Complex sentence: 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.
Split into simpler sentences: In GaAs the drift velocity in the third band is lower than in the usual conduction band. With a small increase in the forward voltage, more and more electrons can reach the third band and current decreases.