Transformers in many RF IC applications are very effective component for impedance transforming, signal coupling, phase splitting (balun), etc. Despite of many applications in RF IC, transformers have not been integrated in silicon RF IC due to lower performance when than that of off-chip transformer. Integrated magnetic elements, such as inductor and transformer, in silicon RF IC have low performance due to relatively high conductivity of silicon substrate and the ohmic loss by thin metallization.

A new spiral-type suspended transformer for silicon RF IC¡¯s has been fabricated by our surface micromachining technology. Because the suspended transformer has sufficient air gap between substrate and transformer and thick copper metal line, substrate and ohmic losses, dominant loss factors in transformer, are reduced. The fabricated transformer on the standard silicon substrate (resistivity ¡í 10§Ù¡¤cm) has shown a very low S21 of -1.9dB at 1GHz. The equivalent circuit for the fabricated transformer has been adequately proposed and its model parameters have been extracted.

Figure 2. Model of inductor and transformer