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A method of designing enhancement-mode m.o.s.t.s having controllable turn-on characteristics is described. The m.o.s.t.s are specifically aimed at performing the functions of voltage-controlled resistors and the design called for a log-linear variation in channel resistance with gate voltage over as wide a range as possible. The devices were made using a conventional p-channel oxide gate process with an additional n+ diffusion stage to give the required variation of threshold voltage at different points within the device. In the final form of the m.o.s.t. the log-linear range of channel resistance variation extended from 5MÂ¿ to 500Â¿ for an applied gate voltage range 4Â¿15 V. The design method used for these devices could equally well be applied to making composite m.o.s.t.s having another form of channel resistance variation from the log-linear variation studied here.