A dense sample of MgB 2 (T c =38.5 K) was synthesized under 200 MPa pressure using a novel hot isostatic pressing (HIPing) technique, and a high-purity single crystal of YBa 2 Cu 3 O 7 - δ (T c =93.5 K) was grown in a home-made BaZrO 3 crucible. Electric field vs current density (E-J) isotherms of the MgB 2 and YBa 2 Cu 3 O 7 - δ high-temperature superconductors were measured and analyzed in terms of the critical scaling model. E-J isotherm sets were taken at fields ranging from 2 to 90 kOe. Scaling and resistivity data reveal a smooth transition from a vortex glass to a vortex liquid state for the bulk MgB 2 sample and the existence of both a vortex lattice melting line and a vortex glass to vortex liquid line in the YBa 2 Cu 3 O 7 - δ single crystal. The critical scaling exponents for both systems are presented. A more complete (H-T) phase diagram, containing the upper critical field H c 2 (T), the vortex-glass transition line H g (T), and the magnetic irreversibility line H i r r (T) was established for bulk MgB 2 .