Graded-Junction Gate/n⁻ Overlapped LDD MOSFET Structures for High Hot-Carrier Reliability

A newly developed gate/n⁻ overlapped LDD MOSFET has been investigated. The MOSFET was fabricated by an oblique rotating ion implantation technique. A new formula of impurity ion profile was derived to analyze the lowering of substrate current and improvement of the degradation caused by hot-carrier effect of the MOSFET. As the results, it was proved that impurity ion profile near the drain edge is remarkably graded in the direction both along channel and toward substrate even just after the implantation, so that maximum lateral electric field is remarkably relaxed as compared with conventional LDD MOSFET. Also, the maximum point of lateral electric field at the drain edge is located apart from the main path of channel current. Moreover, the maximum point of lateral electric field at the drain edge is located under the gate electrode far from the gate edge and deep in the substrate. This tendency turned out to be promoted by an increase of both the oblique angle and the energy of implanted ions.