METASTABLE IMPACT IONIZATION OF TRAPS MODEL FOR LOCK-ON IN GAAS PHOTOCONDUCTIVE SWITCHES

A model for lock-on in GaAs photoconductive switches is proposed based on avalanche breakdown from impact ionization of deep EL2 traps. Compared to band-to-band impact ionization, the model predicts an order-of-magnitude reduction in the threshold electric field from the combination of three effects: (i) The EL2 trap is located in the middle of the gap and requires half the band-gap energy to access a band edge; (ii) avalanching a single carrier type from the trap requires approximately half the energy that is split between electron-hole pairs in band-to-band transitions; (iii) thermally assisted tunneling and Poole-Frenkel barrier lowering account for the remaining threshold reduction. Light triggering is provided by optical ''bleaching'' of EL2 and related traps into metastable states. This causes an increase in the mean free path for optical phonon emission and a decrease the threshold electric-field strength for avalanche. Model approximations, that retain order-of-magnitude level accuracy, give a calculated current rise time of 138 ps compared to a 200 ps measured value, a lock-on delay time of 1.1 ns compared to a measured value of approximately 1 ns, and an optical threshold energy value of 10 mJ compared to a 2.1 mJ experimental value. Calculated thermal recovery rates from lock-on of approximately 1 mus are consistent with experimental repetition rates of up to 1 kHz that require recovery in less than 1 ms.