Structure and fabrication of field-effect transistor having source/drain extension defined by multiple local concentration maxima

A structure comprising a field-effect transistor provided along an upper surface of a semiconductor body having body material of a first conductivity type, the transistor comprising:

• a channel zone of the body material;
• a source and a drain situated in the semiconductor body along its upper surface, laterally separated by the channel zone, and doped with composite semiconductor dopant of a second conductivity type opposite to the first conductivity type such that the source and drain are of the second conductivity type so as to form respective pn junctions with the body material;
• a gate dielectric layer overlying the channel zone;
• and
• a gate electrode overlying the gate dielectric layer above the channel zone, the source comprising a main source portion and a more lightly doped lateral source extension laterally continuous with the main source portion and extending laterally under the gate electrode, the dopant of the second conductivity type having a concentration that locally reaches at least three subsurface maximum concentrations in the source such that (i) at least one of the subsurface maximum concentrations in the source is attendant to a dopant distribution that largely defines the main source portion and (ii) at least two of the subsurface maximum concentrations in the source are attendant to a dopant distribution that largely defines the source extension.

A method of fabricating a plurality of like-polarity field-effect transistors (“FETs”) from a semiconductor body having body material of a first conductivity type, the method comprising:

• defining a gate electrode for each FET such that the gate electrode is situated above, and vertically separated by a gate dielectric layer from, a part of the body material intended to be a channel zone for that FET;
• and
• introducing composite semiconductor dopant of a second conductivity type opposite to the first conductivity type into the semiconductor body to form, for each FET, a pair of source/drain (“S/D”) zones of the second conductivity type laterally separated by that FET's channel zone such that each S/D zone forms a pn junction with the body material and comprises a main S/D portion and a more lightly doped lateral S/D extension laterally continuous with the main S/D portion and extending laterally under the gate electrode and such that the channel zone is terminated by the S/D extensions directly below that FET's gate dielectric layer wherein the act of introducing the composite dopant of the second conductivity type includes (a) introducing first semiconductor dopant of the second conductivity type substantially simultaneously into first and second portions of the semiconductor body respectively intended for the S/D extensions of one of the S/D zones of a first of the FETs and one of the S/D zones of a second of the FETs and (b) introducing second semiconductor dopant of the second conductivity type into the body's first portion while simultaneously substantially preventing the second dopant from entering the body's second portion.