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MOS snapback confusion

Discussion in 'Electronic Design' started by Roger Bourne, Dec 5, 2008.

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  1. Roger Bourne

    Roger Bourne Guest

    Hello all,

    Lately, I've been going over esd protection schemes. Many ESD
    protection are referred to as *snap* type or *snapback*. Looking into
    the snapback caracteristics of a MOS device, I found myself extremely
    confused. (There a very helpful thesis link for this topic which I
    obtained by an earlier post. I forgot which post).

    This is the snapack I-V of NMOS (view in fixed font):


    (Ids)
    /
    /
    /
    /
    /
    ----------- <--2nd knee
    \
    /
    /
    /
    ------------------------ <-- 1st knee
    Vsb Vbd
    (Vds)

    If the the ESD protection if of *snap* type (esd protection consists
    only of grounded nmoses) does it mean (according to the above diagram)
    that the device is protected by the fact that an ESD event is expected
    to place the ESD nmos to the snapback voltage?
    In other words, the current will increase causing the vds too also
    increase until it reaches the part in the curve where there is
    negative resistance (2nd knee), at which point the Vds will snap back
    to the Vsb voltage. Hence, during an ESD event the pad voltage will be
    pulled back to Vsb.

    Won't that place the device in a state from which it cannot exit ?
    I mean that if the ESD current discharge stops when the mos device is
    beyond the 2nd knee, won't the nmos be stuck at at voltage below Vbd?
    I have trouble imagining that the Vds voltage will increase as the
    current lowers in order for the nmos to go back to the origin of the I-
    V curve.

    Also, in order to reach the 1st knee of the curve, won't the esd mos
    allow voltages to pass that are above the oxide breakdown voltage
    (BVox), thus causing irreparable harm to any gates connected to that
    pad ?

    Any help will be appreciated.

    Thank you
    -Roger
     
  2. Guest

    Are you assuming that the ESD event occurs when there is a power
    supply also connected to the pin? Otherwise it is pretty clear how
    the current and hence voltage will return to zero after the ESD, when
    the stored energy has all been dissipated.

    In general (perhaps unlike equipment) chips themselves are not ESD
    tested with any power supplies attached, so the chip vendors will not
    see the problem of the ESD cell staying latched on, even if it really
    could occur in an application. (Separate latch-up testing is done,
    but the peak current is much lower in those tests.) I have even heard
    of SCRs used for ESD protection of power rails, which allows good
    robustness when the device is ESD tested without power supplies
    attached, but which might be entertaining if there is a low-resistance
    battery attached (using leads with some inductance so that the ESD
    voltage can still trigger the SCR).

    Regarding the drain voltage being too high for the device during the
    ESD event, I think that the snapback is not triggered by oxide
    breakdown but more like some kind of parasitic bipolar device caused
    by the drain and source n-type diffusions and the p-type substrate
    between them. I think that some method of biasing up the gate from
    the ESD pulse is sometimes used, and somehow this helps the snapback
    to start, but I don't know.

    Certainly the voltage on the drain can still reach values that could
    damage the gates of small FETs connected to those pins, and it is
    normal to interpose secondary ESD protection consisting of some series
    resistance and smaller clamp diodes or similar wherever this does not
    reduce the circuit performance too much. The secondary ESD protection
    does not have to be made of such big devices because the current has
    mostly been diverted by the main ESD cell.

    You could probably find out quite a lot about ESD protection by
    reading patents because all of the most obvious solutions have been
    patented by now, and for many companies, the choice of which structure
    to use is probably dictated by avoiding the remaining un-expired
    patents.

    Chris
     
  3. Roger Bourne

    Roger Bourne Guest

    Hello,

    Thank you for your reply.
    What concerns me is that snapback will begin at voltage that exceeds
    the oxide breakdown voltage. Like you mentioned, snapback is caused by
    the parasitic bipolar device of the nmos and does not seem depend on
    the oxide breakdown. This is why (from an ESD-novice point of view) a
    classic clamping mechanism to the power rails seems to be offering a
    more reliable protection to the mos gates - even though it limits the
    input voltage range to -0.5V to Vcc+0.5V. But I must be missing
    something otherwise these ESD snapback ccts would not be used,

    -Roger
     
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