  We report three-well, resonant-phonon depopulation terahertz quantum cascade
lasers with semi-insulating surface-plasmon waveguides and reduced active
region (AR) thicknesses. Devices with thicknesses of 10, 7.5, 6, and 5 {\mu}m
are compared in terms of threshold current density, maximum operating
temperature, output power and AR temperature. Thinner ARs are technologically
less demanding for epitaxial growth and result in reduced electrical heating of
devices. However, it is found that 7.5-{\mu}m-thick devices give the lowest
electrical power densities at threshold, as they represent the optimal
trade-off between low electrical resistance and low threshold gain.
