An efficient continuous wave (CW) THz source working at nominal room temperature is described. Optically pumped room temperature THz emission is observed from various kinds of semiconductor bulk crystals. In order to investigate the emission mechanism, temperature dependences of terahertz emission intensity in various semiconductors are measured. Semiconductor samples used are InSb, InSb:Ge, InAs, GaSb, Ge, and Si. From these results, it is shown that the temperature dependences of emission characteristics are different between direct and indirect transition semiconductors, and that the high resistive Ge is suitable for room temperature THz emitter via intracenter transitions excited by IR pump lasers.