Optical measurements of the electric field and temperature distribution in photoconductive GaAs switches

The temporal development of electric field distribution and temperature distribution in photoconductive, gallium arsenide (GaAs:Si:Cu) switches was studied by means of absorption measurements near the bandedge of gallium arsenide. Regions of high absorption, corresponding to enhanced field strength, were recorded close to the cathode contact for low applied fields (E < 20 kV/cm), and at both contacts for higher fields. Breakdown was observed at voltages where the absorption patterns merged. For low intensity laser activation the absorption became temporarily (during laser activation) homogeneous over the switch area, but the pattern emerged again during the tail current phase and became even more pronounced than before. After turning the voltage off, the absorption in certain regions of the switch decayed only slowly, with a time constant of about hundred nanoseconds, indicating local heating of the switch. At high laser intensities the absorption pattern, generated through pulse biasing of the sample, disappeared completely during laser activation. The switch stayed homogeneous, where electric field and temperature are concerned, even during the following lock-on phase, and recovered right after the switch voltage was turned off. The results show that energies in excess of 1 mJ/cm2 are needed to eliminate field inhomogenetics in photoconductive GaAs:Si:Cu switches and to obtain nanosecond switch recovery.