| High-speed photodetectors are very important components for fiber communication. In conventional photodetectors, the bandwidth is limited by the carrier transit time across the intrinsic region. As the transit time decreases by thinning intrinsic region, the higher RC roll-off frequency, however, limits the device speed. The inevitable trade-off between RC-circuit and carrier transit time limits this kind of photodetector bandwidth. Although the traveling wave photodetector (TWPD) can overcome the RC-imposed limitation by impedance and velocity matching, the design of the intrinsic region is restricted by the geometry of transmission line. By using low-temperature grown GaAs (LT-GaAs), the material response is dominated by the carrier-trapping instead of carrier-transit time. The design of material limits to bandwidth can thus be independent from the RC-distributed effects. In this dissertation, a novel p-i-n photodetector incorporating the LT-GaAs and TWPD circuit structure is demonstrated. The performance is found to be enhanced by the taking advantage of LT-GaAs and TWPD structure.;A distributed photodetector model is used to optimize and design the waveguide structure. The microwave loss and dispersion, velocity mismatching, and the microwave reflection on waveguide are considered in this model. An equivalent circuit model is used to calculate the properties of microwave propagation. The optimum performance can be achieved at the impedance matching, velocity matching and the low loss microwave transmission. An impulse response with 530 fs FWHM is measured by an electro-optic sampling technique. The corresponding -3dB bandwidth is as high as 560GHz. The optical power dependent and bias dependent measurement reveals that the high-speed performance is mainly attributed to the short carrier trapping time in LT-GaAs.;Long wavelength (1300nm ∼ 1550nm) light can be absorbed in the LTGaAs due to the subbandgap defects and As precipitates. An LT-GaAs waveguide photodetector is used to improve the quantum efficiency by increasing the device length. High speed (18GHz bandwidth) with 1% quantum efficiency was demonstrated at long wavelength absorption on GaAs-based material. This opens up the possibility for long wavelength communication using AlGaAs material. |
