Two-dimensional Drift-diffusion Research Articles

Traps and Interface Fixed Charge Effects on a Solution-Processed n-Type Polymeric-Based Organic Field-Effect Transistor

Organic field-effect transistors based on poly{[N,N0-bis(2-octyldodecyl)- naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,50-(2,20-bithiophene)}, [P(NDI2OD-T2)n], were fabricated and characterized. The effect of octadecyltrichlorosilane (OTS) a self-assembled monolayer (SAM) grafted on to a SiO2 gate dielectric was investigated. A significant improvement of the charge mobility (μ), up to 0.22 cm2/V s, was reached thanks to the OTS treatment. Modifying some technological parameters relating to fabrication, such as solvents, was also studied. We have analyzed the electrical properties of these thin-film transistors by using a two-dimensional drift–diffusion simulator, Integrated System Engineering-Technology Computer Aided Design (ISE-TCAD®). We studied the fixed surface charges at the organic semiconductor/oxide interface and the bulk traps effect. The dependence of the threshold voltage on the density and energy level of the trap states has also been considered. We finally found a good agreement between the output and transfer characteristics for experimental and simulated data.

Electric control of spin transport in GaAs (111) quantum wells

We show by spatially and time-resolved photoluminescence that the application of an electric field transverse to the plane of an intrinsic GaAs (111) quantum well (QW) allows the transport of photogenerated electron spins polarized along the direction perpendicular to the QW plane over distances exceeding 10~$\mu$m. We attribute the long spin transport lengths to the compensation of the in-plane effective magnetic field related to the intrinsic spin-orbit (SO) interaction by means of the electrically generated SO-field. Away from SO-compensation, the precession of the spin vector around the SO-field decreases the out-of-plane polarization of the spin ensemble as the electrons move away from the laser generation spot. The results are reproduced by a model for two-dimensional drift-diffusion of spin polarized charge carriers under weak SO-interaction.

Numerical method for a 2D drift diffusion model arising in strained n-type MOSFET device

This paper reports the calculation of electron transport in metal oxide semiconductor field effects transistors (MOSFETs) with biaxially tensile strained silicon channel. The calculation is formulated based on two-dimensional drift diffusion model (DDM) including strain effects. The carrier mobility dependence on the lateral and vertical electric field model is especially considered in the formulation. By using the model presented here, numerical method based on finite difference approach is performed. The obtained results show that the presence of biaxially tensile strain enhances the current in the devices.

Simulation of the performance of organic electronic devices based on a two-dimensional drift-diffusion approach

Organic electronic devices will play an important role in future technical applications because of their easy production and low costs. Besides intrinsic material properties, the performance of organic solar cells, light emitting diods and thin-film transistors depends strongly on the morphology, the occurring hetero-junctions, as well as on achieving an appropriate alignment of transport levels. To optimize the efficiency of organic devices it is, therefore, important to understand what limits their performance. The paper supports this concern by providing a general model for simulating the exciton and charge carrier transport in such building blocks. Due to the complex internal structure and the large aspect ratios of the active device regions, the procedure is based on drift-diffusion equa- tions. All of the necessary source, generation and recombination terms are included in the developed model. The applied numerics is based on finite difference approaches and especially on the Scharfetter-Gummel algorithm. Three examples, which prove the appli- cability of the established formalism to enhance the performance of organic devices are given. The performed simulations reveal interesting particularities of the carrier transport in these devices.

Measurement and simulation of top- and bottom-illuminated solar-blind AlGaN metal-semiconductor-metal photodetectors with high external quantum efficiencies

A comprehensive study on top- and bottom-illuminated Al0.5Ga0.5N/AlN metal-semiconductor-metal (MSM) photodetectors having different AlGaN absorber layer thickness is presented. The measured external quantum efficiency (EQE) shows pronounced threshold and saturation behavior as a function of applied bias voltage up to 50 V reaching about 50% for 0.1 μm and 67% for 0.5 μm thick absorber layers under bottom illumination. All experimental findings are in very good accordance with two-dimensional drift-diffusion modeling results. By taking into account macroscopic polarization effects in the hexagonal metal-polar +c-plane AlGaN/AlN heterostructures, new insights into the general device functionality of AlGaN-based MSM photodetectors are obtained. The observed threshold/saturation behavior is caused by a bias-dependent extraction of photoexcited holes from the Al0.5Ga0.5N/AlN interface. While present under bottom illumination for any AlGaN layer thickness, under top illumination this mechanism influences the EQE-bias characteristics only for thin layers.

