MODULATION BANDWIDTH AND NOISE LIMIT OF PHOTOCONDUCTIVE GATES

The modulation bandwidth and noise limit of a photoconductive sampling gate are studied by reducing the parasitic capacitance and leakage cu rrent of the sampling circuit using an integrated junction field-effec t transistor (JFET) source follower. The modulation bandwidth of the p hotoconductive sampling gate is limited by the external parasitic capa citance, and its efficiency is found to saturate at a laser gating pow er of about 1 mW. It is determined that the noise of the photoconducti ve sampling gate is dominated by the photovoltaic current due to the g ating laser amplitude fluctuation. A minimum noise level of 4 nV Hz(-1 /2) has been measured, and an enhancement in signal-to-noise ratio by a factor of >45 has been achieved after the integration of the source follower with the photoconductive sampling gate. The JFET source follo wer serves to increase the modulation bandwidth of the photoconductive sampling gate by about 15 times and buffer the charge of the measured signal using its extremely high gate input impedance. The performance of the photoconductive sampling gate in regard to invasiveness and ga ting efficiency has been optimized, while a picosecond temporal resolu tion has been maintained and the signal-to-noise performance has been enhanced using a gating laser power as low as 10 mu W.