Drain-extended transistor

A transistor comprising:

• a buried layer having a second conductivity type in a semiconductor substrate having a first conductivity type, the buried layer spaced apart from a top surface of the semiconductor substrate and defining a drift region between the buried layer and the top surface;
• a body region having the second conductivity type located in the semiconductor substrate and extending from the top surface toward the buried layer;
• a source having the first conductivity type formed in the body region;
• a drift well extending from the buried layer to the top surface of the semiconductor substrate and touching the body region;
• a drain having the first conductivity type and extending from the top surface of the semiconductor substrate into the drift well;
• a drift surface layer having the first conductivity type between the top surface and the drift well;
• and
• a gate over the body region and between the source and the drift well;
• wherein the drift surface layer extends under the gate toward the source further than the drift well extends under the gate toward the source.

A method comprising:

• forming a buried region in a substrate, the substrate having a first conductivity type and the buried region having a second conductivity type, the buried region defining a drift region between the buried region and a surface of the substrate;
• forming a body region having the second conductivity type extending from the surface of the substrate toward the buried region;
• forming a drain having the first conductivity type and extending from the surface of the substrate toward the buried region, wherein at least a portion of the drift region is between the body region and the drain;
• forming a drift well having the second conductivity type in the drift region;
• forming a drift surface layer having the first conductivity type between the drift well and the surface of the substrate;
• forming a source having the first conductivity type in the body region;
• and
• forming a gate proximate to the body region between the source and the drift region;
• wherein the drift surface layer extends under the gate toward the source further than the drift well extends under the gate toward the source.

A transistor comprising:

• a buried layer having a second conductivity type in a semiconductor substrate having a first conductivity type, the buried layer spaced apart from a top surface of the semiconductor substrate and defining a drift region between the buried layer and the top surface;
• a body region having the second conductivity type located in the semiconductor substrate and extending from the top surface toward the buried layer;
• a source having the first conductivity type formed in the body region;
• a drift well extending from the buried layer to the top surface of the semiconductor substrate and touching the body region;
• a drain having the first conductivity type and extending from the top surface of the semiconductor substrate into the drift well;
• a drift surface layer having the first conductivity type between the top surface and the drift well;
• and
• a gate over the body region and between the source and the drift well;
• wherein the drift surface layer has a first dopant concentration at least four orders of magnitude greater than a second dopant concentration of the drift well.