Fermi threshold field effect transistor including doping gradient regions

Claim 1

A field effect transistor comprising: a semiconductor substrate of first conductivity type; a tub region of second conductivity type in said substrate at a surface thereof, and extending a first depth from said substrate surface; spaced apart source and drain regions of said second conductivity type in said tub region at said substrate surface; a channel of said second conductivity type in said tub region, between said spaced apart source and drain regions, and extending a second depth from said substrate surface, wherein said second depth is less than said first depth, at least one of said first and second depths being selected to produce zero static electric field perpendicular to said substrate surface, from said substrate surface to said second depth, and at least one of said source and drain regions being doped said second conductivity type facing said channel at a doping concentration which is sufficiently high to maximize the saturation current of said field effect transistor; a gate insulating layer on said substrate surface, between said spaced apart source and drain regions; and source, drain and gate electrodes contacting said source and drain regions and said gate insulating layer, respectively; wherein said channel is doped at a second doping concentration which is less than the source doping concentration, said field effect transistor further comprising: a source doping gradient region of said second conductivity type between said source region and said channel, and being doped at a doping gradient which decreases from said source doping concentration adjacent said source region to said second doping concentration adjacent said channel, said source doping gradient region having a thickness greater than 300 Ångstroms.