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Question about Walter Brattain’s field-effect transistor principle.

  1. Feb 3, 2009 #1
    Question about Walter Brattain’s “field-effect transistor principle”.

    The Wikipedia says “On 17 November 1947 John Bardeen and Walter Brattain observed that when electrical contacts were applied to a crystal of germanium the output power was larger than the input.“

    Can someone please elaborate on how electrical contacts can be applied to a germanium crystal to make a power output larger than the input?

    I don't see how you can get the current or the field to intensify just by putting a crystal inside a circuit.
     
    Last edited: Feb 4, 2009
  2. jcsd
  3. Feb 4, 2009 #2
    Re: Question about Walter Brattain’s “field-effect transistor principle”.

    A transistor (effectively a switch or an amplifier) is very closely related to a pair of diodes (effectively one-way valves).

    The simplest way to see this is to look up vacuum tube valves and then triodes.

    Rather than using vacuum tubes, another way to produce these devices is by joining two types of crystal (see n and p semiconductors). A less intuitive way to get the same result is by using surface effects on a single crystal (see "point contact" or "cat's whisker" diodes and triodes).

    Note here that a crystal amplifier works the same as a car stereo amplifier: the amplifier does not produce power (it just controls the flow of an applied power source, like a switch). But the technology is pretty cute: you can build a "foxhole radio" with nothing more than a rusty razor blade, a length of wire with a pointy end, and an earphone.

    ..as for the FET, wasn't that Shockley?
     
    Last edited: Feb 4, 2009
  4. Feb 4, 2009 #3
    Re: Question about Walter Brattain’s “field-effect transistor principle”.

    How much semiconductor physics have you had? How about e/m field theory, circuit theory? You ask a broad question. It is hard to answer in a paragraph. The FET you refer to is explained all over the web. Search using the words "field effect transistor operation holes electrons". Also, you may include "majority carrier inversion insulated gate".

    Are you reasonably familiar with diode theory of operation, i.e. electrons and holes? If not, I'd begin with that, and then work into transistors, FET or bjt. BR.

    Claude
     
    Last edited: Feb 5, 2009
  5. Feb 4, 2009 #4
    Re: Question about Walter Brattain’s “field-effect transistor principle”.

    If I remember correctly the material Lilienfeld used for his mesfet wasn't germanium nor silicon but cadmium sulfide. With it he was able to make an amplifier that inverted the input signal but unfortunately the gain was always less than 1.
     
  6. Feb 4, 2009 #5
    Re: Question about Walter Brattain’s “field-effect transistor principle”.

    Seems like a typical use of the patent system: condemn everyone [else] to only using vacuum tubes for the next two decades ("for the good of society and progress").
     
  7. Feb 4, 2009 #6
    Re: Question about Walter Brattain’s “field-effect transistor principle”.

    Thanks for the response,

    I understand that when you use a crystal amplifier that it just modulates the output of the power supply. I just don’t understand what they ment when they said…

    “John Bardeen and Walter Brattain observed that when electrical contacts were applied to a crystal of germanium the output power was larger than the input.”

    How can you have more output power than input power? That sounds like “overunity”. I guess this was just a typo?
     
  8. Feb 4, 2009 #7
    Re: Question about Walter Brattain’s “field-effect transistor principle”.

    A transistor has three contacts. One provides constant access to a large amount of power input. The next inputs a small amount of power in the form of a signal. The third is the signal output. By definition, the output signal must have more power than the input signal, or it wouldn't be an amplifier. There is no typo and no overunity, you're just forgetting about one of the three contacts.
     
  9. Feb 11, 2009 #8

    marcusl

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    Re: Question about Walter Brattain’s “field-effect transistor principle”.

    Stating cesiumfrog's comment another way, you are also supplying a DC bias in addition to the AC signal. The AC output power is less than the sum of AC + DC input powers.
     
    Last edited: Feb 11, 2009
  10. Feb 16, 2009 #9
    Re: Question about Walter Brattain’s “field-effect transistor principle”.

    in which field transistor is used widely
     
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