Improved Equivalent Circuit Model of p-i-n Diodes for Amplitude and Phase Controllable mmWave Reconfigurable Intelligent Surfaces

An improved equivalent circuit model (ECM) of p-i-n diodes is developed and applied to amplitude and phase controllable mmWave reconfigurable intelligent surfaces (RISs). The proposed ECM takes into account the variable junction resistance and capacitance of forward-biased p-i-n diodes in the transition state (instead of a conventional simplified resistance model) that is critical at mmWave frequencies. The flip-chip AlGaAs p-i-n diodes have been characterized in a 2-port coplanar measurement setup. Both the improved ECM and the electromagnetic model are constructed and validated. The ECM is used for a 28-GHz RIS design with amplitude and phase control of the reflection coefficient. The observed non-linear performance combined with an additional phase shift of up to 84° introduced by the variable junction capacitance indicates the value of the proposed model.