Zener breakdown vs Avalanche breakdown

  1. Hello,

    I am familiar with both of terms that i speak of in title. But I cannot find a full answer, so I might as well ask the PhD'ers here.


    What is REALLY happening in Zener and Avalanche breakdown? I have read this
    http://cnx.org/content/m1009/latest/

    And yes I get that impact ionization thing etc. But still, how does Zener differ from Avalanche breakdown? Why is doping so important? How come it doesn't damage the diode? Why is current constant? Why would it be constant when u exceeded the depletion zone(reverse bias), current should be proportional to voltage? (more voltage, more energy, more charges pulled out, more current)

    I am trying to get a full picture here. You may post links with detailed quantum mechanics, semiconductor theory. I am very eager to learn.

    Thanks
     
    Last edited: Jun 18, 2011
  2. jcsd
  3. Same phenomena except that the Zener breakdown process has a negative temperature coefficient so it's self-limiting thermally while avalanche has a positive temperature coefficient so it's not and it can be destructive because it causes self-heating over time which accelerates failure mechanisms (damage) of all sorts including but also going beyond damage caused by the avalanche current flow.

    The difference occurs based on voltage. Above a certain value the temperature coefficient changes from negative to positive. The doping will have some effect on this but it's also the balance of thermal conduction vs. IR losses.
     
  4. Thanks. Can you provide any link with more detail to it?
     
  5. Relevant to this post:

    If I control the voltage at which Zener diode works by doping, (effectively what is going on is quantum tunneling in reverse bias) how come zener diode doesn't behave like Tunnel diode in forward bias?

    Heavily doped p and n regions allow quantum tunneling to happen in reverse bias ergo I will have zener breakdown and not avalanche breakdown. But this is same for tunnel diode, what is the difference?
     
  6. Relevant to this post:

    If I control the voltage at which Zener diode works by doping, (effectively what is going on is quantum tunneling in reverse bias) how come zener diode doesn't behave like Tunnel diode in forward bias?

    Heavily doped p and n regions allow quantum tunneling to happen in reverse bias ergo I will have zener breakdown and not avalanche breakdown. But this is same for tunnel diode, what is the difference?
     
  7. dlgoff

    dlgoff 3,123
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    Check out the difference in their IV curves.

    [​IMG]

    [​IMG]

    http://hyperphysics.phy-astr.gsu.edu/hbase/solids/zener.html
     
  8. But that is the question. WHY don't they both behave same? Zener effect occurs because of Quantum Tunneling appearing. Very important to differentiate from avalanche breakdown.

    Tunnel diode uses quantum tunneling not only in inverse but in forward too. Both diodes are made by highly doping regions. How come they behave different ?
     
  9. Tunnel diodes are much more heavily doped than voltage reference diodes.

    I will see if I can post some Fermi diagrams later.
     
  10. dlgoff

    dlgoff 3,123
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    Like Studiot said.

    http://en.wikipedia.org/wiki/Tunnel_diode
     
  11. But in order to quantum tunneling to occur(in reverse bias) you need lots of doping, so that the depletion region is thin and fermi levels are in valence and conductance bands(degenerate semiconductors).
    This is, how I learned Zener diodes are made. Lots of doping, so that you have Zener breakdown(through quantum tunneling, and not avalanche effect). Is there any limit in doping,when this quantum tunneling starts dominate in forward bias too?
     
    Last edited: Jun 20, 2011
  12. dlgoff

    dlgoff 3,123
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    The more you dope the semiconductor the more it becomes like a conductor. So I would think there is a limit where conduction begins and tunneling stops. IMO
     
  13. Some facts & figures

    Normal PN junctions both materials doped at 1013 to 1017 impurity atoms per millilitre.

    In tunnel diodes both Pand N material doped at 1019 to 1020 impurity atoms per mL.

    Voltage reference diodes are doped differentially.

    Low reference voltages

    N material 1019 donor atoms per mL

    P material 1017 acceptor atoms per mL

    High (50V) reference voltages

    N material 1019 donor atoms per mL

    P material 1015 acceptor atoms per mL

    These different values change the relative positions of the valence & conduction bands and Fermi level. Note there are two Fermi different levels in the normal N and P material. These levels coalesce across the junction depletion zone to a single level.
     
  14. Hmmmm to me, these are very subtle changes in doping, between tunnel and zener diodes.

    So these small changes in doping can dictate how diode will behave in reverse bias? Just to confirm: If I have regular doping, regular diodes, they have avalanche breakdown, because depletion region is too wide for quantum tunneling to occour?

