Advanced Semiconductor Laboratory
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Alternative current in conductor

Kuang-Hui Li

October 26, 2016 - Posted in Discussion
​​When one applies alternative current on a conductor, most of the current will flow through a thin layer near the surface instead of through the whole conductor. This phenomenon is called skin effect, and that is exactly the reason why power cable contains copper fiber bundle , as shown in FIG. 1.
Power cable cross section.jpg

FIG. 1

How deep is this thin layer? Based on classical electromagnetics, the wave number is a complex number when EM wave travels within a conductor. The imaginary part, κ ,is


, where ω is the frequency of EM wave, ϵ is permittivity of the conductor, μ is permeability of the conductor, σ is conductivity of the conductor. Physicists define skin depth 1/κ. By definition, skin depth is a function of frequency once conductor is fixed. Skin depth is decreasing when the frequency is increasing.

However, what if the conductor is a thin layer? Especially, when one applies low frequency (radio frequency) which makes skin depth longer, skin depth might be at the same scale of thin conducting layer.

Why is this question interesting? For MOCVD system, the heating module is either resistive heater or induction heater. The resistive heater heats susceptor by thermal radiation, but with low efficient. Induction heater heats susceptor by inducing Eddy current on the susceptor, and with high efficient. However, since skin depth is only few millimeter, the temperature distribution of susceptor might not be as uniform as a susceptor heating by the resistive heater.

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