Design of a Multi-GHz SiGe HBT EO Modulator

Silicon-based electro-optic (EO) modulator is an indispensable building block for integrated lightwave circuits. In this paper, we report an EO modulator that incorporates a heterojunction bipolar transistor (HBT) with Ge composition graded base. The emitter is n-type doped silicon with a doping concentration of 10 21 /cm 3 . The width of the emitter strip is 0.2μm and the thickness of the emitter layer is 0.16μm. The base has a thickness of 40nm with varying Ge composition from zero at the emitter-base junction side to 20% at the base-collector junction side. Raised extrinsic base is incorporated for base contact. The intrinsic base is p-type doped with a concentration of 4×10 19 /cm 3 . The HBT is biased at V CE = 0.5 V whereas VBE is switched between -1.0V and 1.0V. The carrier distribution at "ON" state of the EO modulator and the transient analysis are performed by MEDCI simulation. The changes of the refractive indices of the HBT are computed from the carrier density in all regions, and then the refractive index map is imported into an optical mode solver (RSoft BeamProp). The HBT EO modulator that supports only one optical mode is ideal, but a trade-off between modal property and device speed is observed. For current design, we achieved a π-phase modulation length of less than 600μm, and a switching delay less than 62ps.