The terahertz gap, lying roughly between 300 GHz (0.3 THz) and 30 THz in the electromagnetic spectrum, exists because the frequencies generated by semiconductor devices based on transistors and lasers do not overlap. Generation of coherent terahertz radiation has traditionally involved either extending electronic techniques to. higher frequencies or extending photonic sources to longer wavelengths. In both cases, the efficiency drops rapidly as the frequency approaches the terahertz region. We recently fabricated GaAs/AlGaAs quantum cascade lasers, in which a high-confinement metal-metal waveguide was employed and fabricated using In-Au metallic bonding technique. The devices demonstrated lasing operation at a wavelength of around 104.6 mu m (or about 2.9 THz in frequency). In this article, we first present the fabrication and electrical and optical characterizations of the terahertz quantum cascade lasers. We then characterized a set of terahertz quantum cascade lasers with otherwise identical device parameters but the doping concentration. The delta-doping density for each period was varied from 3.2 x 10(10) to 4.8 x 10(10) cm(-2). We observed that both the lasing threshold and the free carrier absorption caused the waveguide loss increase monotonically. Interestingly, however, the observed maximum lasing temperature displayed an optimum at a doping concentration of 3.6 x 10(10) cm(-2). (c) 2006 American Vacuum Society.