Manipulation of the symmetry of localized terahertz field in an AlGaN/GaN high-electron-mobility transistor integrated in a waveguide.

A terahertz wave coupled to a field-effect transistor is usually strongly localized in the gated electron channel. While a strongly localized and asymmetrically distributed terahertz field is welcomed to induce a direct-current (DC) photocurrent and hence helps to improve the sensitivity of homodyne detection, it limits the heterodyne detection sensitivity since the shot noise induced by the DC photocurrent increases with the local oscillator (LO) power. Therefore, the localized field distribution must be carefully tailored to maximize the heterodyne mixing efficiency while minimizing the homodyne mixing of the LO signal. This paper reports the manipulation of the symmetry of a localized terahertz field in a waveguide-coupled AlGaN/GaN high-electron-mobility transistor (HEMT) to improve the heterodyne detection sensitivity in the 340 GHz band. Simulation and experimental results demonstrate that both heterodyne and homodyne mixing are strongly localized at the ends of the gated electron channel; the heterodyne mixing signals are in phase, while the localized homodyne mixing signals (DC current) are out of phase. By regulating the symmetry of the localized terahertz field distributions excited by the radio frequency (RF) terahertz signal and the LO terahertz signal, a fairly symmetric distribution of the LO field is achieved to suppress the DC homodyne current induced by the LO signal, thereby effectively lowering the overall shot noise. The overall detector noise is elevated by only 2 dB when the LO power is increased by 20 dB into saturation. While the responsivity is kept as low as 6.7 mA/W at 336 GHz for the LO signal, the homodyne responsivity and the heterodyne noise-equivalent power (NEP) for the RF signal are maintained at 72 mA/W and -146 dBm/Hz, respectively.