Ldmos transistor with split gate field plate tapered toward drain region and related method

A laterally-diffused metal-oxide semiconductor (LDMOS) transistor, comprising:

• a semiconductor substrate;
• a channel gate in the semiconductor substrate;
• a source region to a first side of the channel gate;
• a drain region to a second, opposite side of the channel gate;
• a split gate field plate over the semiconductor substrate, wherein the split gate field plate is spaced apart from the channel gate, has no lateral overlap with the channel gate and has a tapered surface that is thinner toward the drain region;
• and
• an L-shaped gate dielectric separating the split gate field plate from the channel gate and the semiconductor substrate.

A method, comprising:

• forming a channel gate including a first polysilicon gate body layer over a first gate dielectric layer over a semiconductor substrate, wherein the first polysilicon gate body layer has an end over the first gate dielectric layer;
• forming a second gate dielectric layer over the end of the first polysilicon gate body layer, wherein the second gate dielectric layer is thicker than the first gate dielectric layer;
• forming a second polysilicon gate body layer over the second gate dielectric layer over the end of the first polysilicon gate body layer;
• forming a split gate field plate from the second polysilicon gate body layer over the second gate dielectric layer over the end of the first polysilicon gate body layer;
• and
• forming a source region and a drain region configured for a laterally-diffused metal-oxide semiconductor (LDMOS) transistor in the semiconductor substrate,
• wherein the split gate field plate has a tapered surface that is thinner toward the drain region.