Disclosed is a tri-gate field effect transistor with a back gate and the associated methods of forming the transistor. Specifically, a back gate is incorporated into a lower portion of a fin. A tri-gate structure is formed on the fin and is electrically isolated from the back gate. The back gate can be used to control the threshold voltage of the FET. In one embodiment the back gate extends to an n-well in a p-type silicon substrate. A contact to the n-well allows electrical voltage to be applied to the back gate. A diode created between the n-well and p-substrate isolates the current flowing through the n-well from other devices on the substrate so that the back gate can be independently biased. In another embodiment the back gate extends to n-type polysilicon layer on an insulator layer on a p-type silicon substrate. A contact to the n-type polysilicon layer allows electrical voltage to be applied to the back gate. A trench isolation structure extending through the polysilicon layer to the insulator layer isolates current flowing through the polysilicon layer from other devices on the silicon substrate.