Field-effect transistors are capable of detecting electromagnetic radiation from less than 100 GHz up to very high frequencies reaching well into the infrared spectral range. Here, we report on frequency coverage of up to 30THz, thus reaching the technologically important frequency regime of CO2 lasers, using GaAs/AlGaAs high-electron-mobility transistors. A detailed study of the speed and polarization dependence of the responsivity allows us to identify a cross over of the dominant detection mechanism from ultrafast non-quasistatic rectification at low Terahertz frequencies to slow rectification based on a combination of the Seebeck and bolometric effects at high frequencies, occurring at about the boundary between the Terahertz frequency range and the infrared at 10THz.