Carrier Transit Time Research Articles

Impedance Investigation of Polymer Layers for Electroluminescent Devices

AC electrical properties of polymer light-emitting devices with poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and polyvinylcarbazole (PVK) layers are studied. Polymer nanosized films are deposited by the spin-coating method. Frequency-dependent real and imaginary parts of the impedance are measured and fitted using RC equivalent circuits. Important parameters for such types of structures, like capacitance and contact resistance, are evaluated at different frequencies in the range from 100 Hz to 8 MHz and bias voltages between 0 and 4 V. Transit times of charge carriers at different bias are calculated using the capacitance-frequency curves.

A Comprehensive Model to Predict the Timing Resolution of SiPM-Based Scintillation Detectors: Theory and Experimental Validation

Silicon photomultipliers (SiPMs) are expected to replace photomultiplier tubes (PMTs) in several applications that require scintillation detectors with excellent timing resolution, such as time-of-flight positron emission tomography (TOF-PET). However, the theory about the timing resolution of SiPM-based detectors is not yet fully understood. Here we propose a comprehensive statistical model to predict the timing resolution of SiPM-based scintillation detectors. It incorporates the relevant SiPM-related parameters (viz. the single cell electronic response, the single cell gain, the charge carrier transit time spread, and crosstalk) as well as the scintillation pulse rise and decay times, light yield, and energy resolution. It is shown that the proposed model reduces to the well-established Hyman model for timing with PMTs if the number of primary triggers (photoelectrons in case of a PMT) is Poisson distributed and crosstalk and electronic noise are negligible. The model predictions are validated by measurements of the coincidence resolving times (CRT) for 511 keV photons of two identical detectors as a function of SiPM bias voltage, for two different kinds of scintillators, namely LYSO:Ce and LaBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> :5%Ce. CRTs as low as 138 ps ± 2 ps FWHM for LYSO:Ce and 95 ps ± 3 ps FWHM for LaBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> :5%Ce were obtained, demonstrating the outstanding timing potential of SiPM-based scintillation detectors. These values were found to be in good agreement with the predicted CRTs of 140 ps FWHM and 95 ps FWHM, respectively. Utilizing the proposed model, it can be shown that the CRTs obtained in our experiments are mainly limited by photon statistics while crosstalk, electronic noise and signal bandwidth have relatively little influence.

Analyzing two electroluminescence modes of indium tin oxide/α-NPD/Alq3/Al diodes by using large alternating current square voltages

By applying large A.C. square voltages to double-layer ITO/α-NPD/Alq3/Al diodes, we studied the generation of electroluminescence (EL) in terms of the carrier injection and transport. The EL intensity gradually decayed with the increase of the frequency of the applied A.C. square voltage, and reached a minimum at the frequency corresponding to the carrier transit time. Interestingly, the EL intensity was again increased by the application of further higher frequency A.C. square voltages, relying on the D.C. component of the A.C. square voltages. The results suggest the presence of two EL modes. Using the Maxwell-Wagner effect model analysis, we have proposed a model that accounts for the two EL modes.

Femto-second electron transit time characterization in GaN/AlGaN quantum cascade detector at 1.5 micron

The ultra fast carrier dynamic in GaN/AlGaN quantum cascade detector was investigated using a time-resolved bias-lead monitoring technique. It is demonstrated that the intrinsic speed limitation, governed by the carrier transit time, is smaller than 1 ps, corresponding to a frequency cut off above 200 GHz.

Low-Power-Consumption High-Eye-Margin 10-Gb/s Operation by GaInAsP/InP Distributed Reflector Lasers With Wirelike Active Regions

Low-driving-current and high-eye-margin 10-Gb/s operation was demonstrated by a GaInAsP/InP distributed reflector (DR) laser with wirelike active regions. The reduction of modulation bias currents was realized by the high modulation efficiency of five-stack quantum wirelike active regions as well as thin optical confinement layers for a shorter carrier transit time. A mask test of 10 GbE with 20% margin was passed with a low bias current of 10 mA, and the maximum 3-dB bandwidth was over 15 GHz.

On the role of minority carriers in the frequency dependence of organic magnetoresistance

In this work we investigate the frequency dependence of organic magnetoresistance (OMAR) both in small molecule-based (Alq3) and polymer (PPV derivative) materials, and investigate its thickness dependence. For all devices, we observed a strong decrease in magnetoconductance (MC) with increasing frequency of the ac component of the applied magnetic field. Moreover, we observed a strong reduction of the cut-off frequencies for increasing film thickness. By means of admittance spectroscopy and device simulations, we show that the cut-off frequency is related to the inverse transit time of the minority charge carriers. These observations confirm the important role of minority carriers in OMAR, and show that changes in OMAR are not only due to microscopic mechanisms, but also device physics is of significant relevance.