Effect of humidity on negative corona Trichel pulses

The effects of environmental parameters, e.g., humidity, on the corona discharges in practical applications are important. A two-dimensional (2D) hydrodynamic drift-diffusion model has been used to investigate the effects of humidity on the negative corona Trichel pulses (TPs). The simulations are performed with a conventional needle-to-plate configuration in humid air. It is found that the magnitude of TPs grows gradually with increasing humidity and the frequency of TPs increases with humidity. The movements and formations of charged particles are faster in higher humidity. The numerical results are consistent with the experimental observation.

Physical insights of body effect and charge degradation in floating-body DRAMs

Floating Body one transistor Dynamic Random Access Memories (FBRAMs) have been widely studied and proposed in the literature as an alternative for conventional one transistor/one capacitor DRAMs. FBRAM performance depends on charge degradation during READ and HOLD operations and on the body effect during READ operation, the first setting the amount of the residual non-equilibrium charge during READ operation, and the second setting the effectiveness of this residual charge to modulate the source-body barrier during READ operation. In this work it is proposed a simple analytical charge-based compact model for the body-effect in FBRAMs which is able to reproduce device performance in terms of READ Sense Margin and current ratio. Physical insights of the body effect and charge degradation mechanisms, with particular emphasis to their bias dependence, are discussed in detail. Conclusions can be useful for the choice and the optimization of the bias in FBRAMs. All the discussion is supported by two-dimensional drift–diffusion device simulation on a template double-gate MOSFET.

Electrical properties of plasma enhanced chemical vapor deposition a-Si:H and a-Si1−xCx:H for microbolometer applications

Electrical properties for resistive microbolometer sensor materials including resistivity, temperature coefficient of resistance (TCR), and normalized Hooge parameter were explored in n-type a-Si:H and a-Si1−xCx:H prepared by plasma enhanced chemical vapor deposition. The complex dielectric function spectra (ε = ε1 + iε2) and structure were measured by spectroscopic ellipsometry. Two-dimensional drift-diffusion simulations were used to understand the band-tail slope dependency of TCR and 1/f noise.

Impact of the Inhomogenous Structure of Polysilicon TFT's Active Layer on the Electrostatic Potential Distribution

Recently polycrystalline silicon (pc-Si) thin film transistors (TFT’s) have emerged as the devices of choice for many applications. The TFTs made of a thin un-doped polycrystalline silicon film deposited on a glass substrate by the Low Pressure Chemical Vapor Deposition technique LPCVD have limits in the technological process to the temperature < 600°C. The benefit of pc-Si is to make devices with large grain size. Unfortunately, according to the conditions during deposition, the pc-Si layers can consist of a random superposition of grains of different sizes, where grains boundaries parallels and perpendiculars appear. In this paper, the transfer characteristics IDS-VGS are simulated by solving a set of two-dimensional (2D) drift-diffusion equations together with the usual density of states (DOS: exponential band tails and Gaussian distribution of dangling bonds) localized at the grains boundaries. The impact of thickness of the active layer on the distribution of the electrostatic potential and the effect of density of intergranular traps states on the TFT’s transfer characteristics IDS-VGS have been also investigated.

Analyzing the physical properties of InGaN multiple quantum well light emitting diodes from nano scale structure

We report on the influence of nanoscale indium fluctuations on physical properties for multiple quantum well (QW) light emitting diodes (LEDs). A commercial grade c-plane LED was analyzed by atom probe tomography, and the indium composition distribution was extracted. The influence of the degree of fluctuation and number of quantum wells were analyzed by a two-dimensional Poisson and drift-diffusion solver with very fine mesh and compared to the experimental result and a simple normal quantum well model. The studies show that the indium fluctuation will significantly impact the device’s internal quantum efficiency, droop behavior, and current-voltage curves. Including the influence of indium-fluctuation gives a better prediction of the device performance.

Relation between injection barrier and contact resistance in top-contact organic thin-film transistors

We theoretically investigate the carrier injection into top-contact bottom-gate organic thin film transistors. By means of a two-dimensional drift–diffusion model, we explicitly consider thermionic and tunneling injection in combination with subsequent carrier transport into the device. Based on numerical simulations with this model, we determine the contact resistance as a function of the nominal hole injection barrier height and temperature. Depending on the barrier height or the operating temperature, we find three distinct injection regimes. Our work reveals that in all three regimes self-regulating processes exist due to which the influx of current is adjusted according to the needs of the channel at the given point of operation.We explain why the transmission/transfer line method (TLM) for the determination of the contact resistance, Rc, quantitatively fails for non-quasi-ohmic injection. Self-regulation links the contact resistance to the channel resistance and the contact resistance becomes dependent on the channel length. For larger channel lengths, Rc is underestimated by TLM; the method yields overestimated values for small channel devices.

