For getting the electric field in this case we use the Gauss's law. we get the equation
$$ 2EA = pA/\epsilon_0 $$
- $E$ is electric field,
- $A$ is the cross sectional area,
- $p$ is the uniform surface charged density,
- $\epsilon_0$ is permittivity of the vacuum.
left hand side of the equation is understandable but in the right hand side of the equation it is $pA$, why it is not $2pA$? since infinite sheet has two side by side surfaces for which the electric field has value.