A design theory for the high-frequency p-n junction variable capacitor

One of the more significant uses of the p-n junction variable capacitor is as an HF (> 100 mc) tuning element. Structures which have been available have had rather low Q at these frequencies. In order to determine the limits imposed on Q by the physics of the device, a fundamental study has been made of the problems of the HF junction capacitor. Equations have been developed for the prediction of Q in alloyed and diffused structures. Optimization criteria are proposed which permit design of capacitors in which Q is no longer a significant limitation. In support of the theory, experimental results are presented on units designed and fabricated in accordance with the optimization criteria. Q´s in excess of 500 have been observed at 100 mc.