TABLE I: Maximum electric field and areal charge density calculated for several values of the diffuseness parameter a . a E max [ MV / fm ] E$_{max}$ / E$_{c}$ σ$_{+}$ [ e/fm $^{2}$] 1 fm 1 . $^{82}$· 10 − 0 6 . $^{89}$· 10 +2 1 . $^{01}$· 10 − 1 10 fm 1 . $^{16}$· 10 − 0 4 . $^{38}$· 10 +2 6 . $^{41}$· 10 − 2 100 fm 2 . $^{05}$· 10 − 1 7 . $^{74}$· 10 +1 1 . $^{13}$· 10 − 2 1 pm 2 . $^{08}$· 10 − 2 7 . $^{86}$· 10 0 1 . $^{18}$· 10 − 3 10 pm 2 . $^{08}$· 10 − 3 7 . $^{86}$· 10 − 1 1 . $^{18}$· 10 − 4 100 pm 2 . $^{08}$· 10 − 4 7 . $^{86}$· 10 − 2 1 . $^{18}$· 10 − 5 1 nm 2 . $^{08}$· 10 − 5 7 . $^{87}$· 10 − 3 1 . $^{25}$· 10 − 6 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$_{0}$ . 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] E$_{max}$ ≡ E( z = z$_{0}$ ) = e z 0 ∫ −∞ ρ$_{ch}$ ( z ) dz = σ$_{+}$ , (9) V ( z ) =     V$_{0}$ [ $^{1}$− c$_{1}$ exp ( z − z$_{0}$ λ )] , z < z$_{0}$ V$_{0}$ c$_{2}$ + ( z − z$_{0}$ ) √$_{6}$$_{λ}$ ] − 1 , z > z$_{0}$ (10) λ = π eV$_{0}$ ≈ π ek$_{F}$$_{0}$ . (11) where 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 7