Delay of field collapse in photoconductive gaps fabricated on GaAs/AlGaAs MODFET material

We report the fabrication of a 1 μm photoconductive gap voltage step generator with constant output voltage of 0.10 V for 100 ps and a 2 ps rise time. The photoconductor is fabricated on GaAs/AlGaAs MODFET material and uses the two-dimensional electron gas in the FET channel as the conductive medium. It is fully process-compatible with MODFETs. In devices with standard ohmic contacts, the output current stayed constant for up to 50 ps, even though the electric field in the gap is expected to collapse in less than 1 ps. Two-dimensional device simulations show that, although the field in the gap does collapse on the expected time scale, the current is initially determined by the contacts. A new photoconductor design, in which the ohmic contacts are laterally recessed from the etched gap region, further delayed field collapse, extending constant voltage operation to 100 ps