Power MOSFETs and methods for forming the same

A method comprising:

• providing a substrate with a plurality of insulation regions surrounding a first region, wherein the first region extends from a top surface of the substrate into the substrate, the first region is of a first conductivity type with a first impurity concentration;
• depositing a gate dielectric material over and in contact with the first region;
• depositing a gate electrode material over and in contact with the gate dielectric material;
• removing a first part of the gate dielectric material and a first part of the gate electrode material to expose a first portion of the first region;
• forming a second region within the first portion of the first region, wherein the second region is of the first conductivity type and has a second impurity concentration;
• forming a gate dielectric and a gate electrode over the first region by removing a second part of the gate dielectric material and a second part of the gate electrode material to expose a second portion of the first region;
• forming a first drift region within the second portion of the first region, wherein the first drift region is of a second conductivity type opposite to the first conductivity type, the first drift region has a third impurity concentration;
• forming a second drift region within the first region, wherein the second drift region is of the second conductivity type and of the third impurity concentration, the second drift region is beneath the gate electrode, adjacent to the first drift region, with a depth less than a depth of the first drift region, and the first drift region and the second drift region together form a stepwise shape, and wherein an edge of the first drift region adjacent the second drift region is substantially aligned with an edge of the gate electrode;
• forming a source region of the second conductivity type within the second region;
• and
• forming a drain region of the second conductivity type within the first drift region.

A power Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) comprising:

• a substrate;
• a first region extending from a top surface of the substrate into the substrate, wherein the first region is of a first conductivity type with a first impurity concentration;
• a gate dielectric over and in contact with the first region;
• a gate electrode over and in contact with the gate dielectric;
• a second region within the first region at a first side of the gate electrode, wherein the second region is of the first conductivity type, and has a second impurity concentration;
• a third region within the first region at a second side of the gate electrode, wherein the third region is of a second conductivity type opposite to the first conductivity type, and has a third impurity concentration;
• a stepped drift region of the second conductivity type, of a fourth impurity concentration, and within the first region, wherein the stepped drift region comprises a first part formed above the third region, and a second part beneath the gate electrode, adjacent to the first part, with a depth less than a depth of the first part;
• a source region of the second conductivity type within the second region;
• and
• a drain region of the second conductivity type within the first part of the stepped drift region.