Figure 1. Schematic diagram of STNMOS.
This type of NMOS injects a portion of the current into the substrate of the Protection NMOS to assist in the activation of the parasitic NPN transistor in the Protection NMOS (the NPN transistor present in the NMOS has been discussed in the previous sections). The body trigger is to inject the maintenance current Ih into the base stage of the parasitic triode, so that the triode can be turned on without the formation of an avalanche breakdown between the drain and the substrate, and the trigger voltage can also be reduced. And because there is no need to drain ESD current from the channel, STNMOS itself is more robust than GCNMOS.
相对于Protection NMOS而言，Substrate Triggering NMOS的尺寸要小很多，这样确保了两者的开启先后顺序。 两者的一次击穿电压相同，但是Substrate Triggering NMOS的trigger voltage要小于Protection NMOS的trigger voltage，确保先于Protection NMOS开启。 然后进入holding状态将维持电流Ih注入Protection NMOS的衬底中，促使其直接进入导通状态。 Substrate Triggering NMOS的TLP特性决定了Protection NMOS的TLP特性。
Because the bulk parasitic transistor does not conflict with the channel, the body trigger is combined with the channel conduction. The result is Gate substrate triggering NMOS, as shown in Figure 2.
图二. Gate-Substrate triggering NMOS
The principle of this ESD protection circuit is to use both a channel and a parasitic transistor as a bleeding channel. The maintenance voltage Vh of GCNMOS opens the channel of the protection NMOS, and the Substrate Triggering NMOS injects the maintenance current Ih into the substrate to open the transistor. This greatly improves the conduction efficiency of the Protection NMOS. It is equivalent to one NMOS and one NPN performing ESD venting at the same time. At the same time, the potential of the substrate can also reduce the threshold voltage of NMOS, which is more conducive to turn-on.
图三.多指"软"镇流电阻GGNMOS。
In this structure, the maintenance voltage vh after the snap-back of GGNMOS is used as the channel opening voltage of the next-stage NMOS. When the outermost GGNMOS is preferentially turned on, the NMOS channel of the first pole after the voltage is turned on is maintained, and so on to form a chain ESD protection structure, and the resistance in the structure is the ballast resistor, which can make the current evenly distributed (the technical details of this have been discussed later).
Figure 4: Domino's ESD protection structure.
This structure is also one of the chained ESD protections, which uses the differential pressure of the source to turn on the next stage of GCNMOS. Because of the skin effect of the circuit, this chain structure must be the outermost NMOS turned on first, and then the outermost NMOS generates a voltage, which is opened from the outside and the inside, just like dominoes, and then continues to open the next one. The resistor at the source side acts as a voltage divider, converting the ESD current into voltage to form GCNMOS.
Although the circuit structure of GCNMOS is relatively simple, there will be different lay out methods for different applications, and the layout of the track for ESD protection should be designed separately for different application scenarios