Query: An analytical model for rectifying contacts on polycrystalline semiconductors

Candidate papers:

author_names: Uwe Rau

J Werner
title: An Analytical Model for Rectifying Contacts on Polycrystalline Semiconductors
year: 1999

abstract: An analytical description of the charge carrier transport, valid for non degenerated and degenerated semiconductors, was developed, critically reviewed, and fitted to the temperature dependent Hall mobility data of magnetron sputtered, degenerately doped ZnO:Al films. Our extended model for grain boundary scattering in semiconductors of arbitrary degeneracy is based on previous models from literature and suitable to describe the Hall mobility of the carriers as a function of the free carrier concentration and the temperature at the same time. It is mathematically simple enough for a fast fit procedure, which is not possible with most of the previous models. Applying a combined transport model consisting of ionized impurity scattering, phonon scattering, and grain boundary scattering in degenerate semiconductors, we were able to determine the trap density at the grain boundaries Nt 3 x 1013 to 5 x 1013 cm 2 and the deformation potential Eac in the range of 5 eV to 9 eV depending on the details of the tra.
author_names: Andre Bikowski

Klaus Ellmer
title: Analytical model of electron transport in polycrystalline, degenerately doped ZnO films
year: 2014

abstract: Polycrystalline thin film solar cells made with absorber materials such as CdTe, CIGS, CZTS and metalorganic halides (perovskites) are currently important alternatives for the silicon solar cell technology, which still dominates the photovoltaic market. Then, it is important to have tools which can be used to design this kind of solar cells. For this purpose, we have developed a unified simple analytical model that can be applied to thin film solar cells. The model is based on the basic physics of hetero junction devices, but it takes into consideration that the space charge region can extend along the major part of the cell length, particularly for very thin cells, causing important effects that typically are not observed in conventional junction devices. Photo generated carriers are collected by electric field drift instead of diffusion, and simultaneously strong recombination at this region may dominate the electrical I V characteristic of the cell. Since the space charge region width varies with the applied voltage, the illumination current density and the saturation dark current density are no longer independent of the voltage as is assumed for conventional solar cells. When the model is applied to CIS and CdTe solar cells as examples, it is found that it is possible to design very thin film solar cells (absorber less than 1 mm thick) with high efficiencies, whenever the recombination velocity at the back surface becomes small (102 cm/s) instead of the highrecombination velocities present at ohmic contacts (107 cm/s) This fact implies the cost reduction of thin film solar cells by reducing absorber material thickness, and therefore it poses a challenge to develop deposition methods for very thin CdTe and CIGS absorber materials without pinholes, so that improved efficiencies are obtained when the surface recombination velocity is made small at the back by having a p+ or an electron blocking region before the ohmic contact. This result also explains the high efficiencies achieved by very thin perovskite solar cells.
author_names: Armando Acevedo Luna

Roberto Bernal Correa

Jorge Montes Monsalve

A Morales Acevedo
title: Design of thin film solar cells based on a unified simple analytical model
year: 2019

abstract: We study theoretically the effects of grain boundary potential barriers and their fluctuations in height on the electrical conductivity and the Seebeck coefficient of polycrystalline semiconductors. On the basis of an energy filtering model, we first derive analytical expressions of the electrical conductivity s and the Seebeck coefficient a for polycrystalline semiconductors of any degeneracy, assuming grain boundary potential barriers of a uniform height. These expressions in closed form enable us to perform numerical calculations of s and a for polycrystalline semiconductors having grain boundary potential barriers of fluctuated heights. The fluctuated barrier model is applied to the reported experimental data on the electrical conductivity and the Seebeck coefficient of a boron doped polysilicon sample. Then, the expected enhancement of the thermoelectric power factor due to the energy filtering effect by the grain boundary barriers is presented with the mean value and the standard deviation of the potential barrier height as variable parameters. The calculated results suggest that diminishing the fluctuations of the potential barrier height is the key issue for enhancing the power factor by energy filtering.
author_names: Yasutomo Kajikawa
title: Effects of potential barrier height and its fluctuations at grain boundaries on thermoelectric properties of polycrystalline semiconductors
year: 2013

