Multiple quantum barriers have been used to suppress the dark current of nanoscale avalanche photodiodes (APDs). The n+–π–p+-structured Si–3C-SiC heterojunction-based multiple quantum barrier (MQB) APDs are considered and a detailed model of dark current has been developed from the self-consistent solution of the coupled Schrodinger–Poisson equations. Four major types of electron–hole pair (EHP) generation mechanisms such as (1) thermal generation, (2) band-to-band tunnelling generation, (3) trap-assisted tunnelling generation and (4) avalanche generation are considered for calculating variation of the total dark current with reverse bias voltage. It is observed that the dark current can be suppressed significantly by increasing both the number and thickness of quantum barriers. However, the authors have also admitted that both the number and thickness of quantum barriers cannot be increased indefinitely, since it will cause deterioration in spectral response of the device in near-infrared range (...

Dark current reduction in nano-avalanche photodiodes by incorporating multiple quantum barriers

With 5G's support for diverse radio bands and different deployment modes, e.g., standalone (SA) vs. non-standalone (NSA), mobility management - especially the handover process - becomes far more complex. Measurement studies have shown that frequent handovers cause wild fluctuations in 5G throughput, and worst, service outages. Through a cross-country (6,200 km+) driving trip, we conduct in-depth measurements to study the current 5G mobility management practices adopted by three major U.S. carriers. Using this rich dataset, we carry out a systematic analysis to uncover the handover mechanisms employed by 5G carriers, and compare them along several dimensions such as (4G vs. 5G) radio technologies, radio (low-, mid- & high-)bands, and deployment (SA vs. NSA) modes. We further quantify the impact of mobility on application performance, power consumption, and signaling overheads. We identify key challenges facing today's NSA 5G deployments which result in unnecessary handovers and reduced coverage. Finally, we design a holistic handover prediction system Prognos and demonstrate its ability to improve QoE for two 5G applications 16K panoramic VoD and realtime volumetric video streaming. We have released the artifacts of our study at https://github.com/SIGCOMM22-5GMobility/artifact.

/pdf/vivisecting-mobility-management-in-5g-cellular-networks-26qjyqos.pdf

Vivisecting mobility management in 5G cellular networks

The highly anticipated 5G mmWave technology promises to enable many uplink-oriented, latency-critical applications (LCAs) such as Augmented Reality and Connected Autonomous Vehicles. Nonetheless, recent measurement studies have largely focused on its downlink performance. In this work, we perform a systematic study of the uplink performance of commercial 5G mmWave networks across 3 major US cities and 2 mobile operators. Our study makes three contributions. (1) It reveals that 5G mmWave uplink performance is geographically diverse, substantially higher over LTE in terms of bandwidth and latency, but often erratic and suboptimal, which can degrade LCA performance. (2) Our analysis of control messages and PHY-level KPIs shows that the root causes for the suboptimal performance are fundamental to 5G mmWave and cannot be easily fixed via simple tuning of network configurations. (3) We identify various design and deployment optimizations that 5G operators can explore to bring 5G mmWave performance to the level needed to ultimately support the LCAs.

An in-depth study of uplink performance of 5G mmWave networks

1.0–10.0 THz Radiation from Graphene Nanoribbon Based Avalanche Transit Time Sources

Three‐Terminal Graphene Nanoribbon Tunable Avalanche Transit Time Sources for Terahertz Power Generation

In this paper, the authors have presented a self-consistent quantum drift-diffusion model for multiple quantum well (MQW) impact avalanche transit time (IMPATT) diodes. The bound states in MQWs have been taken into account by self-consistent solutions of the coupled classical drift-diffusion (CLDD) equations and time-independent Schrodinger equations associated with both the conduction and valence bands. The static and high-frequency properties of MQW DDR IMPATTs based on Si$$\sim $$~3C-SiC material system designed to operate near 94-GHz atmospheric window have been studied by means of the above-mentioned self-consistent solutions of coupled CLDD equations and Schrodinger equations followed by a well-established double-iterative field maximum computational technique. A symmetric and two complementary asymmetric doping profiles for the proposed structures have been taken into account for the present study. The RF power outputs of Si$$\sim $$~3C-SiC MQW DDR IMPATTs near 94 GHz obtained from the simulation are compared with the experimentally obtained power outputs of flat DDR IMPATT diodes based on Si, GaAs, and InP at the same frequency band. It is observed that Si$$\sim $$~3C-SiC MQW DDR IMPATTs are capable of delivering significantly higher RF power compared with IMPATTs based on the above-mentioned materials especially when the doping concentrations of 3C-SiC layers are kept higher than those of the Si layers.

