Semiconductor transistor device with dopant profile

A method for forming a transistor comprising:

• forming a recess in a transistor region of a semiconductor substrate;
• forming a first silicon carbide layer over a channel region in the transistor region;
• forming an SiB or SiP layer over the first silicon carbide layer;
• forming a second silicon carbide layer over the SiB or SiP layer;
• forming a silicon layer over the second silicon carbide layer;
• forming a gate dielectric over the silicon layer above the channel region;
• forming a gate electrode over the gate dielectric above the channel region;
• and
• performing a gradient implant in the transistor region.

A method for forming a transistor, comprising

• forming a recess in a transistor region of a semiconductor substrate, the recess extending continuously from a first shallow trench isolation (STI) region at a first end of the transistor region to a second STI region at a second end of the transistor region, the substrate having impurities of a first impurity type in the transistor region;
• forming a first silicon carbide layer over a channel region in the transistor region after forming the recess;
• forming an SiB or SiP layer over the first silicon carbide layer;
• forming a second silicon carbide layer over the SiB or SiP layer;
• forming a silicon layer over the second silicon carbide layer;
• forming a gate dielectric on the silicon layer above the channel region;
• forming a gate electrode over the gate dielectric above the channel region;
• implanting additional impurities of the first impurity type in the semiconductor substrate under the gate electrode, within the channel region thereof;
• and
• implanting a gradient implant dosage of impurities of a second type opposite the first type in source and drain regions of the transistor region.