110-GHz GaInAs/InP double heterostructure p-i-n photodetectors

Long-wavelength GaInAs/InP graded double heterostructure p-i-n photodiodes are demonstrated with 3-dB bandwidths over 100 GHz. The heterojunction hole trapping problem is significantly improved and the device contact resistivity is greatly reduced by using superlattice graded bandgap layers at the hetero-interfaces to reduce the barrier height. Self-aligned processes are used in the device fabrication to reduce device parasitics. Pulsewidths as short as 3.0 ps full-width-at-its-half-maximum (FWHM) for 2 μm×2 μm device are measured by pump-probe electrooptic sampling. 3-dB bandwidths over 100 GHz are found for 2 μm×2 μm and 3 μm×3 μm devices. The device with the integrated bias tee can be biased without using the external bias tee. The electrical resonance between the photodiode and external circuits was reduced by integrating an impedance matched resistor in parallel with the photodiode. The 7 μm×7 μm device with bias tee and matched resistor has a measured pulsewidth of 3.8 ps and a 3-dB bandwidth over 100 GHz. The calculated pulse shape based on the saturation velocity model fits well with the measured response. A model for different components of the series resistance agrees with the measured area dependence of the series resistance.