Microwave performance of thin film ferroelectric varactors in the wide temperature range of −223 °C to +227 °C

Parallel-plate Au(Pt)/Ba0.25Sr0.75TiO3/(Pt)Au thin film varactors are fabricated on high resistive Si substrate and characterized at 1 MHz and microwave frequencies up to 45 GHz in the temperature range of −223 °C to +227 °C. The relative tunability of capacitance decreases with temperature, as capacitance does, from 80 to 90% at −193 °C down to 10% at +227 °C. The temperature coefficient of capacitance in the temperature range −55 to +125 °C is approximately 0.3% at 20 GHz and zero dc field. The temperature dependence of the varactor loss tangent, in general, follows that of the capacitance. The loss tangent at zero dc field and 20 GHz is less than 0.1 at −193 °C and less than 0.02 at +227 °C. The figure of merit of the varactor, taking into account both the tunability and the loss tangent, at 20 GHz is more than 1000 at −193°C and 50 at +227 °C.