A Voltage-Controlled Oscillator Using Variable Capacitors with a Thin Dielectric Electrolyte Film

Variable capacitance functions are demonstrated utilizing the transport of Li ions in a simple Pt/lithium phosphorous oxynitride (LiPON)/Pt structure. The capacitance originates mainly from the formation of an electrochemical double layer (EDL) at the interfaces between LiPON and Pt at lower frequencies. Voltage applications drive Li ions from the positively biased electrode to the negatively biased one. At higher voltages, the number of Li ions at the positive electrode decreases to zero, which contributes in making the total EDL capacitance vanish. As a result, the LiPON capacitor exhibits decreased capacitance as the voltage bias is increased, similar to what is observed in a varactor diode. The upper frequency limit of the capacitance variation increases with an increase in the temperature, which seems to be determined by the ionic conductivity of the LiPON matrix. A voltage-controlled oscillator (VCO) operation is also demonstrated by incorporating the LiPON variable capacitors into a resistor-capacitor oscillator circuit. The VCO clearly shows that the oscillation frequency of the output waveforms is exponentially increased with an increase in the input direct current voltage, whose behavior is the same as a varactor diode-based VCO. The results indicate that this ionic variable capacitor can be used as a building block in analog and mixed signal electronic circuits.