ESD may cause damage to sensitive electronic devices. As such, ESD protection is generally provided to the semiconductor structures. Semiconductor controlled rectifier (SCR) is a widely used ESD protection structure for the semiconductor structures such as I/O pads, high voltage pads or the like. However, a typical SCR has a trigger voltage that may be too high for the ESD protection application since an avalanche breakdown occurs at the N/P junction before the SCR is turned on. As such, the research and improvement of the ESD protection application have been conducted to this day.