Abstract

Uni-traveling-carrier photodiode (UTC-PD) is very attractive for future fiber-optic systems, since it has exhibited high-speed and high-power performance. In this thesis, the bandwidth and saturation current of UTC-PD is investigated by using physics-based modeling. To further improve the performance, novel device structures are proposed.
On the one hand, a graded bandgap structure is employed in the absorption layer. It is shown that, similar to the effect of graded doping method, the electric field in the absorption layer is increased, and thus the bandwidth is improved. Moreover, both the graded doping and graded bandgap structure are optimized. It is found that, for the considered UTC-PD, combining use of the graded doping and graded bandgap in the absorption layer leads to an improvement of 39.4% in bandwidth.
On the other hand, linear doping profile and Gaussian doping profile are considered to be used in the collection layer. It is shown that, the distribution of electric field in the depletion region is improved, which leads to the better saturation performance. For the considered UTC-PD, by using optimized linear doping profile and Gaussian doping profile, the improvement in saturation current is 18.7% and 25.8% respectively.
Additionally, two types of epitaxial structure have been grown. Both of them are predicted to exhibit excellent broadband and high-output power performance. The benefit of the proposed graded bandgap absorption layer is further verified. Moreover, the major steps of fabrication process flow are described.