Nonuniform current and spin accumulation in a 1 μm thick n-GaAs channel

The spin accumulation in a n-GaAs channel produced by spin extraction into a (Ga,Mn)As contact is measured by cross-sectional imaging of the spin polarization in GaAs. The spin polarization is observed in a 1 μm thick n-GaAs channel with the maximum polarization near the contact edge opposite to the maximum current density. The one-dimensional model of electron drift and spin diffusion, frequently used, cannot explain this observation. It also leads to incorrect spin lifetimes from Hanle curves with a strong bias and distance dependence. Numerical simulations based on a two-dimensional drift-diffusion model, however, reproduce the observed spin distribution quite well and lead to realistic spin lifetimes.

Influence of transport-related material parameters on the I– V characteristic of inorganic–organic hybrid solar cells

The carrier transport in inorganic–organic hybrid cells based on small band gap inorganic semiconductors is theoretically studied. In such cells, photo-carriers are generated at the heterojunction (due to dissociation of the donor excitons) and in the bulk of the acceptor material due to direct generation in the small band gap inorganic material. By means of a two-dimensional drift–diffusion model, we demonstrate that material properties directly related to transport, i.e., (i) the carrier mobilities, (ii) the static dielectric constants, and (iii) transport level offsets give, essentially, rise to marked variations in the open circuit voltage, V oc , and the shape of the I– V curve. As the most striking consequence of the differing generation mechanisms, the role of the electron and hole level offsets entirely differ. A lack of a hole transport level offset represents the most serious loss factor for the power conversion efficiency (PCE) due to a pronounced sensitivity of the shape of the I– V curve towards the carrier mobilities and the dielectric constant. On the other hand, degenerate electron transport levels give rise to (i) appreciably large fill factors and (ii) values of V oc , j sc , and PCE that are insensitive towards the ratio of mobility and ϵ r values. However, a non-vanishing electron transport level offset is necessary to achieve a V oc beyond V built-in at all. Then, V oc is further (i) increased by an amount that corresponds to the smaller value of the transport level offsets and (ii) reduced due to a diffusion term that depends on the ratio of the mobility values in the organic and inorganic phase. The latter detrimental term is the smaller, the less the mobility ratio differs from unity, i.e., the smaller the mismatch in the mobility values becomes. Moreover, any mismatch in the carrier mobilities causes an effective mobility-induced diffusion of holes from the organic to the inorganic layer that gives rise to a s-shaped section in the I– V curve.

Numerical studies of transient gain reduction process in a multi-wire proportional chamber

A gain reduction process caused by successive beam irradiation in a multi-wire proportional chamber was numerically investigated to clarify the relations between the gas gain variation and the ion density distribution. A numerical code was developed based on a two-dimensional drift-diffusion model in order to evaluate the ion and electron density distributions and the electric field variation caused by the space charge effect. In order to consider the gain reduction process which occurs under the high rate and successive irradiation, the simulations were performed for the time period of ∼10-100 μs, which is much longer than the time required for ions to travel from an anode to a cathode. The numerical simulation results showed that for the low gas gain regime of ∼10, quasi-stationary density distribution of the ions was formed by the high-rate beams of ∼10(8)-10(10) particles per second, and that the transient variation of the gas gain became constant after establishment of the quasi-stationary ion density distributions.

Experimental and modeling study of the capacitance–voltage characteristics of metal–insulator–semiconductor capacitor based on pentacene/parylene

The capacitance–voltage (C–V) characteristics of metal–insulator–semiconductor (MIS) capacitors consisting of pentacene as an organic semiconductor and parylene as the dielectric have been investigated by experimental, analytical, and numerical analysis. The device simulation was performed using two-dimensional drift-diffusion methods taking into account the Poole–Frenkel field-dependent mobility. Pentacene bulk defect states and fixed charge density at the semiconductor/insulator interface were incorporated into the simulation. The analysis examined pentacene/parylene interface characteristics for various parylene thicknesses. For each thickness, the corresponding flat band voltage extracted from the C–V plot of the MIS structure was more negative than − 2.4 V. From the flat band voltage the existence of a significant mismatch between the work functions of the gate electrode and pentacene active material has been identified. Experimental and simulation results suggest the existence of interface charge density on the order of 3 × 10 11 q/cm 2 at the insulator/semiconductor interface. The frequency dispersion characteristics of the device are also presented and discussed.

