Gate material of MOSFET

Nickel Titanium Alloy News 2021-06-24 10:50 81
The gate material of

MOSFET

in theory, the gate of MOSFET should choose the conductor with good electrical properties as much as possible. The conductivity of polysilicon after heavily doped can be used in the gate of MOSFET, but it is not a perfect choice. The reasons for using polysilicon in MOSFET are as follows:

1. The critical voltage of MOSFET is mainly determined by the difference between the work function of gate and channel materials. Because polysilicon is semiconductor in nature, its work function can be changed by doping impurities of different polarity. More importantly, because the band gap between the polysilicon and the underlying silicon as the channel is the same, when reducing the critical voltage of PMOS or NMOS, the demand can be achieved by directly adjusting the work function of polysilicon. On the other hand, the work function of metal materials is not as easy to change as that of semiconductors, so it is difficult to reduce the critical voltage of MOSFET. If we want to reduce the critical voltage of PMOS and NMOS at the same time, we need two different metals as gate materials, which is a big variable for the process

After years of research, it has been proved that there are relatively few defects between the two materials. On the contrary, there are many defects in the metal insulator interface, which can easily form many surface energy levels between the two, greatly affecting the characteristics of the components

The melting point of polycrystalline silicon is higher than that of most metals. In modern semiconductor process, it is customary to deposit gate materials at high temperature to improve the performance of devices. Low melting point of metal will affect the upper temperature limit of the process

However, although polysilicon has been the standard for manufacturing MOSFET gate in the past two decades, there are still some disadvantages that make it possible for some MOSFET to use metal gate in the future. These disadvantages are as follows:

1. The conductivity of polysilicon is not as good as that of metal, which limits the speed of signal transmission. Although the conductivity can be improved by doping, the effect is still limited. Some metal materials with high melting point, such as tungsten, titanium, cobalt or nickel, are used to make alloys with polysilicon. This kind of mixed material is usually called metal silicide. The polysilicon gate with metal silicide has good conductivity and high temperature resistance. In addition, because the metal silicide is located on the gate surface and far away from the channel region, the critical voltage of MOSFET will not be greatly affected

The process of metal silicide plating on gate, source and drain is called self aligned silicide process, which is usually referred to as salicide process

When the size of MOSFET shrinks to a very small size and the gate oxide layer becomes very thin, for example, the oxide layer can be reduced to a thickness of about one nanometer in the current process, and a phenomenon that has not been found in the past also occurs, which is called \
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