PhD Oral Exam - Jie Xu, Electrical and Computer Engineering

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.

Abstract

Photodiodes convert optical signals into electrical signals 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 structures become 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 the 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 (without considering the external circuit) 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 further presented for the first time. The MOSJ-PD is proposed from OSJ-PD by inserting a cliff layer into the absorption layer. Compared with 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 is further suppressed. Thus, both 3-dB bandwidth and output current are improved simultaneously.

Based on the newly proposed OSJ-PD structure, an evanescently coupled one-sided junction waveguide photodiode (EC-OSJ-WGPD) is proposed and investigated numerically. The EC-OSJ-WGPD has a simple structure, while the characteristics of high speed and high output power are maintained. The designed EC-OSJ-WGPD with an absorption layer thickness of 350 nm achieves a bandwidth of 44.5 GHz (without considering the external circuit) and a responsivity of 0.98 A/W.

A unique equivalent circuit model (Circuit Model B), which combines the Technology Computer-Aided Design (TCAD) and microwave circuit simulation, is adopted to analyze the frequency response of InGaAs/InP photodiode. This methodology demonstrates high accuracy in the frequency response analysis. The OSJ-PD and MOSJ-PD with a diameter of 5 µm achieve bandwidths of 119 and 120 GHz, which are 5.3% and 6.2% higher than the well known MUTC-PD. The EC-OSJ-WGPD achieves a bandwidth of 65.5 GHz.