Thin- and thick-layer methods

In the thin-layer method, the unipolar current produced by N charges starting at one electrode is given as 

In the thick-layer method the current decreases linearly in time, since charges become neutralized at the electrode. For electronic conduction the current density is given as 

Here jo denotes the initial current density at t = 0. If positive and negative charge carriers are present, each kind leads to a linear decay of the current density according to Equation 39 and the total current density is given as 

If i+ i, jtot(t) consists of two linear decays superimposed on each other. The 

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