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PN Junctions vs Transistors When Building Logic Gates

  1. Oct 27, 2014 #1
    Is there any reason why logic gates are built using transistors instead of PN junctions? Wouldn't it be more cost-efficient to use PN junctions? I am referring to CMOS logic gates.

    Also, what can a transistor do that a PN junction can't?
     
  2. jcsd
  3. Nov 1, 2014 #2
    Thanks for the post! Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?
     
  4. Nov 2, 2014 #3
    It seems to me that both PN junctions and transistors act as switches. And both switches are voltage-controlled. So what advantage does a transistor have over a PN junction? When the voltage is high on a PN junction, it is in forward bias and it allows current to pass though. When the voltage is low on a PN junction, it is in zero bias and it does not allow current to pass through. When a NPN bipolar transistor's base voltage is high, it allows current to pass through. When a NPN bipolar transistor's base voltage is low, it does not allow current to pass through.

    EDIT: Actually, I think I get it now. A PN junction has only two contacts while a transistor has three contacts.
     
  5. Nov 3, 2014 #4
    I'm glad you realized the problem: transistors are made out of PN junctions!
     
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