NOVELTY - The device (10) has a semiconductor module (22) with semiconductor elements e.g. insulated gate bipolar transistor and free-wheeling diodes. Heat sinks (11) comprise sets of main ribs (112), respectively. The main ribs are arranged along a thickness direction (Y) in regular interval. Each heat sink comprises a pair of auxiliary ribs (113), where each auxiliary rib incorporates a part and thickness of the part is measured larger along the thickness direction than thickness of the main ribs. The auxiliary ribs are arranged at the externally located main ribs in the thickness direction.    USE - Semiconductor device for forming a three-phase rectifier device (claimed) and for use as a transducer device e.g. three-phase transducer device, for converting an electrical direct current into alternating current for operating a high-power electric motor i.e. drive power motor, of an electric car and a hybrid car i.e. gas/electric car. Can also be used for operating an electric generator and a compressor drive motor.    ADVANTAGE - The auxiliary ribs incorporate the respective parts, where the thickness of each part is measured larger along the thickness direction than the thickness of the main ribs and the auxiliary ribs are arranged at the externally located main ribs in the thickness direction, thus significantly reducing the hazard of bending of the ribs of the heat sinks due to forces, while ensuring high thermal transmission efficiency. The device enhances effective thermal mass of the heat sinks, thus reducing temperature fluctuations of the semiconductor module due to short fluctuations in heat produced by the semiconductor module and avoiding sudden rise in temperature of the semiconductor elements and hence achieving enhanced cooling performance. The device can be fixed within a housing with reduced number of fastening devices, thus achieving reduction of manufacturing costs of the device.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a three-phase rectifier device comprising a three-phase rectifier circuit.    DESCRIPTION OF DRAWING(S) - The drawing shows an exploded oblique perspective view of a semiconductor device.'(Drawing includes non-English language text)'    Thickness direction (Y)    Semiconductor device (10)    Heat sinks (11)    Semiconductor module (22)    Main ribs (112)    Auxiliary ribs (113)