Method for fabricating a field effect transistor

Claim 1

A method for fabricating a field effect transistor, comprising the steps of: forming a silicon dioxide film having a region where said silicon dioxide film becomes thinner at that area on one surface of a silicon semiconductor substrate of a first conductivity type at which the field effect transistor is to be formed, depositing on said silicon dioxide film a polycrystalline silicon layer which has an impurity concentration higher than that of said silicon semiconductor substrate, removing said polycrystalline silicon layer by selective etching so as to leave only a part which becomes a gate of said field effect transistor, removing a surface part of said silicon dioxide film over the entire area by employing as a mask said part of said polycrystalline silicon layer to become said gate and to the extent that the surface of said silicon semiconductor substrate is exposed at the other part in said region, simultaneously growing a silicon dioxide film on said exposed surface of said semiconductor substrate and said polycrystalline silicon layer to become said gate by thermally oxidizing the whole surface of said polycrystalline silicon layer to become said gate and the exposed surface on said semiconductor substrate, the silicon dioxide film thickness produced at said surface of said polycrystalline silicon layer being formed at a faster rate than the thickness at the exposed surface of said semiconductor substrate due to the higher impurity concentration of said polycrystalline silicon layer, whereby the silicon dioxide film produced at said surface of said polycrystalline silicon layer is thicker than the silicon dioxide film produced at said exposed surface of said semiconductor substrate, forming windows for a source and a drain by removing said silicon dioxide films over the entire area by such thickness that said surface of said silicon semiconductor substrate is exposed at the part in said region other than said part covered by said polycrystalline silicon layer to become said gate, but that said polycrystalline silicon layer is not exposed due to the greater thickness of the silicon dioxide film on said polycrystalline silicon layer, forming the source and the drain by doping surface portions of said semiconductor substrate with an impurity of a second conductivity type opposite to said first conductivity type through the windows formed by the preceding step, removing a part of said silicon dioxide film by selective etching so as to expose a part of said polycrystalline silicon layer, depositing an electric conductor over the entire area, and forming said electric conductor into a predetermined pattern by selective etching, thereby to form source, gate and drain electrodes.

Claim 2

A method for fabricating a field effect transistor, comprising the steps of: forming a silicon dioxide film having a region where said silicon dioxide film becomes thinner at that area on one surface of a silicon semiconductor substrate of a first conductivity type at which the field effect transistor is to be formed, depositing on said silicon dioxide film a polycrystalline silicon layer which has an impurity concentration higher than that of said silicon semiconductor substrate, removing said polycrystalline silicon layer by selective etching so as to leave only a part which becomes a gate of said field effect transistor, removing a surface part of said silicon dioxide film over the entire area by employing as a mask said part of said polycrystalline silicon layer to become said gate and to the extent that the surface of said silicon semiconductor substrate is exposed at the other part in said region, simultaneously growing a silicon dioxide film on said exposed surface of said semiconductor substrate and said polycrystalline silicon layer to become said gate by thermally oxidizing the whole surface of said polycrystalline silicon layer to become said gate and the exposed surface on said semiconductor substrate, the silicon dioxide film thickness produced at said surface of said polycrystalline silicon layer being formed at a faster rate than the thickness at the exposed surface of said semiconductor substrate due to the higher impurity concentration of said polycrystalline silicon layer, whereby the silicon dioxide film produced at said surface of said polycrystalline silicon layer is thicker than the silicon dioxide film produced at said exposed surface of said semiconductor substrate, forming windows for a source and a drain by removing said silicon dioxide films over the entire area by such thickness that said surface of said silicon semiconductor substrate is exposed at the part in said region other than said part covered by said polycrystalline silicon layer to become said gate, but that said polycrystalline silicon layer is not exposed due to the greater thickness of the silicon dioxide film on said polycrystalline silicon layer, forming the source and the drain by doping surface portions of said semiconductor substrate with an impurity of a second conductivity type opposite to said first conductivity type through the windows formed by the preceding step, removing a part of said silicon dioxide film by selective etching so as to expose a part of said polycrystalline silicon layer, depositing an electric conductor over the entire area, and forming said electric conductor into a predetermined pattern by selective etching, thereby to form source, gate and drain electrodes.