Measurement of capacitance transients with attoFarad resolution in a microwave varactor diode after Co/sup 60/ irradiation

A high resolution measurement system was constructed to investigate transient capacitance changes in varactor diodes after exposure to Co/sup 60/ photons. These capacitance transients, while small, adversely affect the phase settling times of phase lock loops used in spread spectrum communications systems. The measurement system has a resolution of better than 10 aF, and details are given concerning its design and operation. It was used to study planar and mesa type Si varactor diodes. Small but significant changes in the capacitive transients were observed after irradiation. The behavior has been ascribed to the generation of bulk defects caused by displacement damage in the active layer of the diodes, although interface state generation at the oxide interfaces has not been ruled out. The effect of the changes in the diode transient responses on the settling time of a typical phase lock loop frequency synthesizer has been modeled. The introduction rate of the bulk traps has been estimated to be 1/spl times/10/sup -3/ to 4/spl times/10/sup -3/ cm/sup -1/ rad/sup -1/.<>