A Monolithic AlGaN/GaN HEMT VCO Using BST Thin-Film Varactor

A monolithic AlGaN/GaN HEMT voltage-controlled oscillator (VCO) using a barium-strontium-titanate (BST) thin-film varactor is presented. The fabrication process of the VCO is fully compatible with the standard process of GaN HEMT monolithic microwave integrated circuits (MMICs). An improved equivalent circuit model of the fabricated BST varactor for microwave application was developed for the MMIC VCO circuit design. The experiment MMIC VCO exhibits 1-GHz tunable bandwidth at the central frequency of 7 GHz when the BST varactor bias changed from 0 to 24 V. An output power of 17 dBm and phase noise of -81 dBc/Hz (at offset of 100 kHz) are obtained at drain bias of 8 V and BST varactor of 0 V. The results indicate that the monolithic BST varactor is suitable for microwave application, and the integration of the BST varactor can be successfully applied to develop various tunable GaN MMICs.