A 6.7-fF/μm2 bias-independent gate capacitor (BIGCAP) with digital CMOS process and its application to the loop filter of a differential PLL

A linear bias-independent gate capacitor (BIGCAP) with large intrinsic capacitance and low parasitic capacitance is proposed. BIGCAP is composed of a pair of accumulation-mode n-poly gate capacitors in an n-well and a pair of pMOS gate capacitors, which requires no additional fabrication process steps. Measured results with 1.5-V 0.13-μm digital CMOS technology show that the intrinsic capacitance is 6.7 fF/μm² (6.7 times bigger than that of typical MIM capacitors) and the parasitic capacitance is 1.9% of the intrinsic capacitance (1/5 that of typical MIM capacitors). The linearity is ±2.9% and capacitance variation across a wafer is as small as σ= 0.096%. For a 0.1-V threshold voltage variation, the capacitance variation was only σ= 0.69% and the linearity ranged from ±2.84% to ±2.93%. For three types of BIGCAP using 1.5-V, 2.5-V, and 3.3-V MOSFETs, less than ±4% linearity is achievable by optimizing the ratio (x) of the pMOS gate capacitors´ area to the area of the n-poly gate capacitors, and the optimum x value is within a range of 15%-25%. BIGCAP has been applied to the loop filter of a differential phase-locked loop (PLL) and reduces the gate area of the largest loop filter capacitor to only 35% of that of the conventional design while achieving reasonable jitter of 7.0 ps (rms) and 74.4 ps (peak-to-peak) at 840 MHz with a 1.5-V supply.