Microwave losses in ferroelectric varactors

Paper i proceeding, 2011

The More Than Moore concept of heterogeneous integration of MMICs with non-semiconductor functional components promotes, in particular, integration with ferroelectric based devices. This allows development of next generation reconfigurable/adaptable microwave devices and intelligent/smart systems. The ferroelectric varactors incorporating BaxSr1-xTiO3 films with performance comparable or better than that of semiconductor analogues have been developed and successfully used in commercially competitive microwave circuit demonstrators. The potentially high Q-factor of the BaxSr1-xTiO3 varactors at microwave frequencies is determined by the low intrinsic loss of the material in the paraelectric state. This talk addresses key issues of the microwave ferroelectric varactors development: extrinsic losses associated with film structural imperfections, resonant acoustic phenomena and circuit parasitics. The major extrinsic loss mechanisms in a ferroelectric film are studied in details and modelled. The analyses of field and frequency responses and charge currier transport mechanisms show that the dominant loss is usually due to charged defects associated with oxygen vacancies. This knowledge allows controlled improvement of the film microstructure via optimization of the growth conditions and subsequent reduction of the microwave loss down to its fundamental limit. The loss due to resonant microwave power absorption, caused by field induced piezoeffect, is studied and simulated using electroacoustic model of the multilayer varactor structure. It is proposed and confirmed experimentally that, by optimizing the varactor layered structure, it is possible to suppress the acoustic resonances and reduce the corresponding microwave loss. The analysis of an equivalent circuit model of the varactor shows that the microwave loss associated with parasitic series resistance of the electrode stack and interconnect strips can contribute substantially (even dominate) to the total loss balance. The series resistance loss is strongly depends on the area and shape factor of the varactor plates. This loss can be minimized by optimizing the varactor layout.