A FET (field-effect transistor) in which the source is formed on the back of a semiconductor wafer and current flows vertically through the device rather than horizontally as in the more usual structure. The cross section of a VFET is shown in Fig. a. A variation of the VFET is the permeable base transistor in which an epitaxial layer is formed over the gate electrodes before the drain is formed (Fig. b).

(a) Cross section of vertical FET (b) Cross section of permeable base transistor