A semiconductor device for transmitting a radio frequency signal along a signal line includes a signal line that extends along a principal axis. On one side of the signal line is a first dielectric, and on the opposite side of the signal line is a second dielectric. First and second ground lines are proximate to the first and second dielectrics, respectively, and the ground lines are approximately parallel to the signal line. The device has a transverse cross-section that varies along the principal axis.