There are three competing factors for which the ionisation potential of sulphur lower than phosphorus:
1. The effective nuclear charge
2. The size of the atom and the force of attraction according to Coulomb's law
3. A pair of electrons in an orbital
In case of phosphorous and sulfur, phosphorous has the greater ionization energy which means that it takes more energy to remove an electron from a phosphorous atom, which means that the electrons are lower in energy, even though the atom is larger and sulfur has a greater effective nuclear charge.
This is what we call as stable half filled configuration of phosphorus.