Patent ID: 7135364

Claim:
A method of fabricating a lateral transistor comprising: selectively forming a first diffusion region of a second conductivity type on a semiconductor substrate of a first conductivity type; growing an epitaxial layer of the second conductivity type on said first diffusion region so as to make said first diffusion region a first buried region; selectively diffusing impurity atoms of the second conductivity type so as to form a first plug region of the second conductivity type, from a top surface of said epitaxial layer such that a bottom of said first plug region reaches to said first buried region; selectively diffusing impurity atoms of the second conductivity type so as to form a graded base region of the second conductivity type, from the top surface of said epitaxial layer, wherein a lateral position of the graded base region is separated from a lateral position of said first plug region and a vertical position of the graded base region is separated from a vertical position of said first buried region, the graded base region has a doping profile such that an impurity concentration decreases towards a peripheral region from a central region of the graded base region; forming a first main electrode region of the first conductivity type in and at a top surface of said graded base region; and forming a second main electrode region of the first conductivity type in and at the top surface of said epitaxial layer at a location separated from said first main electrode region, wherein said first and second main electrode regions laterally sandwich said graded base region, the impurity concentration of said graded base region decreases gradually alonge a lateral direction from said first main electrode region towards said second main electrode region when the lateral transistor is unbiased, and a remaining portion of said epitaxial layer laterally sandwiched between said graded base region and said second main electrode region serves as a uniform base region having a uniform lateral doping profile, the lateral doping profile being measured along a lateral direction parallel to the top surface of said epitaxial layer. wherein a combination of said buried base region, said uniform base region and said graded base region serves as a base region.