As we are told in the books，a diode can be conductive by a forward-bias of about 0.7v. Thus,current through the n region is formed by the conduction electrons（free electrons in the conduction band） toward the junction.But in the p region,current is formed by the valence electrons from holes to holes toward the positive side of the power source.Obviously,the conduction electrons from the n region can easily become the valence electrons in the p region and the valence electrons in the p region can easily become conduction electrons so they can move into the copper cord（as we suggest）which is between the positive side of the power source and the p region. So here is the question：why a diode can not be conductive by a reverse-bias voltage? Can the conduction electrons in the copper cord move into the p region? If not,why?