## 4 Measurement Results and Discussion
### Performance Comparison Between two THz Detectors  
Figure 7 shows the voltage responsivity and NEP for gate bias voltage changes of the two detectors. The effective antenna area to obtain the responsivity was calculated to 5.51 \(\times\) 10\({}^{-7}\) m\({}^{2}\) from the receiving area, the THz frequency, and the antenna gain [28]. Figure 7a illustrates the performance of the detector without a preamplifier. In the figure, the voltage responsivity increases as the gate bias voltage is increased and gradually decreases after the gate bias exhibits a maximum voltage responsivity of 202 kV/W at 0.25 V. This results in a graph similar to the quadratic function form of (28) in Section 2 and exhibits a gate bias voltage with the maximum voltage responsivity. The results also indicate that the detector operates in the sub-threshold region because the maximum voltage responsivity is observed when the gate bias voltage is lower than the threshold voltage level. Conversely, as shown in Figure 7b, the voltage responsivity of the detector with the preamplifier increased when the gate bias voltage increased, although it converged to a constant value by the operating region of the sub-threshold preamplifier. The detector exhibits the maximum voltage responsivity of 482 kV/W and is approximately 2.4 times the maximum voltage responsivity of the detector without the preamplifier. Figure 7 also shows the NEP for the detector by using the total noise spectral density measured using a signal analyzer. The NEP of the two THz detectors at the maximum voltage responsivity is minimized due to decreases in the noise around the signal, although the gate bias point is slightly different. The minimum NEPs of the two detectors in the measured results correspond to 130 \(\mathrm{pW}/\sqrt{\mathrm{Hz}}\) and 39.3 \(\mathrm{pW}/\sqrt{\mathrm{Hz}}\). The results reveal that the detector including the preamplifier exhibits lower NEP since its voltage responsivity exceeds that of the detector without the preamplifier.  
Figure 8 shows the output power spectra of two THz detectors and the noise power. The modulation frequency of 200 Hz is used to decrease the effects on the noise of the power supply which are presented at 60 Hz and 120 Hz. The signal-to-noise ratios (SNR) of the two detectors at 200 Hz are measured to be 30.1 dB and 44.0 dB, respectively. The SNR improvement of 13.1 dB in the detector including the preamplifier is due to the increase of the voltage responsivity and NEP by using the preamplifier with the voltage gain of 1. The difference in the characteristics of the two detectors is caused by the presence or absence of the preamplifier at the output of the detector core. Since the preamplifier has a unity gain, it can be understood that the performance improvement of the detector including the preamplifier is not caused by the gain of the amplifier but the increase of the output impedance at the detector core by using the preamplifier. The measurement results between the two  
Figure 6: Measurement setup to obtain the detector performances.  
detectors, as summarized in Table 1, show that it is reasonable that the method to improve the detector performances based on the proposed analytical results in the fabricated CMOS plasmon detectors.