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An experimental study of the effects of injector barrier thickness on the performance of terahertz quantum-cascade lasers is reported. The injector barrier is varied from 54 to 38 Aring in an otherwise identical 3.4 THz laser structure. It is shown that the threshold current density at 10 K increases monotonically from 445 A/cm2 to 1.29 kA/cm2 with reducing barrier thickness. On the other hand, an optimum injector barrier thickness of 44 Aring is found to give rise to a maximum lasing temperature of 141 K.