Background of High-k Material

    Owing to the rapid development of technology, the transistor density continues with Moore's law, which means the transistor is scaling aggressively. 

    In order to have a higher gate capacitance, which will lead to a higher drive current and better performance, the thickness of the oxide layer is decreased. Today, the thickness of SiO₂ layer is reduced to 1.2nm,  the size of 4~5 atoms, which is hundredths of 30 years ago [1]. 

    However, it still cannot meet the human's demand of smaller devices. As the thickness of the gate oxide below 2 nm, leakage currents due to tunneling increase drastically, which will lead to much higher power consumption and reduce device reliability [2]. Also the effect of Boron penetration should not be ignored. Boron penetration is the penetration of the gate oxide by boron of heavily doped p-type substrate, which will impact the gate oxide reliability and device lifetime [3].

    An alternate way of increasing gate capacitance is replacing the silicon dioxide gate with a material which has a higher dielectric constant (a.k.a high-k material), as indicated in the formula below:

    (1)

    With the use of high-k material, a higher capacitance with the same oxide thickness will be produced, but causing a much smaller leakage current.

Reference:
[1] D. Misra, H. Iwai and H. Wong, “High-k Gate Dielectrics,” The Electrochemical Society Interface, pp. 30-34, 2006. 
[2] H. R. Huff and D. C. Gilmer, High dielectric constant materials, Berlin: Springer-Verlag, 2005.
[3] B.Kim, et al. "Impact of boron penetration on gate oxide reliability and device lifetime in p+-poly PMOSFETs," 1997.