Integrated transistor device (10) formed in a chip of semiconductor material (15) having an electrical-insulation region (31) delimiting an active area (30) accommodating a bipolar transistor (11) of vertical type and a MOSFET (12) of planar type, contiguous to one another. The active area accommodates a collector region (18); a bipolar base region (19) contiguous to the collector region; an emitter region (20) within the bipolar base region; a source region (23), arranged at a distance from the bipolar base region; a drain region (24); a channel region (22) arranged between the source region and the drain region; and a well region (35). The drain region (24) and the bipolar base region (19) are contiguous and form a common base structure (19, 24, 37) shared by the bipolar transistor and the MOSFET. Thereby, the integrated transistor device (10) has a high input impedance and is capable of driving high currents, while only requiring a small integration area.