Effects of the gate-to-drain/source overlap on MOSFET characteristics

The characteristics of MOSFET's with different degrees of gate-to-drain overlap are studied. It is found that there exists a critical length of overlap below which the device hot-electron reliability will suffer. Since a few hundred angstroms change in the overlap length can cause the transition from good to poor reliability, devices designed for minimum gate-to-drain overlap might exhibit grossly nonuniform characteristics as a result of minor variations in the overlap structure across a wafer, such as that due to tilted implant. On the other hand, devices with larger overlap have a higher gate-to-drain/source capacitance. Therefore, there exists only a narrow margin within which an optimal compromise between device performance and characteristics can be achieved. This margin will shrink further as device dimensions are scaled down.