Design and Investigation of High Speed and High Power InGaAs/InP One-Sided Junction Photodiodes

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. \n \nTo 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. \n \nA 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. \n \nBased 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. \n \nA 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.