from the base/gate, creating a bridge of holes near the base/gate...
    this however, doesn't make sense to me because I fell like the same factor for repelling the electrons will obstruct the normal current flow.

    The one thing I was slightly led to believe about the difference between MOSFETs and BJTs is that a BJT allows current to pass through the base, which would mean that applying the opposite charge to the base would allow current to pass.

    I really don't know if any of this is a solid understanding so any help would be greatly appreciated.

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    There's lots of information on the net, and lots of good books. Beware that some books have very mathematical explanations that may cause your head to spin. My normal recommendation (The Art of Electronics) is not awfully good here either (at least not for BJTs). It tells you what they do, and has practical explanations, but not theoretical at all.

    Basically all transistors are controlled by an electric field. This is true even of diodes (although there are some diodes that don't directly use this effect).

    Start with a diode. You have a piece of P doped semiconductor joined to a piece of N doped semiconductor. there is an excess of electrons on one side, and an excess of holes (a lack of electrons) on the other.

    If you place an electric field of the correct potential across the junction, the excess electrons can move into the region with a lack of electrons and current can flow. If you place a reversed electric field across it, the electrons are kept away from the electron poor region and current will not flow. The electric field is applied by the same voltage that is trying to cause a current to pass. It happens that with silicon, about 0.6V of electric field is enough to make the electrons flow. Thus the voltage drop across the junction is that which is required to allow the current to pass.

    JFETs are like diodes except that there are 2 connections on the anode (or the cathode). In this case, the electric field generated across the junction determines how much current can flow within the piece of silicon with the 2 electrodes. Because electrons do not pass through the junction as they go between the 2 electrodes on the one side of the junction there is no particular voltage drop. The electric field imposed by the junction determines how conductive the channel between the 2 electrodes is. Because it's a junction device, you can't forward bias it more than 0.6V so there is a limit on how hard you can turn the device on (i.e. how conductive you can make the channel)

    MOSFETS are a class of device similar to a JFET except the gate is actually totally insulated from the piece of silicon that has the 2 electrodes on it. This allows the gate potential to be made positive or negative with respect to the piece of silicon with the 2 electrodes. This means the device can be turned off with an electric field in one direction, or on with an electric field in the other direction. The doping of the silicon in this case determines how conductive the channel is when no field is applied (the doping essentially introduces a sort of static field). Because the gate electrode is insulated, no current flows (except that which is required to charge the small capacitance created by having this small conductor very close to another conductor. The voltage that can be applied to the gate is dependant on the dielectric properties of the insulator used and is frequently around +/- 20 to 30 volts maximum.

    A BJT is a little different. A current flowing into the Base of the transistor through the forward biased BE junction allows a greater current to flow through the reverse biased CB junction and through to the Emitter. This too is controlled by electric fields, but it's far easier to think of it as current through BE allowing a larger current (hfe x Ib) through the collector.

    Mosfets and Jfets are actually a lot easier to understand :)
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