## 4 Results and Discussions  
The TeraFET response on the homodyne excitation at different gate bias voltages is presented in Figure 4a. For low gate voltages (below the threshold point at 445 mV), the voltage response is higher; however a signal-to-noise ratio (SNR) is lower due to an increase in noise. The SNR is defined as the ratio between the rms amplitude of the measured signal and the standard deviation of noise values. The best SNR for the investigated detector was obtained at a gate voltage bias of 475 mV. The graph also shows measured detector noise levels (dashed lines) obtained from the recorded statistics (standard deviation) of the response while blocking THz beams with an opaque metallic plate placed in front of the detector. These data allow calculating the dynamic range of device sensitivity as well as comparing with the theoretically expected thermal Johnson-Nyquist noise for the unbiased TeraFET's channel [35; 36]. The theoretical noise voltage can be calculated using the simple equation \(V_{\text{noise}}=\sqrt{4k_{\text{B}}TR_{\text{DC}}\Delta f}\), where the equivalent noise bandwidth \(\Delta f\) in our case was equal to 1.25 Hz (for 100 ms integration time). \(V_{\text{noise}}\) is shown in Figure 4a with hollow symbols. The measured detector noise levels correspond well to the theoretically expected thermal noise limit, which indicates good noise suppression techniques (such as shielding or removing ground loops) implemented in our setup, and validates the NEP results presented in Figure 2.  
However, even when the detector noise was mainly of thermal origin, response curves showed similar additional signals at high frequencies (above 1.7 THz). The origin of this signal is not yet clearly understood. It can be either below 100 GHz as the result of beating between laser Fabry-Perrot modes or very broadband IR radiation from the local heating of the photomixer. There were several recent reports that the TeraFET detector could act as a detector of passive radiation [37; 38]. The same is expected from Schottky-diode detectors. Figure 4b shows the dynamic range of the measured response's dependency on the frequency for the gate voltage bias of 475 mV. The dynamic range reached a signal-to-noise ratio (SNR) of over 70 dB at 500 GHz and 100 ms integration time. The commercial homodyne system based on the same photomixer emitter and additional photomixer receiver can reach an SNR of ca. 83 dB, as declared by the manufacturer in the specification and confirmed by our independent tests. In our experimental work with one photomixer-based THz source [21], TereFET's SNR was 43 dB at 500 GHz and 100 ms. However, the commercial system uses a setup with four two-inches mirrors, which allows having the smallest losses in the optical path. The investigated system is based on a wire grid polarizer that divides each beam and total power in two. The second optical branch after the polarizer can be used in the future, for example, for additional reflection measurements. The observed improvement in SNR of the TeraFET using the homodyne detection technique compared to the direct one could represent clearly seen water vapor lines [39] for frequencies over 2 THz, as shown in Figure 4b in comparison to Figure 3a. This allows us to conclude that these detectors can find practical application in continuous-wave THz spectrometry systems. Adding the second detector to the same system will allow to simultaneously record transmission and reflection amplitudes as well as propagation delay times, and therefore to measure complex dielectric properties as with heterodyne-based vector network analyzers. TeraFETs have an advantage in comparison with other uncooled semiconductor THz power detectors in that they can be manufactured using mature commercial microchip mass-production technologies and hence can be realized as large-area matrices. Figure 4: (**a**) Comparison of homodyne system signals for different gate biases. The corresponding noise levels for each bias are marked with dashed lines.