Xiaoguang Zheng

Recent advances in avalanche photodiodes

Until the early 2000s, the avalanche photodiode (APD) was widely deployed in high-performance optical receivers that operated up to 10 Gb/s. In subsequent years, the use of APDs for high-capacity systems declined as a result of their limited gain bandwidth, the transition to coherent detection, and the development of high-efficiency modulation techniques. Recently, the rapid growth of optical-fiber communications systems that utilize baud rates up to 25 Gb/s represented by a 100-Gb/s Ethernet has led to a resurgence of research on APDs and the development of low-noise APDs with enhanced gain bandwidth. This paper presents a brief review of APD fundamentals and describes some of the recent advances.

Avalanche photodiodes with an impact-ionization-engineered multiplication region

We demonstrate an impact-ionization-engineered structure for the multiplication region of avalanche photodiodes. By enhancing the control of the impact-ionization position, the structure achieved high gain, low dark current, and very low noise.

High-saturation-current charge-compensated InGaAs-InP uni-traveling-carrier photodiode

Charge compensation is utilized in an InGaAs-InP uni-traveling-carrier photodiode to mitigate the space-charge effect. A 20-/spl mu/m-diameter photodiode achieved a bandwidth of 25 GHz and large-signal 1-dB compression current greater than 90 mA; the output power at 20 GHz was 20 dBm. A smaller /spl sim/100-/spl mu/m/sup 2/ photodiode exhibited a bandwidth of 50 GHz and large-signal 1-dB compression current greater than 50 mA. The maximum RF output power at 40 GHz was 17 dBm.

Very high-responsivity evanescently coupled photodiodes integrating a short planar multimode waveguide for high-speed applications

We report an evanescently coupled photodiode that utilizes a short planar multimode waveguide. Very high responsivity (> 1 A/W) with polarization dependence less than 0.5 dB, 48-GHz bandwidth, and 11-mA saturation current were achieved.

High-saturation-current InP-InGaAs photodiode with partially depleted absorber

A high-saturation-current InGaAs photodiode (PD) with a partially depleted absorber is demonstrated. Optical responsivity of 0.58 A/W and over 1.1 W of dissipated electrical power is achieved. Small signal compression current of 110 and 57 mA is measured at radio-frequency bandwidths of 1 and 10 GHz, respectively. This continuous photocurrent is believed to be the highest reported value for a p-i-n InGaAs PD at these bandwidths.

Low-noise avalanche photodiodes with graded impact-ionization-engineered multiplication region

A novel impact-ionization-engineered multiplication region for avalanche photodiodes is reported. By pseudograding the multiplication region with materials of different ionization threshold energies, the impact ionization of the injected carrier type is localized, while that of the feedback carrier type is suppressed. Low noise (k/sub eff/<0.1), low dark current, and high gain were achieved.

InGaAs/InAlAs avalanche photodiode with undepleted absorber

We report an avalanche photodiode with an undepleted p-type InGaAs absorption region and a thin InAlAs multiplication layer. The motivation for utilizing an undepleted absorption layer, which is similar to that in the unitraveling carrier photodiode, is to reduce the dark current. A dark current below 1 nA at a gain of 10 and a gain–bandwidth product of 160 GHz are demonstrated.

Monte Carlo simulation of low-noise avalanche photodiodes with heterojunctions

A Monte Carlo model is developed to simulate avalanche photodiodes with AlGaAs/GaAs heterojunctions. The experimentally observed ultralow-noise behavior of a center-well avalanche photodiode is successfully reproduced in the model. It is found that the arrangement of different materials in the intrinsic region can modulate the positional dependence of impact ionization events, and hence the gain distribution. Consequently, the noise is sensitive to the structural parameters such as well thickness. Hot and energetic electrons are distinguished by their distribution in k space. This distinction is used to explain why the noise behavior is sensitive to the initial carrier excess energy from photogeneration but relatively insensitive to carrier energy gained from the electric field.

Analysis of breakdown probabilities in avalanche photodiodes using a history-dependent analytical model

The breakdown probabilities of avalanche photodiodes (APDs) working in the Geiger mode are analyzed using a history-dependent analytical impact-ionization model [R. J. McIntyre, IEEE Trans. Electron Devices 46, 1623 (1999)]. The breakdown sharpness in devices with thin and thick multiplication regions is found to follow the same trend in GaAs, InAlAs, and InP material systems. Breakdown characteristics of InP and InAlAs are compared for their applications in photon counting at telecommunication wavelengths.

Low-noise impact-ionization-engineered avalanche photodiodes grown on InP substrates

We report low noise multiplication region structures designed for avalanche photodiodes grown on InP substrates. By either implementing a single heterostructure or using a pseudograded structure in the multiplication region, better control of spatial distribution of impact-ionization for both injected and feedback carriers can be achieved; localization of the carrier impact ionization process has resulted in very low excess noise.