| Summary: | 碩士 === 國立成功大學 === 微電子工程研究所碩博士班 === 92 === CMOS varactors and junction varactors as tunable capacitors are important for CMOS integration circuits especially for the voltage control oscillators (VCOs), and may enable the realization of an inexpensive single chip transceiver. Varactors for RF applications are discussed and compared in this thesis. Three CMOS varactors with different structure were characterized in detail for performing comparisons, i.e., (a) the P+-to-N well junction varactor, (b) the Inversion mode MOS varactor, and (c) the Accumulation mode MOS varactor. The evaluation is mainly based on two major performance parameters, i.e., the quality factor (Q-value) and the capacitance-tuning-ratio (Cmax/Cmin ratio). The varactors were evaluated at frequencies from 200 MHz up to 20 GHz, for the frequency range is the most interesting with respect to wireless applications.
These devices were fabricated in standard CMOS processes with minimum feature size of 0.18 um, and the capacitance, resistance and Q-value versus voltage and frequency curves were measured. Additionally, the layout optimization was also reported. By comparing the three evaluated varactors, the P+-to-N well junction varactor has the highest Q-value, but its Cmax/Cmin ratio is too low to apply in practice. On the other hand, the Accumulation Mode is concluded as the best-suited varactor type for wireless applications.
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