Finite-element Device Simulator Research Articles

The perspectives of diamond for 3H and 63Ni betavoltaic power sources, comparison with 4H–SiC

Recently, power sources that utilize radioisotope energy conversion, specifically the betavoltaic effect, have gained more attention due to the increasing demand for energy-efficient technologies in new electronics. These power sources are capable of delivering electrical power continuously over periods of years. Among various semiconductors, 4H–SiC and diamond are the most desirable materials for building betavoltaic structures. Although 4H–SiC betavoltaics are commercially available and have been demonstrated, diamond is still an emerging material. However, recent advancements in diamond technology, such as the demonstration of bipolar structures—a necessary requirement for high-efficiency betavoltaics—make it a promising candidate for building practical betavoltaic devices in the near future. This study aims to evaluate the potential of diamond bipolar structures for betavoltaic applications using 3H and 63Ni sources and compare the results with 4H–SiC at both room and elevated temperatures, taking into account the current level of materials technology development. The electrical parameters of these structures are estimated using the Monte Carlo transport tool GEANT4 coupled with the finite-element device simulator TCAD SILVACO. The simulation results are compared with existing experimental data for both 4H–SiC and diamond devices.

Self-Aligned van der Waals Heterojunction Diodes and Transistors.

A general self-aligned fabrication scheme is reported here for a diverse class of electronic devices based on van der Waals materials and heterojunctions. In particular, self-alignment enables the fabrication of source-gated transistors in monolayer MoS2 with near-ideal current saturation characteristics and channel lengths down to 135 nm. Furthermore, self-alignment of van der Waals p-n heterojunction diodes achieves complete electrostatic control of both the p-type and n-type constituent semiconductors in a dual-gated geometry, resulting in gate-tunable mean and variance of antiambipolar Gaussian characteristics. Through finite-element device simulations, the operating principles of source-gated transistors and dual-gated antiambipolar devices are elucidated, thus providing design rules for additional devices that employ self-aligned geometries. For example, the versatility of this scheme is demonstrated via contact-doped MoS2 homojunction diodes and mixed-dimensional heterojunctions based on organic semiconductors. The scalability of this approach is also shown by fabricating self-aligned short-channel transistors with subdiffraction channel lengths in the range of 150-800 nm using photolithography on large-area MoS2 films grown by chemical vapor deposition. Overall, this self-aligned fabrication method represents an important step toward the scalable integration of van der Waals heterojunction devices into more sophisticated circuits and systems.

E-061 Towards the Pre-Surgical Treatment Planning of Cerebral Aneurysms Using High Fidelity Simulations

Over the last decade, advances in medical imaging have led to a 75% increase in early diagnosis of cerebral aneurysms (CAs) along with a growing arsenal of medical devices that...

Physical Modeling of Fast p-i-n Diodes With Carrier Lifetime Zoning, Part I: Device Model

This paper presents the development and implementation of a physics-based diode model which can simulate aspects of high-voltage diodes such as snappy recovery during punch-through and the modified carrier density profile due to local lifetime control. It uses a Fourier series solution for the ambipolar diffusion equation in the lightly doped base region. The model is compared with finite-element device simulations. A parameter extraction procedure for the diode with lifetime control is proposed in Part II.

The Influence of Source and Drain Junction Depth on the Short-Channel Effect in MOSFETs

This brief investigates the influence of source and drain junction depth on the short-channel effect (SCE) in highly scaled MOSFETs. The established scale length model represents SCE in terms of an exponential function of gate insulator thickness and gate depletion-layer width, but this model fails to account for any influence of source and drain junction depth on SCE. It is shown using two-dimensional finite-element device simulation that the influence of source and drain junction depth on SCE can be represented by an addition of a pre-exponential term to the established scale-length model. For source and drain junction depths that are deeper than the MOSFET's gate depletion-layer width, SCE is shown with this pre-exponential term to be insensitive to junction depth. Conversely, for source and drain junction depths that are shallower than the MOSFET's gate depletion-layer width, SCE is shown to improve linearly with decreasing junction depth.

A comprehensive analysis of irradiated silicon detectors at cryogenic temperatures

The effect of particle irradiation on high-resistivity silicon detectors has been extensively studied with the goal of engineering devices able to survive the very challenging radiation environment at the CERN Large Hadron Collider (LHC). The main aspect under investigation has been the changes observed in detector effective doping concentration (N/sub eff/). We have previously proposed a mechanism to explain the evolution of N/sub eff/, whereby charge is exchanged directly between closely-spaced defect centres in the dense terminal clusters formed by hadron irradiation. This model has been implemented in both a commercial finite-element device simulator (ISE-TCAD) and a purpose-built simulation of interdefect charge exchange. To control the risk of breakdown due to the high leakage currents foreseen during ten years of LHC operation, silicon detectors will be operated below room temperature (around -10/spl deg/C). This, and more general current interest in the field of cryogenic operation, has led us to investigate the behavior of our model over a wide range of temperatures. We present charge collection spectra from 1064 nm laser pulses as a function of detector bias between temperatures of 115 K and 290 K, using devices irradiated with 23 GeV protons in the range 10/sup 13/-4/spl times/10/sup 14/ protons/spl middot/cm/sup -2/. These data allow a deeper investigation of the influence of defect capture cross sections on N/sub eff/. The model prediction for the reversion to n-type of heavily-irradiated detectors at low temperature is investigated and deviations from the model are explored.

