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Are BJT transistors voltage or current controlled?

  1. Jan 27, 2012 #1
    Hello Forum,

    question about BJT transistors: some books say that they are current (base current) operated...
    Some other books state that the emitter-collector current is controlled by the voltage between the base and the emitter.....which one is correct?
    FET are voltage controlled....

    Holes are not really carriers, correct? They are just empty places that can be filled by electrons...

    Also, a transistor, in general, can act as a switch (ON or OFF) or as an amplifier...
    But it is not an active device that produces extra energy: it simply acts like a switch, but instead of being a binary switch (only ON or OFF), it acts as a continuous switch...that is what is mean by amplification: the transistor allows more or less current in an analog fashion...

  2. jcsd
  3. Jan 27, 2012 #2
    in case of FET we assume the gate resistance is infinitely high. Thus the current through gate is ideally zero. In fact a field across the oxide layer is set up which determines drain current. Thus the current is dependent on the field which in turn is dependent upon the voltage.

    In case of BJT there's a finite amount of base current that flows between the emitter-base junction which also has a voltage drop across is. Thus the base-emitter voltage and the base current are interrelated by a formula. We can say BJT can either be controlled by the base current or the base-emitter voltage.
  4. Jan 27, 2012 #3
    Holes are not actually a particle. But they do carry charge, which is important enough in electronics.
    The transistor works as switch or amplifier, depending on the way you designed the circuit.
    In fact the switch operation of transistor is a very crude approximation of an amplifier.
  5. Jan 27, 2012 #4
    Thanks Kholdstare!
  6. Feb 4, 2012 #5
    Hello Forum,

    I was reading the following explanation about diodes at


    It looks like electrons and holes are moving towards each other, like two diffusing gases.
    As electrons move from the N type region to the left they automatically form a hole that is filled by an electron that is now moving into it.
    On the P type region part of the diode, holes are moving to the right. A hole moving to the right is equivalent to an electron moving to the left, correct?

    So, eventually, all electrons are moving to the left, creating a current, which is actually by convention going in the right direction....

    Must the doping of the N type need to be the same as the doping of the P type? Does the number of holes in the P type region need to be the same as the number of electrons in the N type region?
    If not, why not?

  7. Feb 4, 2012 #6
    If the diode is made of a specific material, everytime an electron jumps into a hole light can be emitted (LED), correct?
    Does the light emission occur both within the P type and N type or only in some specific region of the combination?

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