A theory of generation in a two-subband "Stark ladder" with a coherent electron subsystem is developed. In the proposed model, electrons reach the upper level of a quantum well due to resonant tunneling and pass to the lower level of the well (vertical transitions), emitting a photon hω, then tunnel resonantly to the upper level of a neighboring well, performing a radiative transition, and so on until electrons leave the lower level of the last well. A static electric field applied to the superlattice shifts the levels so that the lower level of the nth well coincides with the upper level of the (n + 1)th well. Analytic expressions are derived for the wave functions and polarization currents of an N-well structure. The possibility of bulk oscillation of the N-well structure in the optimal mode with an efficiency close to unity, weak reflection, and a linear dependence of the power on the pumping current is demonstrated. The total generation power is proportional to the number of wells. For structures with an even number of wells, the energy of electrons from the emitter must simply coincide with the resonance energy for any laser fields; i.e., the energy tuning which is necessary in a single-well structure is not required. Universal relations are derived for parameters of the N-well structure, which ensure the simultaneous fulfillment of resonance conditions in all the wells. The possibility of coherent lasing in a one-subband Stark ladder with a lower gain is also indicated.
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