Measurement and Analysis of Noise Spectra in Terahertz Wave Detection Utilizing Low-Temperature-Grown GaAs Photoconductive Antenna

摘要： Noise power spectral density (NPSD) in time domain terahertz (THz) wave detection systems utilizing GaAs-based photoconductive antennas (PCAs) was investigated quantitatively. The contributions of the PCA noise and the amplifier noises at the amplifier output depend strongly on the resistance of the PCA, the circuit parameters, and the frequency. The PCA has two types of noise: one can be modeled by the Johnson-Nyquist (thermal) noise for the PCA resistance, while the other has an NPSD inversely proportional to the frequency with its intensity dependent on the properties of the GaAs and the metallization. At a high frequency range ~ 100 kHz, voltage-type amplifier noise could appear if the cable capacitance between the PCA and the amplifier is large. As a result, a low-noise range tends to appear in the intermediate frequency range. In comparison with the PCAs with Ti/Au metallization, the PCAs with Pd/Ge/Ti/Au having lower contact resistance lead to lager influence of the Johnson-Nyquist noise at the output.