Double diffused MOS transistors

N-channel enhancement mode, MOS transistors with non-uniform substrate doping built on silicon and on thin films of silicon on sapphire are described. The non-uniform substrate doping is the result of additional boron doping limited to a portion of the channel length. This supplementary substrate doping is achieved by diffusing boron through the same oxide window used to define the phosphorus source diffusion. By confining the supplementary channel doping to the vicinity of the source, n-channel enhancement mode transistors can be fabricated with no degradation of junction breakdown voltage at the drain or increase in drain capacitance. If an n-channel transistor is built on an n-type film, the channel length can be established by adjusting diffusion profiles to give a p-region of the desired thickness.