Self-consistent quantum drift-diffusion model for multiple quantum well IMPATT diodes

5G-NR is beginning to be widely deployed in the mmWave frequencies in urban areas in the US and around the world. Due to the directional nature of mmWave signal propagation, improving performance of such deployments heavily relies on beam management and deployment configurations. We perform detailed measurements of mmWave 5G deployments by two major commercial 5G operators in the US in two diverse environments: an open field with a baseball park (BP) and a downtown urban canyon region (DT), using smartphone-based tools that collect detailed measurements across several layers (PHY, MAC and up) such as beam-specific metrics like signal strength, beam switch times, and throughput per beam. Our measurement analysis shows that the parameters of the two deployments differ in a number of aspects: number of beams used, number of channels aggregated, and density of deployments, which reflect on the throughput performance. Our measurement-driven propagation analysis demonstrates that narrower beams experience a lower path-loss exponent than wider beams, which combined with up to eight frequency channels aggregated on up to eight beams can deliver a peak throughput of 1.2 Gbps at distances greater than 100m.

A Comparative Measurement Study of Commercial 5G mmWave Deployments



The potentiality of Multiple Quantum Well (MQW) Impacts Avalanche Transit
Time (IMPATT) diodes based on Si~3C-SiC heterostructures as possible terahertz radiators have
been explored in this paper.



The static, high frequency and noise performance of MQW devices operating at 94,
140, and 220 GHz atmospheric window frequencies, as well as 0.30 and 0.50 THz frequency
bands, have been studied in this paper.



The simulation methods based on a Self-Consistent Quantum Drift-Diffusion (SCQDD)
model developed by the authors have been used for the above-mentioned studies.



Thus the noise performance of MQW DDRs will be obviously better as compared to the
flat Si DDRs operating at different mm-wave and THz frequencies.



 Simulation results show that Si~3C-SiC MQW IMPATT sources are capable of
providing considerably higher RF power output with the significantly lower noise level at both
millimeter-wave (mm-wave) and terahertz (THz) frequency bands as compared to conventional
flat Si IMPATT sources.


Terahertz Radiators Based on Si~3C-SiC MQW IMPATT Diodes

The static, small-signal and noise characteristics of multiple quantum well (MQW) impact avalanche transit time (IMPATT) diodes operating at 94, 140, 220, 300 and 500 GHz frequencies have been investigated in this paper. The said MQW structures have been implemented by using Si~3C-SiC heterostructures. A self-consistent quantum drift-diffusion (SCQDD) model based simulation method has been used for the above mentioned studies. Simulation results show that Si~3C-SiC MQW IMPATT sources are highly proficient to provide considerably higher power output with significantly lower noise measure at aforementioned frequency bands as compared to conventional flat Si IMPATT sources.

Multiple Quantum Well IMPATT Sources based on Si~3C-SiC Heterostructures Operating at Millimeter-Wave and Terahertz Frequency Bands

Great demand of suitable terahertz (THz) sources requires optimized design and appropriate simulation tool for designing and investigating the device capability at the design spectrum before actual fabrication. Design and simulation of complex devices like THz avalanche transit time (ATT) diodes require comprehensive and accurate simulation model for small-signal, large-signal and noise simulation before the actual fabrication. This chapter is devoted to summarize these three types of simulation methodologies for THz ATT sources.

Monisha Ghosh

Papers

Self-consistent quantum drift-diffusion model for multiple quantum well IMPATT diodes

A Comparative Measurement Study of Commercial 5G mmWave Deployments

Terahertz Radiators Based on Si~3C-SiC MQW IMPATT Diodes

Multiple Quantum Well IMPATT Sources based on Si~3C-SiC Heterostructures Operating at Millimeter-Wave and Terahertz Frequency Bands

On Some Modern Simulation Techniques for Studying THz ATT Sources