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The title says it all: What does this Mosfet Symbol mean (two diodes facing each other at Vgs)? I know its an N-MOS, but I don't understand the meaning/purpose of the two diodes.

Part: AO7404

Datasheet Link: http://www.aosmd.com/res/data_sheets/AO7404.pdf

Picture of the symbol (highlighted part):

enter image description here

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It is an ESD clamp, showing that the gate is ESD protected. Typically the back to back diodes are shown as Zeners. This scheme protects against positive and negative ESD hits.

  • In the event of a positive ESD event, the top diode is forward biased, and the bottom diode's reverse breakdown voltage will be exceeded. The ESD is shunted to the source, which is often ground.

  • In the event of a negative ESD event, the top diode's reverse breakdown voltage is exceeded, and the bottom diode is forward biased. The ESD is again shunted to the source.

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Looks little confusing . After looking that the datasheet is from a Chinese supplier one can guess it. edit: That the zener symbol is not correct. enter image description here

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    \$\begingroup\$ That doesn't really answer the question. \$\endgroup\$ – Matt Young May 17 '14 at 16:35
  • \$\begingroup\$ I thought it migth be clear now as you have already explained it nicely... \$\endgroup\$ – Adi May 17 '14 at 16:36
  • \$\begingroup\$ @Adi it's a valid contribution \$\endgroup\$ – placeholder May 17 '14 at 16:53
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    \$\begingroup\$ Thanks dear placeholder , In the past few years I have dealt with lots of chinese suppliers and they do lots of mistakes in making specs. \$\endgroup\$ – Adi May 17 '14 at 17:06
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For anyone else looking for more information on this topic, this type of component is often called a transient-voltage-suppresion (TVS) diode and helps reject ESD as @Matt Young described.

Another example of its use can be found in the schematic of the u-blox C94-M8P rev B evaluation board. Note that the TVS diodes are protecting header pins that can be touched by the user.

C94-M8P-B Schematic

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