Terahertz responsivity and low-frequency noise in biased silicon field-effect transistors

We report on a comprehensive study of terahertz responsivity and low-frequency noise of current-biased antenna-coupled silicon field-effect transistors. Our measurements corroborate the findings of earlier studies that the current bias dramatically enhances the responsivity by at least an order of magnitude. However, this impressive improvement is accompanied by an equally drastic increase of the spectral density of the voltage fluctuations with the applied current. Therefore, the resulting signal-to-noise ratio (SNR) increases at most by a small amount and only for sub-threshold conditions. We do not register an absolute improvement beyond the best SNR value of the unbiased device. These findings are compared with theoretical absolute-SNR estimates derived using a hydrodynamic transport description. It predicts the maximal enhancement to be limited by a factor of about 1.35. In practical devices, the onset of excess noise suppresses even this little enhancement and limits the applicability of current biasing scheme to applications which exploit modulation frequencies above excess noise corner frequency.