Terahertz Phase Modulation by MES/MIS Gate Capacitor in AlGaN/GaN HEMTs

Terahertz phase shifters, as essential components of phased-array antennas, are vital for dynamic beamforming and communication-sensing scenarios. This paper explores the feasibility and key mechanisms of using AlGaN/GaN high-electron-mobility transistor (HEMT), integrated with an on-chip resonant antenna, for free-space terahertz phase modulation. Continuous and wide-range phase shift up to nearly 180° at 336 GHz is achieved by varying the gate voltage. By comparing the Metal-Semiconductor (MES) HEMT with Schottky gate contact and the Metal-Insulator-Semiconductor (MIS) HEMT with dielectric deposition beneath the gate, we found that the phase modulation arises from the impedance change due to the nonlinear regulation of the two-dimensional electron gas (2DEG) concentration in the sub-threshold region. The phase shift is affected by the synergy of the Schottky capacitance and the edge parasitic gate-channel capacitance. The phase shift and the accompanied amplitude modulation are strongly related to the transconductance tuned by the gate voltage. Direct binary phase shift keying (BPSK) is realized by dynamically coding the gate voltage from a list of stored gate voltage levels.