In GaAs the drift velocity in the third band is lower than in the usual conduction band, so with a small increase in the forward voltage, more and more electrons can reach the third band and current decreases.
The drift velocity in the third band is lower than in the usual conduction band. This is in GaAs. More and more electrons can reach the third band. This is with a small increase in the forward voltage. More and more electrons can reach current decreases. This is with a small increase in the forward voltage.