THE LOCK-ON EFFECT IN ELECTRON-BEAM-CONTROLLED GALLIUM-ARSENIDE SWITCHES

The term "lock-on effect" describes the inability of photoconductive or electron-beam-controlled semiconductor switches to recover to their initial hold-off voltage following the application of the laser or electron-beam pulse, if the applied voltage exceeds a certain value. For GaAs, this threshold voltage corresponds to average electric fields in the range from 4 to 12 kV/cm. Experimental results on semi-insulating GaAs switches indicate that the corresponding lock-on current after e-beam irradiation is identical with the steady-state dark current. The highly resistive state of the switch before e-beam irradiation is shown to be a transient phase towards the much lower steady-state dark resistance, with a duration which depends on the impurity content of the switch material and the applied voltage. The irradiation of the GaAs samples with electrons or photons causes an acceleration of this temporal evolution; at sufficiently high laser or e-beam intensities "lock-on" of the dark current after termination of the driving ionization source is observed. Based on the experimental results, a model is developed which describes the lock-on effect in terms of double injection and carrier trapping in deep intraband levels. The model explains the major characteristics of the lock-on effect and is supported by the qualitative agreement of the calculated current-voltage curves with the experimental data.