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Metal gate
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A metal gate, in the context of a lateral Metal-Oxide-Semiconductor MOS stack, is just that--the gate material is made from a metal.
For decades, the industry had moved away from metal as the gate material in the MOS stack due to fabrication complications. A material called Polysilicon (silicon, highly doped with donors or acceptors) was used instead because it can be deposited easily and is tolerant to subsequent manufacturing steps which involve extremely high temperatures (in excess of 900-1000 degrees Celsius), where metal was not. Furthermore, metal has a tendency to disperse into silicon during these Thermal Annealing steps[1].
However, Polysilicon doesn't offer the near-zero electrical resistance of metals, and is therefor not ideal for charging and discharging the gate capacitance of the transistor. As described in the 2006 Update of the International Technology Roadmap for Semiconductors (ITRS), manufacturers will have to begin to implement metal gates on CMOS transistors once again, in conjunction with High-K dielectrics[2]. This is listed as a near-term obstacle, and they stress it must be implemented in a "timely" manner, for the 45-nm node. Details on exactly how this is being done, and exactly what metal is being used, are considered trade secrets, and may not be known for some time.
- ^ ref needed
- ^ The International Technology Roadmap for Semiconductors, "Process Integration, Devices & Structures", page=6