This study investigated the microwave characteristics of the liquid crystal tunable capacitors for the first time. With the dielectric anisotropy properties, the liquid crystal capacitors presented very different characteristics compared to the semiconductor or MEMS tunable capacitors. For the fractal structures with both the line width and the gap of 5□m, a quality factor of 310 with a control voltage of 5V was achieved at 4GHz. A tuning range of 25.3% for the control voltages from zero to 5V was obtained at 5GHz.
Three different configurations of liquid crystal tunable capacitors were investigated, including fractal structures, comb structures and square structures. The tuning ranges of the fractal structures, the comb structures and the square structures at 4GHz from 0V to 5V were found to be 8.81%, 7.17%, and 11.85%, respectively. The square structures have the largest tuning ranges mostly due to the uniform electric fields produced. The Q-values of the fractal structures, the comb structures, and the square structures at 4GHz with operation voltage of 5V were 68.2, 98.3, and 31.0, respectively. The square structures have the lowest Q-values due to the inductive effects of the structures. In addition, liquid crystals with different chiral doping concentrations were injected into the capacitor structures to investigate the impact of different liquid crystals on the tunable capacitors. The threshold voltages (Vth) increase as the chiral doping concentrations increase. The steepness of the C-V curves increase as the chiral doping concentrations decrease. Furthermore, a liquid crystal capacitor model was developed to qualitatively illustrate the performance of the liquid crystal tunable capacitors. The model prediction and the measurement results were highly correlated.
The results demonstrated the potential applications of liquid crystals as dielectric materials for capacitors with high quality factors and wide tuning ranges at high operation frequencies, particularly suitable for flexible electronics with transparent substrates.

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[31] 李建志，<<液晶與高分子混層結構之雙波混合效應>>，碩士論文，私立中原大學，民國92年。