Evanescently Coupled Waveguide InGaAs UTC-PD Having an Over 21 GHz Bandwidth Under Zero Bias

The design and fabrication of an evanescently coupled waveguide InGaAs unitraveling-carrier photodiode (PD) are presented. Through doping the InGaAs absorption layer gradedly, a large built-in electric field can be obtained. As a result, a 4-μm-wide 15-μm-long PD has a bandwidth larger than 21 GHz at zero bias. Passive input waveguide is integrated monolithically with the PD by a simple process, which eases the integration of PD with other optical components.