Application of RF Varactor Using BaxSr1-xTiO3/TiO2/HR-Si Substrate for Reconfigurable Radio

In this paper, the potential feasibility of integrating Ba_xSr_1-xTiO₃ (BST) films into Si wafer by adopting tunable interdigital capacitor (IDC) with TiO₂ thin film buffer layer and a RF tunable active bandpass filter (BPF) using BST based capacitor are proposed. TiO₂ as a buffer layer is grown onto Si substrate by atomic layer deposition (ALD) and the interdigital capacitor on BST(500 nm)/TiO₂(50 nm)/HR-Si is fabricated. BST interdigital tunable capacitor integrated on HR-Si substrate with high tunability and low loss tangent are characterized for their microwave performances. BST/TiO₂/HR-Si IDC shows much enhanced tunability values of 40% and commutation quality factor (CQF) of 56.71. A resonator consists of an active capacitance circuit together with a BST varactor. The active capacitor is made of a field effect transistor (FET) that exhibits negative resistance as well as capacitance. The measured second order active BPF shows bandwidth of 110 MHz, insertion loss of about 1 dB at the 1.81 GHz center frequency and tuning frequency of 230 MHz (1.81-2.04 GHz).