High leakage currents of ultrashallow junctions formed by B diffusion out of solid-phase epitaxially grown CoSi2 contacts grown from Co/Ti bilayers on Si are explained by the Shannon contact model. Depending on implant condition, the diodes behave either as p-n diodes or Schottky diodes with barrier height enhanced by the p-type diffusion. Diodes implanted with B-11(+) at 7.5 keV and 10(15) cm(-2) dose show leakage current near 100 nA/cm(2) at -5 V and behave like ideal p-n junctions after a 900 degrees C, 30 s postimplant anneal. Diodes implanted at 3.5 keV with 10(15) cm(-2) dose or at 7.5 keV with a 10(14) cm(-2) dose display higher leakage currents and other characteristics like Schottky diodes. Further analysis of the Shannon contact model shows that thermionic emission leakage current of Schottky-like diodes may limit p-n junction scaling : with p-type doping concentrations of 10(18)-10(19) cm(-3), the Shannon contact model predicts that a 20-30 nm p-type junction depth below metal contacts is necessary to keep leakage at acceptable levels. A capacitance-voltage method is suggested for finding the minimum junction depth of p-n junctions.