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To study the substrate effect on transformer performance, several types of spiral transformers were fabricated on p+ silicon substrate with microporous silicon (PS) regions. Our analysis shows that the use of PS significantly reduces the substrate effects including eddy current and capacitive coupling between spirals and the substrate. The reduced substrate effects lead to higher Q-factor and resonant frequency (fr), mutual reactive coupling coefficients (kIm) with larger useable band width and higher available gain (Ga) mainly because of the reduction in power loss to the substrate. The use of PS can greatly increase transformer performances and broaden the parameter space for on-chip transformer layout optimization.