Electro-static discharge protection devices having a low trigger voltage

1. An electro-static discharge (ESD) protection device comprising:

a first PN diode coupled between a pad and a first node;

a second PN diode coupled between a ground voltage and the first node to provide a first discharge current path between the pad and the ground voltage; and

a silicon controlled rectifier (SCR) coupled between the pad and the ground voltage to provide a second discharge current path between the pad and the ground voltage, and

wherein the first node is directly coupled to the SCR.

2. The ESD protection device of claim 1 ,

wherein an anode of the first PN diode is connected to the pad,

wherein a cathode of the second PN diode is connected to the ground voltage, and

wherein a cathode of the first PN diode is electrically connected to an anode of the second PN diode through the first node.

3. The ESD protection device of claim 2 , wherein the SCR includes:

an NPN bipolar junction transistor having a collector connected to the first node, a base connected to a second node, and an emitter connected to the ground voltage; and

a PNP bipolar junction transistor having an emitter connected to the pad, a base connected to the first node, and a collector connected to the second node.

4. The ESD protection device of claim 3 , further comprising a resistive element coupled between the first and second nodes.

5. The ESD protection device of claim 1 , further comprising a third PN diode and a fourth PN diode that are coupled in series between the pad and the ground voltage to provide a third discharge current path.

6. The ESD protection device of claim 5 ,

wherein an anode of the third PN diode is connected to the pad,

wherein a cathode of the fourth PN diode is connected to the ground voltage, and

wherein a cathode of the third PN diode is electrically connected to an anode of the fourth PN diode through the first node.