A research report exploring the changes in MOSFETs' gate electrode material throughout time, as the final research project for both Research Methodologies and AVLSI course, supervised by Prof. Asadollah Shahbahrami and Dr. Mahdi Aminian at the University of Guilan, Department of Computer Engineering. Winter 2024.
As MOSFET transistors were introduced, their gate electrodes were made of aluminum. However, in the late 1970s, aluminum was replaced with polysilicon. This change was made to reduce the overlap between the source/drain and the gate, and to lower the threshold voltage. For around 40 years, polysilicon was the preferred gate material in MOSFETs, but as transistors continued to shrink, several issues emerged. These issues included unwanted capacitance between active regions and the gate, velocity saturation and mobility degradation, and gate leakage. Various solutions were developed to tackle these problems. Unwanted capacitance between the active regions and the gate was addressed by switching from metal gates to polysilicon. Velocity saturation was mitigated using strained silicon. Finally, to deal with gate-channel leakage, the industry saw returning to metal gates as the solution. In 2007, Intel led the way by reintroducing metal gates in their 40nm and 45nm processors. They also introduced High-K Metal Gate (HKMG) technology in CMOS devices. In this approach, the insulating layer was made from high-k materials instead of polysilicon, and the gate material was changed to metal. This research report delves into the reasons for these changes and explores some of the other challenges related to the scaling of MOSFETs.
Transistor, MOSFET, High-K materials, Silicon dioxide, Metal gate, Polysilicon, HKMG, CMOS
- not published
- language: persian