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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.