1. Field of the Invention
The invention relates to a semiconductor device, and more particularly, to a semiconductor device for electrostatic discharge (hereinafter abbreviated as ESD) protection.
2. Description of the Prior Art
Chips and/or Integrated circuits (ICs) are the most essential hardware foundation of the modern information society. As products based on ICs become more delicate, they also become more vulnerable to impacts from external environment. For example, it is found that ESD is a constant threat to modern electronics.
Generally considered ESD has two modes: human body mode (HBM) and machine mode (MM). In the human body mode, static electricity is discharged from human body to a semiconductor device when a user touches a body part of the semiconductor device. Static electricity discharged from a human body to a device typically has a high voltage of about 2000V and is discharged through great impedance. Another source of ESD is from metallic objects, known as the machine mode, is characterized by a greater capacitance and lower internal resistance than the HBM ESD source. The result of ESD on unprotected ICs is often destruction, characterized by melting or explosion of a part of the ICs. Therefore, the ESD protection devices are taken as important components of the protection circuitry provided in today's electronic devices.
Various approaches have been made to provide ESD protection device associated with IC interface pads to prevent a core circuit of the ICs from the ESD damages. Typically, during a normal IC operation, the ESD protection device is turned off. However when an ESD event occurs, the ESD surges will cause the ESD protection device to breakdown and create a substrate current path, through which the ESD current is diverted to ground and thus the core circuit is protected. Therefore, it is conceivable that if the ESD protection device can be quickly triggered, it will be better to bypass the ESD current from the core circuit.
There is therefore a continuing need in the semiconductor processing art to develop an ESD protection device having lower trigger voltage which can be quickly turned on in order to render immediate protection to the core circuit. Furthermore, it is found that the trigger voltage of the ESD protection device is related to the substrate resistance (hereinafter abbreviated as Rsub) : Small Rsub leads to a higher trigger voltage which is undesirable to the ESD protection device. In view of the above, there exists a need for the ESD protection device having increased Rsub that results in reduced trigger voltage.