We present a time-dependent quantum kinetic simulation on the nonequilibrium electron transport of the symmetrical and asymmetrical double-barrier resonant-tunneling devices with various thickness of barriers. We obtain two bistabilities and hystereses of current found in experiments. Our calculations show that the physical mechanisms are different for the two bistabilities. The first bistability is mainly due to the charge accumulation in the quantum well and the second bistability and hysteresis is related to the current oscillation. Increasing the thickness of the collector barrier can enhance the first bistability while decreasing that of the collector barrier will enhance the second bistability.