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Ultrafast high-field transport in GaAs: direct observation of quasi-ballistic electron motion, impact ionization and avalanche multiplication

Femtosecond electron dynamics in GaAs is investigated tracing ultrafast modifications of the Franz–Keldysh absorption spectrum. A sophisticated heterostructure design allows for a study of unipolar transport as well as a nanometre scale definition of layers for both carrier injection and detection of the propagating electron ensemble. Transit times through the structure are directly measured for electric fields between 7 kV cm−1 and 180 kV cm−1 comparing room temperature operation to results for TL = 4 K. For electric fields F ⩾ 400 kV cm−1 a clear signature of impact ionization and avalanche multiplication on a picosecond time scale is found.