abstract: Abstract Polycrystalline thin film solar cells made with absorber materials such as CdTe, CIGS, CZTS and metalorganic halides (perovskites) are currently important alternatives for the silicon solar cell technology, which still dominates the photovoltaic market. Then, it is important to have tools which can be used to design this kind of solar cells. For this purpose, we have developed a unified simple analytical model that can be applied to thin film solar cells. The model is based on the basic physics of hetero junction devices, but it takes into consideration that the space charge region can extend along the major part of the cell length, particularly for very thin cells, causing important effects that typically are not observed in conventional junction devices. Photo generated carriers are collected by electric field drift instead of diffusion, and simultaneously strong recombination at this region may dominate the electrical I V characteristic of the cell. Since the space charge region width varies with the applied voltage, the illumination current density and the saturation dark current density are no longer independent of the voltage as is assumed for conventional solar cells. When the model is applied to CIS and CdTe solar cells as examples, it is found that it is possible to design very thin film solar cells (absorber less than 1 mm thick) with high efficiencies, whenever the recombination velocity at the back surface becomes small (102 cm/s) instead of the high recombination velocities present at ohmic contacts (107 cm/s) This fact implies the cost reduction of thin film solar cells by reducing absorber material thickness, and therefore it poses a challenge to develop deposition methods for very thin CdTe and CIGS absorber materials without pinholes, so that improved efficiencies are obtained when the surface recombination velocity is made small at the back by having a p+ or an electron blocking region before the ohmic contact. This result also explains the high efficiencies achieved by very thin perovskite solar cells.
author_names: Armando Acevedo-Luna

Roberto Bernal-Correa

Jorge Montes-Monsalve

Arturo Morales-Acevedo
title: Design of thin film solar cells based on a unified simple analytical model
year: 2017

abstract: In this paper, we present an analytical closed model for the gate to source/drain fringing capacitance (C f of nanoscale metal oxide semiconductor field effect transistors (MOSFETs) with the consideration of layout dependent effects and process fluctuations. A kind of field poly structure on shallow trench isolation (STI) is used to separate C f from other gate around parasitic capacitances. A significant layout dependent effect is found in C f for the case with high contact density. Based on the device structure, C f is divided and analytically modeled by three dual k perpendicular plate capacitances. The effects of gate to contact space (CPS) contact to contact space (CCS) and the process variations, such as the over etching of source/drain contact, are taken into account. The proposed model is validated on 40 nm MOSFETs, with a series of layout parameters, and good agreement is obtained between the modeled and measured data over a large range of CPS and CCS. The proposed model can improve the precision for digital and RF circuit simulation in sub nanometer technology generation.
author_names: Yabin Sun

Ziyu Liu

Xiaojin Li

Jiaqi Ren

Fanglin Zheng

Yanling Shi
title: Analytical gate fringe capacitance model for nanoscale MOSFET with layout dependent effect and process variations
year: 2018

abstract: This work shows an analytical semiconductor diode model suitable to describe photovoltaic cells for a large variety of physical parameters, such as mobility of charge carriers and illumination intensity. The model is based on a simplified drift diffusion calculation assuming a constant electric field and a linear increasing current inside the semiconductor layer. The model also accounts for recombination processes in the active and contact layers. Organic and inorganic solar cells can be accurately modeled, which is confirmed by comparison of experimental data and full drift diffusion calculations with the same physical parameters. In addition, this model shows how physical properties can be directly extracted from the crossing point often found in J V characteristics.
author_names: Arne Muller

Vladislav Jovanov

Veit Wagner
title: Analytical model (CELIC) for describing organic and inorganic solar cells based on drift diffusion calculations
year: 2017

abstract: This paper describes a Transmission Line Model approach to the modeling and analysis of alloyed planar ohmic contacts. It briefly reviews the standard Transmission Line Model (TLM) commonly used to characterize a planar ohmic contact. It is shown that in the case of a typical Au Ge Ni alloyed ohmic contact, a more realistic model based on the TLM should take into account the presence of the alloyed layer at the metal semiconductor interface. In this paper, such a model is described. It is based on three layers and the two interfaces between them, thus forming a Tri Layer Transmission Line Model (TLTLM) Analytical expressions are derived for the contact resistance Re and the contact end resistance Rc of this structure, together with a current division factor, S. Values for the contact parameters of this TLTLM model are inferred from experimentally reported values of Re and Re for two types of contact. Using the analytical outcomes of the TLTLM, it is shown that the experimental results obtained using a standard TLM can have considerable discrepancies.
author_names: G K Reeves