Linear-Cascade Near-Ballistic Unitraveling-Carrier Photodiodes With an Extremely High Saturation Current–Bandwidth Product

Saturation current-bandwidth product (SCBP), the key of figure of merit in high-speed and high-power photodiodes (PDs), is mainly limited by the tradeoff between carrier drift time in depletion layer and RC-limited bandwidth of conventional PDs. Here, we present a revolutionary photodiode structure: linear-cascade photodiodes (LCPDs), designed to further improve the SCBP performance. Our demonstrated LCPD structure can greatly increase the SCBP without using a complex distributed structure of the traveling-wave PD or reducing the load resistance (output RF power). Two flip-chip bonding packaged near-ballistic unitraveling-carrier photodiode (NBUTC-PD) units are employed in our LCPD structure. It exhibits a great improvement in SCBP compared to that of the control device with a single NBUTC-PD. A two-port equivalent-circuit model is established for the LCPDs and the modeling results clearly indicate that the increase in SCBP can be attributed to the significant reduction in its total capacitance due to the serial connection. Furthermore, we find that only when each PD unit in the LCPD structure has the same amount of injected optical power and modulated frequency of optical signal, the whole structure exhibits a carrier transit time as short as that of a single PD. Under the proper optical excitation, we can achieve a record high SCBP (7500 mA·GHz and 100 GHz) for two-element LCPDs under a 50 Ω load.

Using admittance spectroscopy to quantify transport properties of P3HT thin films

The charge transport properties of organic electronic materials are of critical importance for the understanding of the operation of organic photonic devices. We show in this study that admittance spectroscopy (AS), which is a measure of the frequency dependent capacitances of organic electronic materials, can be used to determine charge carrier mobilities and transport parameters in organic photovoltaic materials. A plot of the negative differential susceptance versus frequency yields a maximum at a frequency. The position of the maximum is related to the average carrier transit time. Using AS, we study the hole transport properties of thin films of poly(3-hexylthiophene) (P3HT) based materials for organic photovoltaic applications, including pristine P3HT, and P3HT doped with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). We demonstrate that AS can be used to extract hole mobilities of P3HT and P3HT:PCBM with a thickness of ∼350 nm. For P3HT:PCBM, copper phthalocyanine is used as an electron trapping layer to reduce the effect of negative capacitance.

Physical and Numerical Modelling for Bipolar Charge Transport in Disorder Polyethylene Under High DC Voltage

This paper reports on validation of a numerical model for both transient and steady bipolar charge transport, trapping and recombination in polymeric insulators, such as low density polyethylene. The numerical model is based on the high precision Runge-Kutta method for determining mobile and trapped charge local density within the sample, during the application of a DC voltage. In addition, we have applied the finite element method considered as the most general one, which can be applied to any sample shape and boundary conditions. It is applied to resolve Poisson's equation, thus providing the potential and its gradient within the sample and at the dielectric-electrode interfaces. The principal results show the appearance of charge packets for the first time in modeling works although they have long been reported in experimental studies on polyethylene materials. The conditions for the appearance of such packets are discussed. For the net space charge density and the conduction current, the dynamics are in a good agreement with those observed in some experimental works during the transit time of the bipolar carriers. The model is already numerically validated for the space charge densities and the electric field distribution under low DC voltage.

Microwave and millimeter-wave losses in conventional optoelectronic devices

In this paper, microwave characteristics of conventional optoelectronic devices, with emphasis on devices with microstrip (MS) and coplanar waveguide (CPW) electrode structures, are obtained. This analysis is essential for any improvement in the structure of the conventional optoelectronic devices so as to obtain a high performance. Microwave loss is one of the important bandwidth limitation factors in microwave and millimeter-wave (mmW) optical devices. Different sources of loss including ohmic, dielectric and radiating loss in MS and CPW of conventional optical devices are analyzed and compared. The results show that the total microwave loss increases with frequency in conventional MS and CPW waveguides. Also, in traveling-wave optoelectronic devices, the bandwidth is limited in the optical part by effects such as the carrier transit time effect and in the microwave part by factors such as length of the devices in active and non-active sections. In addition, validation of the results in the paper is performed with published theoretical and/or measurement results.

