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    p-type guard ring, ic design

    hi guys
    can any one tell me what guard rings are and how they work..
    can someone also tell me how they are used to prevent Latchup in cmos circuits
    regards
    rogger123

    •   Alt22nd October 2004, 16:17

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    Re: guard rings

    They were originally used in analog circuits made on PC boards. They surround an area where surface currents are a potential problem. By putting a potential on the ring that is equal to the potential of the circuit portion inside there is a zero potential difference across the PC material and so no leakage currents flow across the PC board surface.


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    Re: guard rings

    hi flatulent,
    does the same concept apply when they are used on the substrate and well for cmos layout ?
    what we basically do is have a p-type diffusion on a p-type substarte and then a p-type ring around this diffusion(is my undrstanding correct?).
    but how will this help in
    a) reducing the substarte resistance
    b) taking care of injected minority carriers



    •   Alt23rd October 2004, 05:08

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    Re: guard rings

    I would expect the P diffusion is of much higher doping density than the substrate. I am not familiar with the details of this structure in MOS. Is some potential applied to these extra diffusions? The higher doping density would cause any stray n carriers to be combined more quickly than they would in the substrate and therefore be less likely to travel a long distance and cause problems at the end of their longer travels.

    I have not done any IC design in 30 years. Back then it was all bipolar and each BJT was in its own area surrounded by the opposite polarity diffusions and substrates.



    •   Alt23rd October 2004, 05:42

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    Re: guard rings

    hi flatulent,

    yes a potential is applied. for a n+ it is connected to Vdd and for a p+ it is connected to Vss.



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    Re: guard rings

    hi flatulent,
    they also have another configuration. where a n+ diffusion is used within a p+ guard ring on a p+ substrate. the p+ guard ring is then tied to Vss. the opposite applies for n+ guard ring. now would it function in a similar manner as the previous configuration which just uses a p+ guard ring on a p+ substrate without any diffusion?



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    Re: guard rings

    It is difficult to guess without more construction details. The reverse biased pn junction may use the depletion region extending into the substrate to block stray carriers. It also may be used to reduce capacitive and conductive coupling between parts of the circuit.



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    Re: guard rings

    hi

    if u see a high performance circuits, a lot of devices will carry a huge current which disturbs other nearby devices. so we protect it by putting a guard ring, so that it will not affect other devices.



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    guard rings

    simply, guard ring can be used to keep signal integrity.
    VDD VSS line, or sandwitch of them can be applied for it



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