Another FDTD area which has gained recent attention is the area of active and passive device modeling. Sui et al. [124] extended the 2-D FDTD method to model lumped elements, including nonlinear elements such as diodes and transistors. Ko [125], meanwhile, used the FDTD technique to model various microwave integrated circuit components, such as branch-line couplers and filters. Wolff et al. [126] used the FDTD method to model planar microwave circuits, containing nonlinear active components. Toland and co-workers [127,128,129] have also used the FDTD method to model nonlinear active elements, including a cavity oscillator and a two element active antenna. Luebbers et al. [130] investigated antennas with both linear and nonlinear loads. Thomas et al. [131] have developed an approach for coupling SPICE lumped elements into the FDTD method and used this method to model the same active antenna that was originally investigated by Toland [132]. Recently, Piket-May et al. [133] have extended Sui's 2-D analysis to model full wave propagation in 3-D circuits containing both active and passive loads.