Design of a K-Band CMOS SPST Switch with Enhanced Isolation

A single-pole single-throw (SPST) switch using cross-coupled inductors is proposed to achieve high isolation and low insertion loss in the K-band. The SPST switch is composed of a series-connected transistor, double-stacked shunt transistors on each side, and cross-coupled inductors that connect the node between the stacked transistors to the opposite node of the series device. The cross-coupled inductors are designed to resonate with the parasitic off-state capacitance of the series transistor, thereby generating a peak in the isolation performance. In the opposite state, the inductors create high-impedance shunt paths with the parasitic capacitance of the shunt transistor, enabling minimum insertion loss. The proposed SPST can tune the peak isolation frequency by changing the gate bias of the circuit. The measurement results of the SPST switch, fabricated using 65-nm CMOS technology on a size of 0.2 mm², show an isolation of 45 dB and an insertion loss of 1.6 dB at 24 GHz.