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We propose a simple expression to relate the total excess noise factor of a single-carrier multiplication staircase avalanche photodiode (APD) to the excess noise factor and gain given by the individual conduction band discontinuities. The formula is valid when electron impact ionization dominates hole impact ionization; hence, it is especially suited for staircase APDs with In-rich multiplication regions, as opposed, for example, to GaAs/AlGaAs systems where hole ionization plays an important role. The formula has been verified by accurate means of numerical simulations based on a newly developed nonlocal history dependent impact ionization model.

We present a Full-Band Monte Carlo (FBMC) investigation of impact ionization in GaAs p-i-n Avalanche Photodiodes (APDs). FBMC simulations have been used to compute the gain and the excess noise factor and a new equation has been derived for the extraction of history-dependent impact ionization coefficients from FBMC simulations. Results from FBMC are then compared with the ones of nonlocal historydependent impact ionization models. We found that at high reverse bias voltages it is important to take into account the fact that secondary carriers are generated with nonzero kinetic energy. Finally, we propose an improved history-dependent model using an energy-dependent mean free path.

Extensive experimental characterization and TCAD simulation analysis have been used to study the dark current in Avalanche Photo-Diodes (APDs). The comparison between the temperature dependence of measurements and simulations points out that SRH generation/recombination is responsible for the observed dark current. After the extraction of the carrier lifetimes in the GaAs layers, they have been used to predict the APD collection efficiency of the photo-generated currents under realistic operation conditions and as a function of the photogeneration position inside the absorption layer.

This work focuses on avalanche photodiodes based on GaAs/AlGaAs with separated absorption and multiplication regions (SAM-APDs). The two regions are separated by a thin p-doped layer which, under the application of a reverse bias, is able to confine the potential drop only in the multiplication region. We realized such layer under the form of either a delta sheet of C atoms or a 50-nm-thick GaAs:C layer. Devices with these two structures will be discussed and compared in terms of capacitance and response to light.

This work reports on the fabrication and characterization of a novel high-speed, low-noise X-ray Avalanche Photodiode based on III-V compound semiconductors operating over an extended photon energy range. These materials were suggested as their higher atomic numbers allow for the absorption of higher photon energies; hence, shorter response times can be achieved by growing APDs with thinner active regions. In addition, the use of staircase hetero-junctions enhances electron multiplication and results in lower noise if compared with conventional p-i-n diodes. In this work, molecular beam epitaxy was used to produce GaAs/AlGaAs APDs with separated absorption and multiplication regions. The multiplication region, separated from the absorption region by a ? p-doped layer of carbon, contains a staircase structure composed of nanometric layers of AlGaAs and GaAs, which alternate periodically. The periodic modulation of the band gap enables a well-defined charge multiplication and results in low multiplication noise. Several devices were characterized in terms of dark current, photocurrents generated utilizing visible and hard X-ray sources as well as noise generated under laser light.

We report simulation results for gain and noise in avalanche photodiodes fabricated using heterojunctions of III-V compound semiconductors. We employ a recently developed nonlocal model for impact ionization. The model has been calibrated and validated on devices fabricated and measured in our labs and data from the literature. The model is then used to explore the design trade-offs related to the number of conduction band steps in staircase APDs for X-ray detection based on the GaAs/AlGaAs material system.

We present an improved Random Path Length algorithm to accurately and efficiently estimate the design space of heterostructure avalanche photodiodes (APDs) in terms of gain, noise and bandwidth without any need of full Monte Carlo transport simulations. The underlying nonlocal model for impact ionization goes beyond the Dead Space concept and it is suited to handle staircase structures composed by a superlattice of III-V compounds as well as thick and thin p-i-n APDs. The model parameters have been calibrated on GaAs and $Al_{x}Ga_{1-x}As$ p-i-n APDs in a previous work. In this work GaAs p-i-n APDs are compared to staircase structures in terms of noise and bandwidth.

We present a nonlocal history-dependent model for impact ionization gain and noise in avalanche photodiodes (APDs) especially suited for staircase APDs. The model uses a simple energy balance equation to define effective electric fields valid also in the presence of band discontinuities which are then used to express the ionization coefficients. The model parameters have been calibrated against literature data for gain and noise in GaAs and Al x Ga1− x As $({x} = {0.2}, {0.6}, {0.8})$ p-i-n diodes. Application to the experimental data for gain and noise in heterojunction and staircase separate absorption and multiplication APDs is reported to demonstrate the ability of the model in describing complex APD structures. It is found that, in spite of conduction band discontinuities being much larger than valence band ones, hole impact ionization contributes a significant degradation of the noise metrics in GaAs/AlGaAs staircase APDs. These nontrivial insights demonstrate the usefulness of the model to steer device design and optimization.

This work focuses on the development and the characterization of avalanche photodiodes with separated absorption and multiplication regions grown by molecular beam epitaxy. The i-GaAs absorption region is separated from the multiplication region by a δ p-doped layer of carbon atoms, which ensures that after applying a reverse bias, the vast majority of the potential drops in the multiplication region. Therein, thin layers of AlGaAs and GaAs alternate periodically in a so-called staircase structure to create a periodic modulation of the band gap, which under bias enables a well-defined charge multiplication and results in a low multiplication noise. The influence of the concentration of carbon atoms in the δ p-doped layer on the device characteristics was investigated and experimental data are presented together with simulation results.