Two parameters are adopted to characterize the transport process in double-barrier quantum-well (DBQW) structures. It is shown that, in general, both coherent resonant tunneling and incoherent sequential tunneling processes are possible. However, for most high-frequency applications, large current densities (>104 A/cm2) are required and, therefore, the sequential process is unlikely to occur. To determine the high-frequency capability, both the adiabatic limit of the dc current-voltage curve as well as the capacitance charging time in an embedding circuit need to be considered. We confirm that the DBQW structures with barrier thickness of 20 Å or smaller can operate up to about 1 THz.