When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.
Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.
Photodiodes convert the optical signal into an electrical signal and are widely used in optical fiber communication systems, photonics generation of millimeter-wave (MMW) and terahertz (THz) wave signals, radio-over-fiber wireless communication systems, etc. In these applications, photodiodes play a key role. Nowadays, the well known uni-travelling carrier photodiodes (UTC-PDs) have been widely used in the aforementioned applications since its first invention in 1997. Over the past two decades, the performance of UTC-PD and its derivatives has been improved continuously. However, the epitaxial layer structure becomes more and more complex.
To simplify the structure and improve the performance of photodiodes, a high-speed one-sided junction photodiode (OSJ-PD) with low junction capacitance is proposed for the first time. The OSJ-PD is proposed based on the structure of InGaAs Shottky barrier photodiode (SB-PD) and UTC-PD. It has been demonstrated that the OSJ-PD has the characteristics of the simple epitaxial layer structure, high speed, high output power, and low junction capacitance. The OSJ-PD with 300 nm absorption layer thickness has achieved a bandwidth of 64 GHz and a photocurrent density of 2.4×105 A/cm2 under a 10 V bias voltage.
A modified InGaAs/InP one-sided junction photodiode (MOSJ-PD) is presented for the first time. The MOSJ-PD is proposed from OSJ-PD by inserting a cliff layer into the absorption layer. Compared to the modified uni-travelling carrier photodiode (MUTC-PD), the MOSJ-PD has the advantages of simpler epitaxial layer structure and lower junction capacitance. In MOSJ-PD, the space charge effect at high light intensity can be suppressed. Thus, both 3-dB bandwidth and output current can be improved simultaneously.
Based on the new OSJ-PD structure, evanescently coupled one-sided junction waveguide photodiodes (EC-OSJ-WGPDs) are proposed and investigated numerically. The OSJ-PD has a simple structure, while the characteristics of high speed and high output power are maintained. The multilayer diluted waveguide and coupling layers have been modified to reduce the sensitivity to the TE and TM polarized light. The designed EC-OSJ-WGPDs (PD3 and PD5) with absorption layer thicknesses of 200 and 300 nm achieve bandwidths of 44 and 41 GHz, respectively. The corresponding responsivities are 0.31 and 0.57 A/W.