[en] Controlled oxygen plasma exposure of multilayer WSe₂ can modulate the carrier type of WSe₂ field effect transistors. XPS shows with increased exposure, the top WSe₂ layer(s) oxidizes to amorphous WO_(x−3) layer-by-layer with a high degree of controllability as both single or 2 layers can be transformed. A systematic study of Raman and photoluminescence signatures clearly demonstrate the processing times necessary to selectively oxidized the top 1 or 2 layers of exfoliated WSe₂ flakes. WSe₂ devices exposed in the channel regions experience an increase in p-type conduction and large layer-dependent n-type suppression. Devices exposed in the contact region prior to metalization show slight increases of both p-type and n-type conduction. Devices made on flakes, which were completely exposed show suppression of p-type conduction and layer dependent suppression of n-type conduction. Based on these results we were able to fabricate self-rectifying lateral p–n homojunctions with rectification ratios exceeding 300. (paper)