Method of manufacturing a reference voltage generator device

A method of manufacturing a semiconductor device with at least a pair of insulated gate field-effect transistors having semiconductor gate electrodes of different conductivity type, comprising the steps of:

• forming an insulating film over a major surface of a semiconductor substrate at a first portion thereof and at a second portion thereof, said first and second portions being of a first conductivity type and being formed in a same part of the semiconductor substrate extending to the major surface thereof, and forming a semiconductor layer over said insulating film overlying said first and second portions;
• introducing an impurity of the first conductivity type into a first portion of said semiconductor layer overlain at said first portion except for into another portion of said semiconductor layer overlain at said second portion, to provide an impurity level of said impurity of the first conductivity type at that portion;
• removing said semiconductor layer to form first and second semiconductor gate electrodes at said first and second portions respectively;
• and
• selectively introducing an impurity of a second conductivity type opposite to the first conductivity type into said first and second portions not covered with said first and second semiconductor gate electrodes and into said first and second semiconductor gate electrodes so as to provide at those portions an impurity level of the second conductivity type lower than said impurity level of the impurity of the first conductivity type at said first semiconductor gate electrode, thereby to form source and drain semiconductor regions of the second conductivity type on opposite sides of each of said first and second gate electrodes and thereby to define the second semiconductor gate electrode as of the second conductivity type without converting said first semiconductor gate electrode from the first conductivity type to the second conductivity type.