  We have constructed MgB2/Pb planar junctions for both temperature and field
dependence studies. Our results show that the small gap is a true bulk property
of MgB2 superconductor, not due to surface effects. The temperature dependence
of the energy gap manifests a nearly BCS-like behavior. Analysis of the effect
of magnetic field on junctions suggests that the energy gap of MgB2 depends
non-linearly on the magnetic field. Moreover, MgB2 has an upper critical field
of 15 T, in agreement with some reported Hc2 from transport measurements.