Numerical Study of Scaling Issues in Graphene Nanoribbon Transistors

ABSTRACTThis paper addresses scaling issues in graphene nanoribbon transistors (GNRFETs) by using a two-dimensional (2-D) Poisson and drift-diffusion solver with finite element method (FEM). GNRFETs with the back gate control and the channel width down to less than 5nm have been reported to have Ion=Ioff ratio up to 106. Our simulations show an agreement with the published experimental work and show a potential to reach unit current gain cut-off frequency, fT , up to more than 1THz with a satisfying Ion=Ioff ratio at the same time. This makes GNRFETs attractive for high speed logic.

Simulation based performance comparison of transistors designed using standard photolithographic and coarse printing design specifications

In this work a simulation based comparative study of organic field effect transistors designed using standard lithographic and printing designs is presented. The device simulations were performed using two-dimensional drift-diffusion equations with a Poole–Frenkel field dependent mobility model. Both photolithographic and coarse printing transistor designs employed common materials such as 150nm thick pentacene, 150nm thick parylene gate insulator, gold source-drain electrodes and aluminum gate electrodes. The major differences between the two fabrication specifications are the minimum source/drain line width and the transistor channel length. The typical specifications for the minimum line width and channel length were 2μm and 5μm for photolithography and 25μm and 20μm for coarse printing techniques, respectively. The gate, source, and drain capacitances and channel on-resistances at various channel lengths and gate overlaps have been extracted and presented specifically for both process schemes. Due to increased channel length and gate-source/drain overlap of printed electrodes relative to lithographic design, the resulting on-resistance and capacitances for coarse printing are significantly higher. These results demonstrate a substantial operating frequency reduction for printing design relative to photolithographic design. For the tested materials and designs it is shown that the cut-off frequency for the photolithographic process was 400kHz but decreased to a much lower 26kHz for the coarse printing process. Since printing technology uses various other materials, which typically have less performance than the ones used for this simulation, the actual printed device might have even lower performance than predicted here.

Impact of semiconductor/metal interfaces on contact resistance and operating speed of organic thin film transistors

The contact resistance of field effect transistors based on pentacene and parylene has been investigated by experimental and numerical analysis. The device simulation was performed using finite element two-dimensional drift-diffusion simulation taking into account field-dependent mobility, interface/bulk trap states and fixed charge density at the organic/insulator interface. The width-normalized contact resistance extracted from simulation which included an interface dipole layer between the gold source/drain electrodes and pentacene was 91 k?cm. However, contact resistance extracted from the simulation, without consideration of interface dipole was 52.4 k?cm, which is about half of the experimentally extracted 108 k?cm. This indicates that interface dipoles are critical effects which degrade performances of organic field effect transistors by increasing the contact resistance. Using numerical calculations and circuit simulations, we have predicted a 1 MHz switching frequency for a 1 μm channel length transistor without dipole interface between gold and pentacene. The transistor with dipole interface is predicted, via the same methods, to exhibit an operating frequency of less than 0.5 MHz.

Mechanisms for the enhancement of the lateral photovoltage in perovskite heterostructures

The mechanisms for the greatly enhanced lateral photovoltaic effect in the perovskite oxide heterostructures are studied by solving time-dependent two-dimensional drift-diffusion equations self-consistently. By our calculations, we find that the lateral photovoltage of p -type material is larger than that of n -type material owing to the larger drift electric field induced in the p -type material than that in the n -type material. Moreover, the built-in electric field at the interface between the thin film and substrate can also enhance the lateral photovoltage. The above two mechanisms can well explain one-order-of-magnitude enhancement of the lateral photovoltaic effect in the perovskite heterostructures. In addition, we find that the materials with larger mobility ratio have a stronger Dember effect. Such an understanding of the mechanisms for the enhancement of lateral photovoltage in oxide heterostructures should be useful in further designing of the structures of position-sensitive detectors and new THz sources.

Understanding the effect of semiconductor thickness on device characteristics in organic thin film transistors by way of two-dimensional simulations

This paper investigates the dependence of organic thin film transistor (OTFT) characteristics with semiconductor (pentacene) thickness by way of two-dimensional drift–diffusion simulations. The extracted field-effect mobility of the as-measured top contact devices exhibits a variation as the pentacene thickness changes from 10 to 100 nm. Device simulation of an ideal OTFT does not exhibit any dependence on semiconductor thickness, as expected. Therefore, in simulations, the physical behavior related to charge transport at the pentacene–insulator interface and in the bulk is systematically introduced in order to model the experimentally observed device characteristics. These effects are simulated by incorporating a low-mobility-layer adjacent to the dielectric, and energy level shift in the same layer as compared to the bulk pentacene to induce confinement towards the insulator, and bulk donor traps in the pentacene film above a certain thickness (∼35 nm). Finally, verification between the simulated and experimental device characteristics is obtained as a function of pentacene thickness.