Fig. 1. AlGaN/GaN HEMT terahertz detector and response characteristics. (a) Microscope image of the detector integrated with a WR2.8 waveguide and a 2 mm silicon lens; (b) Schematic of the input and output signal routes ; (c) Equivalent circuit; (d) SNR of IF signal as a function of the gate voltage; (e) SNR of IF signal as a function of the local power

Fig. 2. Circuit diagram of THz vector test system

Fig. 3. System stability of amplitude and phase vs time

Fig. 4. Test result of quasi-optical system and space distribution. (a) Structure of system; (b) Photo of system; (c) Normalized amplitude distribution of y-z section; (d) Normalized amplitude distribution to the perpendicular propagation direction at 15 mm of the spot & fit by Gaussian function; (e) Phase distribution

Fig. 5. Amplitude and phase distribution of quasi-optical system at x-y plane. (a), (d) Amplitude and phase of 15 mm before waist; (b), (e) Amplitude and phase at waist; (c), (f) Amplitude and phase of 15 mm after waist