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Three questions about ESD

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refugee

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hi guys,
i have some doubts about esd, would you plz be so kind to help me out.
1. which one is more sensitive to esd? oxide or diffusion
2. why we need to protect the drain of nmos to vin, but not the source of pmos?
3. when using ggnmos, suppose there is an zap from input pin to ground, which mechanic is occurred? does parasitic bjt of the nmos discharge the esd charge or the nmos has been turned on? ( because we usually add a resistor between the gate and ground)
 

protonixs

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Re: doubts about esd

refugee said:
2. why we need to protect the drain of nmos to vin, but not the source of pmos?

We need to protect the drain against esd because it is connected to the outside world (pin). The source is connected to power lines that is why there is no need to put esd protection (power lines already have esd protection). Everything that is connected to the outside world should have esd protection and not nmos alone as you have mentioned.
 

deepak242003

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Re: doubts about esd

Although I am yet to get expertie in ESD's I have tried to clear your doubts.. I am open for any discussion.


refugee said:
1. which one is more sensitive to esd? oxide or diffusion
I believe thin layer oxide ( gate oxide is more sensitive with less breakdown voltage
refugee said:
2. why we need to protect the drain of nmos to vin, but not the source of pmos?
Because drain terminal is directly connected to pads . so we need to put protection there in form of balasting resistor or silicide blocking.. If this protectoin is not there,Gate to source voltage would have blown your gate oxide.
 

mikersia

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doubts about esd

1. Breakdown in oxide typically means crush of device, while breakdown of diffusion could be recoverable, depending on current density. So oxide breakdown is more sensitive.

2. It isn't clear what did you mean? If you mean difference in protection efforts regarding NMOS and PMOS devices, than is is explained by fact that NMOS have snapback behaviour, taking about all disharge current through themself and going to thermal fault first.

3. Mechanism of current discharge in that case is defined by design of protection structure.Typically parasitic NPN bjt will conduct most of current. NMOS turning-on will provide earliest switching of NPN.
 

refugee

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Re: doubts about esd

mikersia said:
1. Breakdown in oxide typically means crush of device, while breakdown of diffusion could be recoverable, depending on current density. So oxide breakdown is more sensitive.

2. It isn't clear what did you mean? If you mean difference in protection efforts regarding NMOS and PMOS devices, than is is explained by fact that NMOS have snapback behaviour, taking about all disharge current through themself and going to thermal fault first.

3. Mechanism of current discharge in that case is defined by design of protection structure.Typically parasitic NPN bjt will conduct most of current. NMOS turning-on will provide earliest switching of NPN.

Please forgive me for not explained clearly

I have upload a diagram to explain my sencond question.
Both pmos and nmos are connect to the Vin pin. And also the Vin pin have been protected. The nmos drain which connected to the this power supply is usually inserted by a res to prevent from esd.
So, I don't know why the nmos should be protected, but not the pmos
 

mikersia

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doubts about esd

During ESD event we have to use designated protection structure to conduct almost all discharge current. All other structures shouldn't assist. PMOS doesn't have practically snapback behaviour,- so it can't to take all current themself. But another NMOS could, and it will be broken, if current will not be limited by resistor.
 

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