Barry Harrison
title: An analytical model for alloyed ohmic contacts using a trilayer transmission line model
year: 1995

abstract: The interest in organic semiconductors, especially organic field effect transistors has grown enormously during the last years. They feature properties far beyond these of their inorganic counterpartners essential for devices like flexible displays or printable electronics. However, analytical compact models of organic semiconductors are rarely to find and mostly not able to describe the physical and chemical facts clearly. In the most cases they just exist for semiconductors with a well matching HOMO level. In dependence of different source and drain materials, varying workfunctions, HOMO LUMOand trap levels the parasitic contact resistance effects became a major problem in organic transistors [1] [2] So the source semiconductor junction was considered to be the limiting factor for the charge carriers. Exemplarily for the case of bottom contact, bottom gate OFETs we present an analytical model for the electrostatic potential, the electric field as well as the source and drain injection current which is able to describe typical characteristics in the output and transfer curves. These are, for instance, an s like shape in the output characteristic or kinks in the transfer characteristic (Fig. 2,4) especially of devices fabricated with organic semiconductors whose HOMO level does not match well with the electrode's workfunction. In general the charge injection into organic semiconductors is described by the thermionic driftdiffusion theory. Mostly tunneling processes are neglected. In our model we combine thermal diffusion and tunneling processes to describe the injection into a random hopping system. The organic semiconductor data set is leaned on Pentacene but kept variable regarding the barrier height for holes PhBp. A band like transport within the semiconductor is assumed in this step, as well as a gate bias dependent mobility [3] As it is most commonly used, we assumed gold as source and drain electrode's material. In accordance with the different charge transport mechanisms we separated the whole device into different subparts: The source semiconductor contact, whose properties highly influence the charge carrier injection, the transistor channel itself and the drain semiconductor contact. Each of these parts can limit the total device current. We assumed three operational states clearly to be separated and influenced just by the barrier height PhBp (Fig. 1) At first if the gate and drain biases are low and the bands at the source channel and the channel drain junction are bent upwards (p semiconductor) Secondly when the drain bias raises the bands at the source channel junction bend downwards and finally the bands on both junctions bent downwards for high VGS and comparatively low VDS. If we assume that the injection current consists of a diffusion and a tunneling contribution the band bending has to be derived at the junctions. This is done by computing the electric field on the basis of a 2D potential solution analytically by solving the Poisson equation via the conformal mapping technique [4] [5] The total device current is calculated by superposition, where IsTE, IsTU, IdTE, and IdTU, are the source or drain related thermionic and tunneling currents, respectively (Fig. 3)
author_names: Franziska Hain

Michael Graef

Thomas Holtij

Alexander Kloes

Benjamin Iniguez
title: Investigation of the Influence of the Source Drain Barrier Height on the I V Characteristics of OFETs by Using a 2 D Analytical Model for the Injection Current
year: 2013

abstract: An analytical model is presented to determine the potential and electric field distribution along the semiconductor surface of new silicon on insulator (SOI) reduced surface field (RESURF) device. The SOI structure is characterized by a semi insulating polycrystalline silicon (SIPOS) layer inserted between a silicon layer and a buried oxide. An improvement in the breakdown voltage due to the presence of the SIPOS shielding layer is demonstrated. Numerical simulations using medici are shown to support the analytical model.
author_names: Chi-hon Ho

C N Liao

Feng-Tso Chien

Yao-Tsung Tsai
title: An Analytical Model for Silicon on Insulator Reduced Surface Field Devices with Semi Insulating Polycrystalline Silicon Shielding Layer
year: 2008