The concept of THz detection based on excitation of plasma waves in two-dimensional electron gas in Si FETs is one of the most attractive ones, as it makes possible the development of the large-scale integrated devices based on a conventional microelectronic technology including on-chip antennas and readout devices integration. In this work we report on investigations of Terahertz detectors based on low-cost silicon technology field effect transistors and asymmetric unit cell double grating gate field effect transistor. Double-grating-gate field-effect transistors have a great potential as terahertz detectors. This is because the double grating gate serves not only for carrier density tuning but also as an efficient THz radiation coupler. In this paper, we present characterization of these transistors using high magnetic fields. Low and high magnetic field data are used to determine the electron mobility and electron concentration, respectively, in different parts of the transistor channel. We show that detectors, consisting of a coupling antenna and a n-MOS field effect transistor as rectifying element, are efficient for THz detection and imaging. We demonstrate that in the atmospheric window around 300 GHz, these detectors can achieve a record noise equivalent power below 10 pW/Hz0.5 and responsivity above 90 kV/W once integrated with on-chip amplifier. We show also that they can be used in a very wide frequency range: from ̃0.2 THz up to 1.1 THz. THz detection by Si FETs paves the way towards high sensitivity silicon technology based focal plane arrays for THz imaging.