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Title: Numerical analysis of light absorbing semiconducting devices beyond the conventional 3dB bandwidth
Author: Yi, Qian
ISNI:       0000 0004 2727 6947
Awarding Body: University of Bath
Current Institution: University of Bath
Date of Award: 2012
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This thesis describes an investigation by computer simulation of the performance of two semiconductor device types at the heart of optical high speed data communications, namely the PIN photodiode and the electroabsorption modulator. Both device types operate by light absorption and are therefore likely to have similar factors that limit their performance at high speed. In order to have high speed detection, the PIN photodiode has been investigated through varying the materials, and used a photodiode structure to improve its bandwidth. If output signals beyond the 3dB frequency limit can be well detected by the photodiode, then significant improvements in the detection speed can be achieved. This possibility is a motivation of this thesis. In this study the InP /InGaAs/InP PIN photodiode is chosen because the light at 1.55 urn wavelength can be absorbed by InGaAs. At 1.55J.lm, the fibre is on low dispersion and low loss. A numerical model of a PIN photodiode has been written in C. Comprehensive modelling ofthe PIN photodiode requires a self-consistent solution of the Poisson's equation for calculating the electrostatic potential and the continuity equations for the electron and hole currents. The PIN photo diode model has included the therm ionic current over the hetero-junction, drift current and diffusion current that other models often ignore. After completing an extensive study of the large signal performance of PIN photodiodes at data rates much higher than the conventional 3dB bandwidth, the model was extended to investigate InP/InGaAsP/lnGaAs MQW -EAM under high speed applied bias pulses. The numerical modelling of the MQW-EAM requires a self-consistent solution of the Poisson's equation, the Schrcdinger 's equation and the current continuity equation. The Schrodinger 's equation is for the estimation of carrier concentration in the quantum wells. The MQW-EAM numerical model has applied a special technique for adding the carrier concentration in the quantum wells to the charge density. 11 Large performance has been successfully analysed on PIN photodiode to reveal that optical pulses at repetition frequencies are substantially higher than the conventional 3dB limit can detect (1 ps FWHM and up to 240Gb/s repetition rate) to give photocurrent pulses with an open eye diagram even in the presence of simulated noise, however, these output current pulses tend to spread and merge together sometimes. This tendency can be counteracted to a reasonable extent by using a suitable repetition time, and a large input average power. Similar Gaussian shaped applied bias pulses have also applied to the MQW-EAM, in order to generate fast Gaussian shaped light power, however; the output light pulses spread out nearly 3 times compared with its applied bias pulses under 5ps FWHM and 48Gb/s repetition rate. Thinner l-layer and less quantum wells in the MQW-EAM might be the solution. iii
Supervisor: Allsopp, Duncan Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: numerical analysis ; semiconducting devices