A gate electrode is formed inside a trench via a gate insulating film. The gate insulating film formed on a semiconductor substrate is removed. An insulating film is formed on the semiconductor substrate. A p-type base region is formed in the semiconductor substrate. An n-type emitter region is formed in the base region. Hydrogen annealing process is performed to the semiconductor substrate. A boundary between the base region and the emitter region is located at a position deeper than the insulating film formed between a side surface of the trench and the gate insulating film.

The invention provides a semiconductor structure, the semiconductor structure includes a substrate, two shallow trench isolation structures are located in the substrate, a first region, a second region and a third region are defined between the two shallow trench isolation structures, the second region is located between the first region and the third region. Two thick oxide layers are respectively located in the first region and the third region and directly contact the two shallow trench isolation structures respectively, and a thin oxide layer is located in the second region, the thickness of the thick oxide layer in the first region is greater than that of the thin oxide layer in the second region.

A transistor structure including the following components. An isolation structure defines an active region in a substrate. The isolation structure has a first recess and a second recess located on two sides of the active region. The first recess and the second recess respectively expose a first sidewall and a second sidewall of the substrate in the active region. A first epitaxial layer and a second epitaxial layer are respectively located on the first sidewall and the second sidewall. The first epitaxial layer is located in the first recess and is located on the isolation structure. The second epitaxial layer is located in the second recess and is located on the isolation structure. A gate dielectric layer is located on the substrate, the first epitaxial layer, and the second epitaxial layer. A gate electrode is located on the gate dielectric layer.

The present application discloses a fabrication method for an integrated structure of transistors with different operating voltages. A high-voltage transistor area and a low-voltage transistor area are protected by a retained hard mask layer before a medium-voltage gate oxide layer is grown, so as to avoid additional growth of gate oxide layers above active areas of the high-voltage transistor area and the low-voltage transistor area, thereby avoiding the deterioration of a step height of the low-voltage transistor area due to the subsequent use of a large amount of acid to remove the gate oxide layer additionally grown above the active area of the low-voltage transistor area, and preventing the electrical property and the reliability of a low-voltage device from being subsequently influenced while avoiding the influence of the etching with the large amount of acid on the thickness of a high-voltage gate oxide layer which has already been grown.