Picosecond photoconductive probe

We have developed and characterized a voltage-sensitive picosecond probe that can make non-invasive measurements either with direct contact to a metal electrode or through a passivation layer. While the utility of freely-positionable photoconductive probes has been substantiated for both high- and low-impedance testing of electronics, the invasiveness (in terms of both the field disturbed and the charge extracted), mechanical robustness, and sensitivity at low light levels have hitherto limited their usefulness. The probe described in this work has two important differences over other unamplified probes based on semiconductor substrates. First, although the gate in this probe is formed of nonstoichiometric (i.e. low-temperature MBE grown) GaAs, all but a small mesa of this photoconductor is removed, leaving as a support a flexible polyimide film. Second, we take advantage of the low capacitance and high resistance provided by a JFET source follower to amplify the sampled (quasi-dc) electrical signal immediately after it is gated.