Interdefect charge exchange in silicon particle detectors at cryogenic temperatures

Silicon particle detectors in the next generation of experiments at the CERN Large Hadron Collider will be exposed to a very challenging radiation environment. The principal obstacle to long-term operation arises from changes in detector doping concentration (N/sub eff/), which lead to an increase in the bias required to deplete the detector and hence achieve efficient charge collection. We have previously presented a model of interdefect charge exchange between closely spaced centers in the dense terminal clusters formed by hadron irradiation. This manifestly non-Shockley-Read-Hall (SRH) mechanism leads to a marked increase in carrier generation rate and negative space charge over the SRH prediction. There is currently much interest in the subject of cryogenic detector operation as a means of improving radiation hardness. Our motivation, however, is primarily to investigate our model further by testing its predictions over a range of temperatures. We present measurements of spectra from /sup 241/Am alpha particles and 1064-nm laser pulses as a function of bias between 120 and 290 K. Values of N/sub eff/ and substrate type are extracted from the spectra and compared with the model. The model is implemented in both a commercial finite-element device simulator (ISE-TCAD) and a purpose-built simulation of interdefect charge exchange. Deviations from the model are explored and comments made as to possible future directions for investigation of this difficult problem.

InGaP/GaAs heterojunction bipolar transistor optical and electronic band structure characterization

In this work we investigate the optical and band structure properties of full InGaP/GaAs based heterojunction bipolar transistor (HBT) epitaxial structures grown by metalorganic chemical vapour phase epitaxy (MOVPE). In related work, full HBTs have been fabricated from the two wafers studied, which exhibit high and low common-emitter current gain (hFE) parameters on electrical test. The focus of this study is to investigate and compare the photoluminescence and photoreflectance spectroscopy response of these known good and bad epitaxial wafers. The results of low temperature (10–300 K) spectral and transient photoluminescence (PL) analysis are presented, revealing evidence of the nature of the InGaP ordering induced non-radiative loss mechanism. The results also demonstrate the modification to the PL lineshape arising from the InGaP/GaAs interfacial conditions. The experimental results are supported by X-ray diffraction data and finite-element device simulation, showing the effect of intermixing layers on the interfacial band potentials. The optical modulation technique of photoreflectance (PR) spectroscopy was employed to investigate the band structure and interfacial electric fields, Fs, of the HBT structures. Following the polarisation–[110] and [11̄0]–dependence of the sub-lattice ordering PR response, it was found necessary to include an emitter/base intermixing layer in order to account for the InGaP/GaAs Fs data. It is concluded that non-optimal MOVPE growth conditions for one of the structures resulted in both sub-lattice ordering and layer intermixing effects, consistent with the low hFE of the HBTs fabricated from this material.

InGaP/GaAs based heterojunction bipolar transistor characterisation using non-contact optical spectroscopy

In this work we have characterised InGaP/GaAs based heterojunction bipolar transistor (HBT) structures, fabricated by metalorganic chemical vapour phase epitaxy (MOVPE), using non-contact electro-optic spectroscopic techniques, photoreflectance (PR), photoluminescence (PL) and ellipsometry. PR analysis details both band structure and interfacial electric field data for both the GaAs collector and InGaP emitter regions. Including sub-lattice ordering for the InGaP alloy, the PR analysis also indicates the presence of an interfacial or intermixing layer between the emitter and GaAs base as a result of non-optimal MOVPE growth. The results are compared with room temperature PL spectra, demonstrating in particular the modification to the PL lineshape arising from the InGaP/GaAs interfacial conditions. The experimental data are also supported with finite-element device simulation, showing the effect of mixing layers on the interfacial band potentials. To interpret the experimental HBT ellipsometric response it was found necessary to include InGaP/GaAs layer intermixing within the optical model, consistent with the previous data. The implications of these results for HBT device performance are also discussed.

Time Dependence of Spatial Defect Profiles in a-Si:H Solar Cells with Light-Soaking

ABSTRACTWe report on modeling the evolution of the spatial defect profile in the i-layer of a-Si:H solar cells combined with tests of the model against the efficiency decay of amorphous silicon (a-Si:H) p-i-n solar cells under various light-soaking conditions. We use a finite-element device simulator. The defect density during light-soaking is calculated as a function of time and of position. The defect density evolves due to the combined effects of light-induced generation, assumed proportional to the product of free carrier densities, and of light-induced annealing, assumed proportional to the sum of carrier densities. We find that in thick cells defects close to the p-i interface affect cell efficiency less than bulk defects, and that in thin cells the defect density increases significantly in the central portion of the i-layer. However, in thin cells the high electric field masks the effect of the defect density increase.

Implementation of heterojunctions into the 2-D finite-element simulator prism: Some scaling considerations

In this paper a very efficient way to obtain a unified scaling system while implementing heterojunctions in a 2-D finite-element device simulator is discussed. A real device is simulated using the technique described. It is shown that discrepancies between the simulation and the measurement can be explained by the inaccuracy of the available models and material parameters.

The solution of a numerical problem encountered when adding a mobility model to a finite-element device simulator

The addition of a mobility model to a finite-element simulator was found to create a convergence problem identified with the addition of positive terms to the off-diagonal elements in the solution matrix. Fast convergence rates were recovered by carefully adjusting the solution matrix such that the off-diagonal elements in question were all nonpositive. >