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Homework Help: Convention in a circuit

  1. May 7, 2008 #1
    Hey guys, was sitting through a physics class today and something bothered me. They refered to the p material of a diode as being the anode and the n material being a cathode. Being a chemistry student im aware that the cathode is + while the anode is -ve. Why has it changed around, due to the use of conventional current? Or is it because of the electric field in the depletion layer?
     
    Last edited: May 7, 2008
  2. jcsd
  3. May 7, 2008 #2

    alphysicist

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    Hi sanado,

    I don't think it is correct to say that the cathode is positive and the anode is negative. The anode is where the current is entering the device, and the cathode is where the current is leaving the device. (For example, if you are using a battery, the negative terminal is the anode, but if you are recharging the battery the positive terminal is the anode.)

    For the cathode side of a pn junction: When you forward bias a pn junction to get current to flow, you attach a positive voltage to the p side. This causes the extra positive charges (the holes) in the p side to move away to the n side, where they exit the crystal. Since the positive current leaves the n side, the n side is the cathode.
     
  4. May 7, 2008 #3
    You are right, alphysicist. For any device, electrons flow out of the anode and conventional current flows out of the cathode. Which is the positive side and which is the negative side will depend on whether it's an active device like a voltage source (cathode positive, anode negative) or a passive device like a diode (anode positive, cathode negative).
     
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