LOW VOLTAGE SILICON CONTROLLED RECTIFIER STRUCTURE FOR ESD INPUT PAD PROTECTION IN CMOS IC'S

An electrostatic discharge (ESD) protection structure for protection of a circuit to which an operation voltage is to be applied, comprising a silicon controlled rectifier (SCR) connected between ground and a pad of the circuit to be protected, the SCR including a resistor apparatus connected to the pad for controlling the breakdown voltage of the SCR, and apparatus for controlling the resistor apparatus to a high resistance value in the absence of the application of the operation voltage whereby the SCR is controlled to break down at a low ESD voltage which is lower than a circuit damaging voltage, and to be of low resistance value upon the application of the operation voltage whereby the SCR is controlled to break down at an ESD voltage which is higher than the low ESD voltage.