Modeling Surface Recombination with Enhanced Devices Network for Optoelectronics

We present a lumped devices network approach to simulate surface recombination effects in optoelectronics devices. The network is composed of generalized lumped devices where the excess carrier concentrations and gradients are mapped on electrical quantities, i.e. equivalent voltages and currents respectively, so that the overall simulation is SPICE-compatible. A novel enhanced device is derived and used as an additional element to account for the influence of the surface in SPICE-like simulations. Thus, in addition to generation, propagation and collection that were included in a former work, recombination at the surface of the semiconductor can be taken into account with standard circuit simulators. Optoelectronic devices performance degradation due to surface recombination is analyzed with this approach and compared with full numerical TCAD simulations.

Prediction of optically-triggered amplification in phototransistor with SPICE circuit simulators

Chiara Rossi, Jean-Michel Sallese

Modeling of optoelectronic devices involves long and complex numerical simulations, usually performed with TCAD tools. Numerical simulations provide accurate results for a single device but are not feasible when dealing with a full circuit comprising several nodes. To solve this issue, we developed a novel approach to simulate optoelectronic devices in standard SPICE circuit simulators, thus avoiding using TCAD tools. The concept is based on a coarse meshing of the semiconductor whose nodes are interconnected with the so-called Generalized Lumped Devices. The Generalized Lumped Devices are four ports devices: two ports simulate real currents and voltages in the semiconductor while two additional ports simulate excess carrier density and gradient through the definition of equivalent currents and voltages. The model behind the Generalized Devices is physics based and can correctly simulate optical generation of excess carriers, drift-diffusion transport, bulk and surface recombination as well as capacitive effects, without the need to introduce fitting or empirical parameters. Moreover, since all inputs and outputs are electrical quantities, the model is fully SPICE-compatible and can be merged with SPICE netlists of circuits. In this work, we use the Generalized Devices approach to simulate a bipolar phototransistor and prove that we can predict the photocurrent versus the collector voltage, for different illumination intensities. The model accurately takes into account the optically-triggered current amplification in the phototransistor. Sentaurus TCAD numerical simulations are in agreement with the Generalized Devices approach. Finally, since the model is physics based, we could assess the impact of different semiconductor parameters, such as doping concentrations, lifetime, surface recombination velocity, on the output characteristics of the phototransistors, directly with SPICE circuit simulators.

SPIE-INT SOC OPTICAL ENGINEERING2020

A novel approach for SPICE modeling of light and radiation effects in ICs

Chiara Rossi

Modeling the interaction of ionizing radiation, either light or ions, in integrated circuits is essential for the development and optimization of optoelectronic devices and of radiation-tolerant circuits. Whereas for optical sensors photogenerated carriers play an essential role, high energy ionizing particles can be a severe issue for circuits, as they create high density of excess carriers in ICs substrate, causing parasitic signals. In particular, recent advances in CMOS scaling have made circuits more sensitive to errors and dysfunctions caused by radiation-induced currents, even at the ground level. TCAD simulations of excess carriers generated by light or radiation are not dedicated to large scale circuit simulations since only few devices can be simulated at a time and computation times are too long. Conversely, SPICE simulations are faster, but their accuracy is strictly dependent on the correctness of the compact models used to describe the devices, especially when dealing with photocurrents and parasitic radiation-induced currents. The objective of this thesis is to develop a novel modeling approach for SPICE-compatible simulations of electron-hole pairs generated by light and by high energy particles. The approach proposed in this work is based on the Generalized Lumped Devices, previously developed to simulate parasitic signals in High Voltage MOSFET ICs. Here, the model is extended to include excess carriers generation. The developed approach allows physics-based simulations of semiconductor structures, hit by light or radiation, that can be run in standard circuit simulators without the need for any empirical parameter, only relying on the technological and geometrical parameters of the structure, and without any predefined compact model. The model is based on a coarse mesh of the device to obtain an equivalent network of Generalized Lumped Devices. The latter predicts generation of excess carriers and their propagation, recombination and collection at circuit nodes through the definition of equivalent voltages, proportional to the excess carrier concentrations, and equivalent currents, proportional to the excess carrier gradients. The model is validated with Sentaurus TCAD numerical simulations for different scenarios. Regarding light effects, the proposed strategy is applied to simulate various optoelectronic devices. Complete DC I-V characteristics of a solar cell and transient response of a photodiode are studied. Next, phototransistors are considered. After, a full pixel of a 3T-APS CMOS image sensor is analyzed. The photosensing device, described with Generalized Devices, is co-simulated with the in-pixel circuit, described with compact models. The impact of semiconductor parameters on pixel output and on crosstalk between adjacent pixels is predicted. Finally, radiation-induced soft errors in ICs are examined. Alpha particles at different energies hitting the substrate are simulated. Parasitic currents collected at contacts are studied as a function of particles position and energy. Funneling effect, which is a phenomenon specific to high injection, is also included in the model. This work shows that the Generalized Lumped Devices approach can be successfully used for SPICE simulations of optoelectronic devices and for prediction of radiation- induced parasitic currents in ICs substrate. This thesis is a first step towards a complete and flexible tool for excess carriers modeling in standard circuit simulators.

EPFL2021

Methodology for 3-D Substrate Network Extraction for SPICE Simulation of Parasitic Currents in Smart Power ICs

Pietro Buccella, Maher Kayal, Jean-Michel Sallese, Camillo Stefanucci

A 3-D simulation of substrate currents is crucial to analyze parasitic coupling effects due to minority carrier injection in smart power ICs. In this paper, a substrate parasitic extraction methodology is introduced by dividing the IC layout into elementary elements to solve the continuity equation for minority carriers in the volume based on the finite-difference method. A substrate parasitic network is derived from the mesh generated through the existing mixed-signal design flow. The induced substrate model is included in circuit simulators such as SPICE to predict the effects of substrate couplings during the design phase. Furthermore, this analysis enables optimization of layout with minimal parasitic effects. By linking the substrate model to the active components, the couplings between the integrated circuit with the substrate parasitic currents can be analyzed during circuit simulations. Simulations and measurements on an high voltage driver reveal consistent results and therefore confirm the validity of the method. Therefore, the approach developed herein is effective to predict parasitic couplings due the injection of minority carriers.