Method of fabricating bipolar transistors and insulated gate field effect transistors having doped polycrystalline silicon conductors

DETAILED DESCRIPTION It is an object of the present invention to provide a method of fabricating a bipolar transistor device, an MOSFET device or a composite device made up of a bipolar transistor and an MOSFET which method can overcome the above-mentioned drawbacks of the prior art.
Accordingly to an aspect of the present invention, there is provided a method of fabricating a semiconductor device comprising the steps of: forming a layered structure of a desired pattern on a main surface of a single crystal semiconductor layer of one conductivity type, the structure having a side wall which defines said pattern and including a lower layer disposed at a lower level, a first insulating layer disposed at an intermediate level, and a first polycrystalline semiconductor layer heavily doped with n-type impurity and disposed at a higher level; forming a second insulating layer on the side wall of the layered structure; forming a second polycrystalline semiconductor layer on exposed surfaces of said single crystal semiconductor layer, said first polycrystalline semiconductor layer of the layered structure and said second insulating layer; diffusing said n-type impurity in said first polycrystalline semiconductor layer of the layered structure into part of said second polycrystalline semiconductor layer disposed contiguous to the first polycrystalline semiconductor layer; selectively etching away said part of the second polycrystalline semiconductor layer diffused with said n-type impurity and said first polycrystalline semiconductor layer, using said first and second insulating layers as an etching stopper; doping the remaining part of said second polycrystalline semiconductor layer with conductivity-affording impurity; and heat-treating the device to drive the doped impurity in said second polycrystalline semiconductor layer to a surface region of said single crystal semiconductor layer, thereby forming a doped surface region in the single crystal semiconductor layer and a doped polycrystalline semiconductor layer electrically connected to said doped surface region and extending therefrom