TIME OF TUNING OF FERROELECTRIC DEVICES
The time required to tune microwave devices is significant parameter for a number of applications such as phase shifters, variable delay lines, and tunable filters. Theory of dielectric constant relaxation in ferroelectrics which was confirmed by experiments with ideal structures (single crystals) demonstrates that the time of the dielectric constant variation under harmonic E-field is limited by the relaxation time of the soft ferroelectric mode of crystalline lattice ~10-11 sec. However, defects in ferroelectric films (boundaries of grains) and in near electrode regions can radically complicate the process of time response. Most probable phenomena that could suppress the fast response of dielectric constant are the processes of e-injection from electrodes and capture of charge carries in different traps of defects. These processes are characterized by a large spectrum of different time constants in the range from a few microseconds to a few minutes. In order to distinguish the time relaxation processes in the ferroelectric films under application of unipolar voltage video pulses (generally used to control the tunable devices), the pulses with different amplitudes (1-60V/µm) and time scale (1ns - 10s) were used for our experiments presented below.
Response time of ferroelectric permittivity to unipolar voltage pulses of microsecond scale
for tunable elements based on (Ba,Sr)TiO3 films with different structures the relaxation time can be in the range (10-11-10-9)s, that makes possible to design the fast acting microwave devices.