The authors present the effects of the doping concentration on the performance of a set of terahertz quantum-cascade lasers emitting around 2.75 THz. The chosen design is based on the longitudinal-optical-phonon depopulation of the lower laser state. An identical structure is regrown varying the sheet density from 5.4×109 to 1.9×1010 cm-2. A linear dependency of the threshold current density on the doping is observed. The applied field where lasing takes place is independent of the doping. The field is responsible for the alignment of the cascades and therefore the transport of the electrons through the structure.