We present experimental results of THz detection signal dependence on the magnetic field. THz radiation was detected with AlGaAs/GaAs high electron mobility transistor where 2D plasma excitations are responsible for the detection signal appearance. We observe both Shubnikov - de Haas oscillations and cyclotron resonance in the detection signal. Fourier transform analysis of the oscillations allowed us to observe two parts of the 2D electron gas: gated and ungated. This allows us to conclude that 2D electrons in the transistor channel are necessary for detection process and both gated and ungated parts takes part in that process.