Design of Coil Length of On-Chip Transformer With High Turn Ratio and High Coupling Performance

This paper examines the effect of coil length on the turn ratio and coupling coefficient of an on-chip transformer with multiple metal layers. This paper develops a transformer layout that utilizes one winding loop in a metal layer with a ladder where the pieces of metal between two coils crossed. According to the relevant analytical formula, a transformer is properly arranged with two coil locations to ensure high performance. To demonstrate the proposed algorithm, a total of five devices are fabricated using 0.13-μm CMOS technology. Measurements reveal that the proposed transformer has a turn ratio of n = 5.59, a coupling coefficient of k = 0.7, and an inductance density of 1052 nH/mm².