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Homework Help: Static or Dynamic equlibrium in a PN junction?

  1. Nov 1, 2015 #1
    1. The problem statement, all variables and given/known data
    We are considering an UNBIASED junction.

    I've read in a textbook that electric field created in the junction STOPS diffusion BUT in the same text its written the drift and diffusion currents cancel each other.
    1. Does the diffusion and drift currents occur simulataneously and continues forever maintaing a dynamic equilibrium between the two? Or is it a static equilibrium where the electric field just stops the diffusion completely?

    2. Also another doubt that i have is that what carriers constitute the drift current? Is it the majority carriers or the minority carriers or both??

    3. If the drift current is ONLY due to minority carriers, how can it balance the magnitude of diffusion current since there are only a few minority carriers?

    4. In the same textbook i also read there will a diffusion of minority carriers. What will be the direction of this diffusion, say of electrons(minority carriers) in the P side of the junction and how will it effect the equlibrium?

    2. Relevant equations
    Its a qualitative analysis.

    3. The attempt at a solution
    I tried drawing diagrams of the lattice structure to visualise the electrostatic repulsion forces to get an idea of the movement of all the carriers but its utterly confusing and i couldn't reach any conclusions. I don't know what else to do about these problems.
  2. jcsd
  3. Nov 1, 2015 #2
    I went through the books again and the diffusion of minority carriers is not discussed under the PN junction. There isn't any reason for it to occur in a diode unless it is unevenly heated or something.

    Also after more thinking i think the equilibrium is static or else the width of the junction will keep increasing due to diffusion.

    But under a static equilibrium the diffusion of minority carriers will still happen, giving a non zero net current in equilibrium. So where am i going wrong?
  4. Nov 1, 2015 #3
    Is it like, under static equilibrium(unbiased) the minority carriers WON'T diffuse to the junction and get sweeped away by the junction and form a current simply because of the attraction from its parent silicon atom. But when an external voltage is applied (reverse bias), the minority carriers gain enough energy to break away from the attraction of parent silicon atom(and move through the Conduction band) leaving behind CHARGED or UNCOMPENSATED silicon atoms which MAY OR MAY NOT get compensated later ??
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