How does the zener diode protect IC from negative discharges

[faisal78]

ESD and Zener diodes

All,
Help me out here and educate me.

The typical ESD protection to an IC would be using a (monopolar) zener diode. It is pretty straight foward that the high voltage caused by a positive high voltage discharge (e.g. +15kV) ESD would be clamped down to Vz (e.g. 5.6v) and excessive high current would be discharged to ground through the zener during this time.

The question that has been bugging me is, how does the zener protect and IC from a negative discharge, -15kV maybe?

Let me know.... :?:

Just recently I also just found out there is such thing as a bi-directional zener diodes, but not as widely used as monopolar ones.

 

[DoctorProf]

Re: ESD and Zener diodes

A diode has a forward and reverse characteristic. The method you describe is the reverse characteristic. The voltage across the diode increase until the zener voltage is reached and the zener clamps the voltage to the zener voltage + I*R based on its internal resistance up to the point it is destroyed by ESD.

A zap of the opposite polarity will put the zener diode in forward conduction. Here, the voltage will be much lower and the protection capability much higher. The forward votlage drop is ~0.7 volts + I*R. The volume where the energy is dissipated is much larger in the forward conduction mode rather than in the reverse conduction mode.

Hope this helps.

 

[Colbhaidh]

Re: ESD and Zener diodes

Typical IC processing allows each input to be attached to an N+ to PWell diode to ground and a P+ to NWell diode to VCC. These rae typically large fingered diodes. This takes care of +/- zaps. Note that for 0.18um and below, modern ESD protection is lucky to reach 4kV - sometimes it can only protect to 2kV depending on the application.