Abstract:
In this dissertation we used digital-to-analog (D/A) conversion technique, which uses the analog quantity polarization as a D/A conversion medium. It was implemented by CMOS capacitors or by ferroelectric capacitors, which exhibit strong nonlinearity in charge versus voltage behavior. Because a ferroelectric material inherently has spontaneous polarization and generally has a large dielectric constant, the effective capacitance of a ferroelectric capacitor is much larger than that of a CMOS capacitor of the same size. The data converter based on ferroelectric capacitors possesses the potential nonvolatile memory function owing to ferroelectric hysteresis. Charge scaling DAC architecture is used for polarization-switching digital-to-analog converter (PDAC), the two PDACs implemented are 4-bit CMOS capacitor based DAC and the 4-bit Ferroelectric capacitor based DAC.
To simulate charge switching in ferroelectric capacitors, a pair of exponential growth and decay currents is mapped to the process of polarization reversal. This is based on the fact that these exponential currents, are completely specified by two constants Im and A each accommodates an integral charge Q = I.* 2. Equating this charge to the remanent spontaneous polarization allows for the modeling of switching current. For practical circuit simulations for charge switching, this modeling of switching current is simplified to an exponential decay current whose integral charge is set equal to the total reversed spontaneous polarization. This is because an exponential decay current can be conveniently implemented by charging a series resistor and capacitor (RC) circuit with a pulse-voltage source. The voltage transitions of the pulse source are associated with the polarization reversal and can be controlled with a non-inverting Schmitt trigger that toggles at the positive and negative coercive voltages of a ferroelectric capacitor. The model for the ferroelectric capacitor is designed and this capacitor is used in the polarization switching digital to analog converter.