    Zener, or as you said voltage reference diodes, have significant more doping involved. Depletion region is thinner, and fermi levels are moved to conductance/valence band. But still these diodes behave just like normal ones do in forward bias.


    Tunnel diodes have degenerate doping, so much that the quantum tunneling appear both in reverse and in forward bias?

    I understand how this quantum tunneling works, quiet well. Can you confirm/correct this?

    Thank you
     
  15. Not mentioned simple fact

    Zener breakdown is a result of very high field intensity [voltage over the junction]
    which pulls the electrons right out of their shell.

    Avalanche breakdown results from kinetic electrons colliding with atoms and knocking them out of their outer shells.

    Zener process occurs first, then the Avalanche process.
    Not sure if this can be reversed.
     
  16. Seeing as you have exams soon I will post what I can.

    First installment


    My fig1

    we should start with the current voltage characteristic of a PN junction (diode).
    Points to note are that it may be forward biased (C-D-E), reverse bised (A-B-C) or zero biased.

    In Portions AB and DE of the curve the junction has broken down and is acting as a conductor. Note both sections look like the curve for a resistor.
    AB exhibits avalanche breakdown at sufficient reverse voltage, depending upon the doping.
    DE the forward bias is sufficient to boost electrons into the conduction band. this leaves holes in the valence band. Both contribute to conduction.



    My Fig2
    By increasing the doping of we can reduce the reverse bias point at which B occurs thus creating high value reference diodes.
    Further increasing the doping introduces a small tunnel effect as fig 3 creating lower voltage zenere diodes. The next installment will show that the zener and tunnel effects are very similar.

    MyFig3

    The tunnel diode I - V curve can be seen to be made up from a normal diode characteristic plus the tunnel effect which only occurs over a small range of about 0 - 1 volt. Since the normal diode has almost no conduction in this region the tunnel effect dominates here
    The doping is so high that the diode is in breakdown in both forward and revers biase so there is a resistor like line through the origin.
    then there is the characteristic tunnel hump which occurs as the forward bias first brings the conduction and valence bands into alignment, then drives them out of alighment again.

    It should be noted that only electrons are involved in tunneling. Holes play no part in this.
     

    Attached Files:

    Last edited: Jun 21, 2011
  17. @ Figure 2: Ultimately, increasing the doping, you move the point where avalanche effect is starting. Do voltage reference diodes work with avalanche or with tunnel effect in reverse bias, or in both?

    At some point in doping, you made the depletion region so small that the tunnel effect can occur? Those are Low voltage reference diodes? Ok from you post that makes sense.

    Isn't avalanche breakdown bad? Because it is not self limiting, and it can destroy the diode?

    Sorry if i repeated some statements of yours, I am trying to be precise.
     
  18. And I must say THANK you again Studiot. I really have a good knowledge of Electronic elements, just I have a lot of gaps which result in trivial questions like this. I am a fast learner but, you know, I have to be supplied with a pretty damn good explanation. I don't just accept things.

    Thank you for your time and effort for trying to explain things (that usually one would pay for) to a complete stranger.

    I must say that my knowledge of electronic elements and circuits wouldn't be as good if I wasn't active on this forum.

    Thank you all, especially Studiot. He is gem of this forum.
     
  19. That is why you employ a series current limiting resistor.
    Hopefully the answers to the others will become clear. I am trying to get several pages in.

    This attachment shows cross sections of an PN junction for normal doping and heavy doping and explains how tunneling can occur.

    The energy bands are at slightly lower levels in N material than P.
    The fermi levels are also different so there are two fermi levels, one in the bulk N and one in the bulk P, when they are separated.
    The first effect of creating a junction is to create a single unified fermi level.
    The second is to create a 'depletion zone' .




    First sketch normally a doped PN junction

    There is an energy gap that needs to be overcome between the highest valence level in the P and the lowest conduction level in the N. This is why the junction needs a forward bias to operate.
    The 'depletion zone' is about 1000 nanometres wide.

    Second sketch show what happens when heavy doping add carriers to the material.

    Firstly the depletion zone squashes up by up to 100 times in size.
    Secondly the energy levels are brought together so the forbidden zone disappears and the bands overlap.
    The fermi level cuts both the valence band of the P material and the conduction band of the N material. This allows electrons to 'tunnel through'.

    Note there are more conditions to be met which will appear in the next post.
     

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