  We present experimental evidence and physics-based simulations of the lock-on
effect in high-voltage GaAs avalanche diodes. The avalanche triggering is
initiated by steep voltage ramp applied to the diode and in-series 50 Ohm load.
After subnanosecond avalanche switching the reversely biased GaAs diode remains
in the conducting state for the whole duration of the applied pulse (dozens of
nanoseconds). There is no indication of the p-n junction recovery that is
commonly expected to develop on the nanosecond scale due to the drift
extraction of non-equilibrium carriers. The diode voltage keeps a constant
value of ~70 V much lower than the stationary breakdown voltage of 400 V.
Numerical simulations reveal that the conducting state is supported by impact
ionization in narrow high-field collapsing Gunn domains as well as in
quasi-stationary cathode and anode ionizing domains. Collapsing Gunn domains
spontaneously appear in the dense electron-hole plasma due to the negative
differential mobility of electrons in GaAs. The effect resembles the lock-on
effect of GaAs bulk photoconductive switches but is observed in reversely
biased p-n junction diode switched by a non-optical method.
