The first contribution is due to the applied electric field, the second is due to the electrostatic attraction between the ionic trap site and the conduction electron. The potential has now a maximum at a distance r 0 from the Coulomb trap center, given by q 2 / ( 4 π π ϵ ϵ r 0 2 ) = q E . Therefore r 0 = q / ( 4 π π ϵ ϵ E ) and