| 摘要 |
Methods of forming field effect transistors include the steps of implanting first conductivity type dopants at a first dose level into a first portion of a relatively lightly doped drift region of first conductivity type semiconductor and then oxidizing the first portion of the semiconductor drift region to form a relatively thick field oxide isolation region and simultaneously form a drain region extension of first conductivity type semiconductor (e.g., N0) underneath the field oxide isolation region by driving the dopants implanted at the first dose level into the drift region. A body region of second conductivity type semiconductor (e.g., P-type) is then formed in a second portion of the semiconductor drift region. A gate electrode is then formed on the drift region to extend opposite the body region and the field oxide isolation region. Source and drain regions of first conductivity type semiconductor (e.g., N+) are then formed in the body region and in a third portion of the drift region spaced from the second portion, respectively, by implanting first conductivity type dopants at a second dose level using the gate electrode and field oxide isolation region as an implant mask. The third portion of the drift region overlaps the first portion of the drift region so that a composite drain region having a laterally graded doping profile can be formed. The first portion of the drift region preferably extends between the third and second portions so that the doping profile of the drain region (e.g., N+) and drain region extension (e.g., N0) monotonically decreases in a direction towards the body region.
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