A semiconductor layer (10) provided on a BOX (buried oxide) layer (2)
includes a first P-type region (11), an N+-type region (12),
and an N-;type region (13) which together form a diode. A plurality
of second P-type regions (14) are provided on a bottom part of the semiconductor
layer (10). A plurality of insulating oxide films (21) are interposed
between the plurality of second P-type regions (14). When the diode is in
a reverse-biased state, the second P-type region (14) directly below the
N+-type region (12) is approximately the same in potential as
the N+-type region (12). The second P-type region (14)
will be lower in potential relative to this second P-type region (14) directly
below the N+-type region (12), as the second P-type region (14)
gets nearer to the first P-type region (11). Electric field concentration
can thus be relaxed at an interface between the semiconductor layer (10)
and the BOX layer (2), whereby improvement in breakdown voltage of the diode
is realized.
