Semiconductor device having a well electrically insulated from the substrate

DETAILED DESCRIPTION Accordingly, the object of the present invention is to provide a semiconductor device that has an excellent crystal quality in the device active layer so that highly reliable microdevices can be formed, and a manufacturing method therefor.
The semiconductor device according to the present invention comprises a semiconductor substrate; a semiconductor layer different in conductivity type from that of said semiconductor substrate, lower in oxygen concentration than said semiconductor substrate, and formed uniformly on said semiconductor substrate; a well region different in conductivity type from that of said semiconductor layer and formed into an island shape in said semiconductor layer so that a bottom surface thereof is 1 to 20 .
mu.
m away from a surface of said semiconductor substrate; and both or either of a MOS-type field effect transistor or a capacitance formed in said semiconductor layer or said well region so as to be electrically insulated from said semiconductor substrate.
The oxygen concentration of said semiconductor substrate lies within a range from 8.
times.
10.
sup.
17 to 12.
times.
10.
sup.
17 cm.
sup.
-3, and that of said semiconductor layer is 3.
times.
10.
sup.
17 cm.
sup.
-3 or less.
The semiconductor layer is an epitaxial growth layer (referred to as epitaxial layer, hereinafter), and the thickness thereof is less than 30 .
mu.
m.
The impurity concentration of said semiconductor substrate lies within a range from 1.
times.
10.
sup.
14 to 5.
times.
10.
sup.
18 cm.
sup.
-3.
The junction depth of said well region lies within a range from 1 to 8 .
mu.
m.
Furthermore, the method of manufacturing a semiconductor device according to the present invention comprises the steps of: heat-treating a surface of a semiconductor substrate within a reducing atmosphere; diffusing impurities on the surface area of the heat-treated semiconductor substrate, to form a semiconductor layer of a conductivity type different from that of the semiconductor substrate; forming a well region of a conductivity type different from that of the semiconductor layer, in the semiconductor layer; and forming both or either of a MOS type field effect transistor or a capacitance in the semiconductor layer or the well region