Experimental and Theoretical Studies of Transient Electron Velocity Overshoot in GaN

We employ an optically detected time-of-flight technique with femtosecond resolution that monitors the change in the electroabsorption due to charge transport in an AlGaN/GaN heterojunction p–i–n diode to measure the electron velocity overshoot in GaN at room temperature. It has been found that electron velocity overshoot occurs at electric fields as low as 105 kV/cm, with the peak transient velocity increasing with E up to ∼320 kV/cm, at which field a peak velocity of 7.25 × 107 cm/s is attained within the first 200 fs after photoexcitation. At higher fields, the increase in transit time with increasing field suggests the onset of negative differential resistance due to intervalley transfer. The existence of transient velocity overshoot at fields lower than the calculated peak steady-state velocity suggests that it occurs while the electrons are primarily in the Γ valley. Full zone Monte Carlo calculations imply that the overshoot is associated more with band nonparabolicity in the Γ valley than with intervalley transfer at fields less than 325 kV/cm, and, in conjunction with theoretical calculations employing a semiclassical transport model, confirm the importance of this nonparabolicity for the determination of the temporal shape of the transient velocity overshoot curves.