Temperature dependence of reconfigurable bandstop filters using vanadium dioxide switches

In this Letter, we report and investigate the temperature dependency of various radio frequency (RF) parameters for a fabricated reconfigurable bandstop filter with vanadium dioxide (VO2) switches measured up to 55 GHz. Here, the insulator-to-metal (ITM) and metalto- insulator transition (MIT) hysteresis of the VO2 thin film influence on the RF characteristics of the filters is analyzed from 25 C and 120 C in heating and cooling. The resonance frequency and maximum insertion loss (IL) stability and sensitivity with temperature variations are explored. It is noticed that increasing the temperature to 50 C from 25 C (or decreasing it to 50 C from 120 C) will result in a less than 1% fractional frequency shift with respect to the off and on resonance frequencies. The sharp DC conductivity level variations of the VO2 thin film around the transition temperatures translate into sharp effects on the resonance characteristics of the filters. On the contrary, the maximum IL levels are less sensitive to the sharp conductivity changes of DC films around the VO2 transition temperature. A unique behavior is reported when successively heating-up and cooling-down, over and below, respectively, the transition temperature of VO2: the fabricated filter exhibits completely different resonance frequencies. This suggests that in the temperature dependence of the VO2 RF design, the practical use of reconfigurable RF functions has to take into account the history of thermal effects and increase or decrease in the device temperature when crossing the IMT/MIT transition point.