Varactor structures

An improved varactor diode having first and second terminals is obtained by providing a substrate having a first surface in which are formed isolation regions separating first and second parts of the diode. A varactor junction is formed in the first part and having a first side coupled to the first terminal and a second side coupled to the second terminal via a sub-isolation buried layer (SIBL) region extending under the bottom and partly up the sides of the isolation regions to a further doped region ohmically connected to the second terminal. The first part does not extend to the SIBL region. The varactor junction desirably comprises a hyper-abrupt doped region. The combination provides improved tuning ratio, operating frequency and breakdown voltage of the varactor diode while still providing adequate Q.