Layout-optimized inductors with gradually varying width and spaced (tapered) spirals are well known for their higher quality factor ( Q ) characteristics. For the first time, the significance of tapered inductors in achieving higher inductance ( L ) density is brought out in this article. Instead of having a constant spiral pitch (width + space), gradually reducing pitch with increased turns is shown to significantly benefit L density without compromising on Q. Following this approach, inductor figure-of-merit (FOM) is shown to increase with L density using variable pitch spirals while it is reduced in other spiral configurations. It is also revealed that for fixed inner and outer diameters, significant FOM improvement can be achieved only through increased L density and not through higher Q. Moreover, the proposed approach is shown to have improved inductor FOM across substrate resistivities and metal thickness values. Prototype-tapered inductors (shown in Fig. 1) designed using the proposed approach are fabricated using 0.35- mu textm BiCMOS technology. Measurements (which are in good agreement with electromagnetic (EM) simulations) show higher L values of 6.6 nH with a high- Q of 23 usable at 3.8 GHz, thereby achieving a very high FOM of 4.3. Fig. 1.Chip images of prototype variable width and constant spacing inductors. (a) D1. (b) D2. © 1963-2012 IEEE.