A FIELD-SENSITIVE PHOTOCONDUCTIVE PROBE FOR SAMPLING THROUGH PASSIVATION LAYERS

A field-sensitive photoconductive sampling technique has been demonstr ated in measurements performed through an insulating layer without the need for conductive contact. Sampled signals are sensed by a virtual- ground, floating-gate amplifier without draining charge from the devic e under test or the photoconductive switch. The minimum detectable sig nal is 2.5 mu V/Hz(1/2) with a spatial resolution of 7 mu m, while the sampling bandwidth is essentially that observed using photoconductive sampling with a conductive contact to the device under test. The phot ovoltaic and shot current noise are negligible in comparison with the lock-in amplifier noise since the current flowing in this high-impedan ce, floating-gate probe is negligible. (C) 1996 American Institute of Physics.