  We report the first formation of the metallic $p$-$n$ junctions, the
ferroelectric (Ba,Sr)TiO$_3$ (BST) switched optimally electron-doped ($n$-type)
metallic T'-phase superconductor, (La,Ce)$_2$CuO$_4$ (LCCO), and hole-doped
($p$-type) metallic CMR manganite (La,Sr)MnO$_3$ (LSMO) junctions. In contrast
with the previous semiconductor $p$-$n$ ($p$-$I$-$n$) junctions which are
switched by the built-in field $V_0$, the present metallic oxides $p$-$I$-$n$
junctions are switched by double barrier fields, the built-in field $V_0$, and
the ferroelectric reversed polarized field $V_{rp}$, both take together to lead
the junctions to possess definite parameters, such as definite negligible
reversed current ($10^{-9}$ A), large breakdown voltage ($>$7 V), and ultrahigh
rectification ($>2\times10^4$) in the bias voltage 1.2 V to 2.0 V and
temperature range from 5 to over 300 K. The related transport feature, barrier
size effect, and temperature effect are also observed and defined.
