Time-resolved electroabsorption measurement of the transient electron velocity overshoot in GaN

A femtosecond time-resolved electroabsorption technique employing an AlGaN/ GaN heterojunction p-i-n diode with a p-type AlGaN window layer and a semit ransparent p contact has been used to measure the transient electron veloci ty overshoot in GaN. A peak transient electron velocity of 7.25x10(7) cm/s within the first 200 fs after photoexcitation has been observed at a field of 320 kV/cm. The increase in electron transit time across the device with increasing field beyond 320 kV/cm provides experimental evidence for a nega tive differential resistivity region of the steady-state velocity-field cha racteristic in this high field range. (C) 2001 American Institute of Physic s.