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calculated by using the formula which follows from the Gauss law: 

where σ$_{+}$ is the areal density of the net positive charge at z < z$_{0}$ . 

<img src='content_image/330993.jpg'>

<img src='content_image/330997.jpg'>

For our discussion below we need to estimate the electric field and other related quantities on a qualitative level. This can be done by using the ultrarelativistic limit, V ≫ m$_{e}$ , which is justified everywhere except of the asymptotic region at z → ∞ . Taking for simplicity the limit a → 0, we get the exact solution of the Poisson equation (2)[27] 

where 

<img src='content_image/330998.jpg'>

<img src='content_image/330999.jpg'>

The constants c$_{1}$ = 0 . 2374 and c$_{2}$ = 1 / (1 − c$_{1}$ ) = 1 . 3113 are obtained from the continuity conditions for V ( z ) and dV/dz at z = z$_{0}$ . It is interesting that the decay length λ is determined by the electron Fermi momentum in the bulk of the medium k$_{F}$$_{0}$ . One can see that the potential changes much faster at z < z$_{0}$ than at z > z$_{0}$ . The electric field can be 