Charge Carrier Lifetime and Recombination in Bulk Heterojunction Solar Cells

In this paper, the main photocurrent density and power conversion efficiency limiting mechanisms in bulk heterojunction solar cells are discussed with the emphasis on recombination processes. Charge extraction by linearly increasing voltage, time of flight, and other methods that allow the carrier lifetime and recombination to be studied experimentally in operating solar cells are discussed. It is shown that non-Langevin recombination is required for high-performance organic photovoltaic devices, which typically have low charge carrier mobility. Long charge carrier lifetime, exceeding carrier transit time through the film, can be achieved when non-Langevin recombination is observed. Langevin-type recombination dominates in most low-efficiency solar cells, whereas non-Langevin recombination is present in high efficient, e.g., annealed poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester blend devices. The film nanomorphology plays a crucial role governing the charge transport and the carrier lifetime. Double injection current with non-Langevin carrier recombination is demonstrated in high-efficiency devices, which strongly exceeds the injection current with Langevin recombination, due to the high carrier concentration attainable under non-Langevin recombination. Several different models explaining the non-Langevin recombination in organic solar cells are reviewed. Requirements for charge carrier mobility and recombination to maximize power conversion efficiency in organic photovoltaic devices are outlined.

Frequency dependence of organic magnetoresistance

To identify the microscopic mechanisms of organic magnetoresistance (OMAR), the dependency on the frequency of the applied magnetic field is explored, which consists of a dc and ac component. The measured magnetoconductance decreases when the frequency is increased. The decrease is stronger for lower voltages, which is shown to be linked to the presence of a negative capacitance, as measured with admittance spectroscopy. The negative capacitance disappears when the frequency becomes comparable to the inverse transit time of the minority carriers. These results are in agreement with recent interpretations that magnetic field effects on minority carrier mobility dominate OMAR.

Influence of carrier transport on Raman amplification in silicon waveguides

This paper analyze for the first time the impact of the charge carrier transport on the continuously pumped stimulated Raman amplification in silicon waveguides. A novel analytical model is developed using which the coupled differential equations of the pump and the probe optical signals and those of carrier transport are solved concurrently. The simulation and analysis suggest that the neglect of the carrier transport phenomenon, reported in the previously published works, is approximately justified only if the effective carrier lifetime is comparable to the carrier transit time, otherwise it can result in substantial overestimation of the free carrier density at the optical mode center.

A Novel Equivalent Circuit Model for Separate Absorption Grading Charge Multiplication Avalanche Photodiode (APD)-Based Optical Receiver

A novel equivalent circuit for separate absorption grading charge multiplication (SAGCM) APDs composed of basic circuit components is developed. The model is applied to simulate the frequency performance of APDs, and the simulating results show a well-reasonable agreement with the physical model calculation and experimental data. The influence of three important factors on frequency performance, including carrier transit time, avalanche buildup time, and parasitic elements (consisted of resistance, inductance, and capacitance of APDs) are investigated. Using this model, the cosimulation of the packaged APD\transimpedance amplifier (TIA)-module is also carried out. We found that the resonance of parasitic inductance and capacitance can be utilized to compensate the attenuation of high-speed APD\ TIA circuit board.

Limits on vanadium oxide Mott metal–insulator transition field-effect transistors

There have been numerous proposals for use of metal–oxide materials as an alternative to semiconductors in field-effect transistors (FET), as current Si FET technology inevitably encounters intrinsic scaling limitations. We report on device-independent power–delay characteristics of potential VO 2-based field induced Mott transistors and compare scaling limits to that of Si. Since the critical electric field for metal–insulator transition (MIT) in VO 2 is similar to the breakdown field of Si, and due to the inherent possibility of further scaling along one direction in VO 2, both materials exhibit similar lower bounds on switching energy. MIT in VO 2 results in free carrier concentration several orders of magnitude larger than that of Si, easily overcoming the carrier transit time limits of conventional semiconductor MOSFETs. VO 2 switching speed is constrained by the kinetics of the phase transition and more importantly limited thermal dissipation. Our simple model predicts an intrinsic VO 2 material lower bound switching time of the order of 0.5 ps at a power transfer of 0.1 μ W .

