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单词 Fick’s law
释义
Fick’s law

Chemistry
  • A law describing the diffusion that occurs when solutions of different concentrations come into contact, with molecules moving from regions of higher concentration to regions of lower concentration. Fick’s law states that the rate of diffusion dn/dt, called the diffusive flux and denoted J, across an area A is given by: dn/dt = J = –DA∂‎c/∂‎x, where D is a constant called the diffusion constant, ∂‎c/∂‎x is the concentration gradient of the solute, and dn/dt is the amount of solute crossing the area A per unit time. D is constant for a specific solute and solvent at a specific temperature. Fick’s law was formulated by the German physiologist Adolf Eugen Fick (1829–1901) in 1855.


Electronics and Electrical Engineering
  • If a concentration gradient of mobile particles exists, there will be a flow of such particles – diffusion – from the region of high concentration to the region of low concentration:

    f=D(Nx)

    where f is the particle flux through a plane parallel to the surface, in numbers per square metre per second. D is a constant of proportionality – the diffusion constant – and N/x is the concentration gradient.

    Since N is also a function of time during the diffusion process, Fick’s second law can be derived from the first law and is given as:

    Nt=D(2Nx2)

    Diffusion also takes place in the y- and z-directions, parallel to the plane of the surface, as soon as a concentration gradient is established below the surface. Diffusion flow will continue until the concentration becomes uniform across the specimen.

    This effect is utilized in producing a desired impurity profile in a particular specimen of semiconductor. At normal temperatures the impurity atoms are immobile until heated to a high temperature. A concentration gradient is formed by heating the semiconductor wafer in a gaseous atmosphere of the impurity atoms so that a high concentration exists at the surface. Under such conditions the impurity atoms diffuse into the semiconductor. In practice the diffusion process is stopped by reducing the temperature when the desired profile has been achieved.


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