An SOI substrate, a semiconductor device, and a method of backgate work function tuning. The substrate and the device have a plurality of metal backgate regions wherein at least two regions have different work functions. The method includes forming a mask on a substrate and implanting a metal backgate interposed between a buried oxide and bulk regions of the substrate thereby producing at least two metal backgate regions having different doses of impurity and different work functions. The work function regions can be aligned such that each transistor has different threshold voltage. When a top gate electrode serves as the mask, a metal backgate with a first work function under the channel region and a second work function under the source/drain regions is formed. The implant can be tilted to shift the work function regions relative to the mask.