PROPERTIES OF ELECTROMAGNETIC FIELDS AND EFFECTIVE PERMITTIVITY EXCITED BY DRIFTING PLASMA WAVES IN SEMICONDUCTOR-INSULATOR INTERFACE STRUCTURE AND EQUIVALENT TRANSMISSION LINE TECHNIQUE FOR MULTI-LAYERED STRUCTURE

Strong interests are recently emerging for development of solid-state devices operating in the so-called \terahertz region for possible application in radio astronomy, industry and defense. To flll the THz gap by using conventional electron approach or transit time devices seems to be very di-cult due to the limitation that comes from the carrier transit time where extremely small feature sizes are required. One way to overcome this limitation is to employ the traveling wave type approach in semiconductors like classical traveling wave tubes (TWTs) where no transit time limitation is imposed. In this paper, the analysis method to analyze the properties of drifting plasma waves in semiconductor-insulator structure based on the transverse magnetic (TM) mode analysis is presented. Two waves components (quasi-lamellar wave and quasi-solenoidal wave), electromagnetic flelds (Ey, Ez and Hx) and !- and k-dependent efiective permittivity are derived where these parameters are the main parameters to explain the interaction between propagating electromagnetic waves and drifting carrier plasma waves in semiconductor. A method to determine the surface impedances in semiconductor-insulator multi-layered structure using equivalent transmission line representation method is also presented since multi- layered structure is also a promising structure for fabricating such a so-called plasma wave device.

Transient Charge Carrier Transport Effects in Organic Field Effect Transistor Channels

AbstractTransient phenomena that impact the speed of electron devices fabricated from organic semiconductor materials are of critical importance for the development of this new technology. Here we investigate theoretically and experimentally the effects of traps on the establishment and the depletion of the conducting channel in organic semiconductor field effect transistors (OFETs). The device structures explored resemble typical organic thin-film transistors with one of the channel contacts removed. The channel length is varied, and generally is longer than in typical OFETs, in order to vary the carrier transit time. By measuring the displacement current associated with charging and discharging of the channel in these capacitors, transient effects on the carrier transport in organic semiconductors may be studied. When carriers are injected into the device, a conducting channel is established with traps that are initially empty. Gradual filling of the traps then modifies the transport characteristics of the injected charge carriers. In contrast, DC experiments as they are typically performed to characterize the transport properties of organic semiconductor channels investigate a steady state with traps partially filled. Numerical and approximate analytical models of the formation of the conducting channel and the resulting displacement currents are presented. Two simple scenarios are considered first: the spatially-averaged carrier density model neglects carrier dynamics but shows the salient trapping/detrapping effects. Second: a simple geometric model neglects traps but elucidates the effect of carrier dynamics and is used to define an effective mobility that can be extracted from experiments. Finally, numerical simulations are used to study the dynamical and trap effects together. Comparing the average trapping time with the time scale of the current transient sheds light on the influence of traps on the effective mobility. If the average trapping time is very long the effect of traps is negligible and the mobility is largely unaffected by traps on the timescale of the experiment. If the average trapping time is short the effective mobility is reduced but it is time-independent. However, the effective mobility changes with time if the average trapping time is comparable to the current transient interval, eventually reaching a constant value once the trapping and detrapping events balance. Furthermore, the temperature dependence of the effective mobility arising from the temperature dependence of the trap emission rate is explored. Experimental data for pentacene based devices of different channel length and obtained over in the temperature range 120K &lt; T &lt; 300K support the concepts of the physical model.

Variations in Hole Injection due to Fast and Slow Interfacial Traps in Polymer Light‐Emitting Diodes with Interlayers

AbstractDetailed studies on the effect of placing a thin (10 nm) solution‐processable interlayer between a light‐emitting polymer (LEP) layer and a poly(3,4‐ethylenedioxythiophene)/poly(styrenesulfonic)‐acid‐coated indium tin oxide anode is reported; particular attention is directed at the effects on the hole injection into three different LEPs. All three different interlayer polymers have low ionization potentials, which are similar to those of the LEPs, so the observed changes in hole injection are not due to variations in injection barrier height. It is instead shown that changes are due to variations in hole trapping at the injecting interface, which is responsible for varying the hole current by up to two orders of magnitude. Transient measurements show the presence of very fast interfacial traps, which fill the moment charge is injected from the anode. These can be considered as injection pathway dead‐ends, effectively reducing the active contact surface area. This is followed by slower interfacial traps, which fill on timescales longer than the carrier transit time across the device, further reducing the total current. The interlayers may increase or decrease the trap densities depending on the particular LEP involved, indicating the dominant role of interfacial chain morphology in injection. Penetration of the interlayer into the LEP layer can also occur, resulting in additional changes in the bulk LEP transport properties.

Advantages of admittance spectroscopy over time-of-flight technique for studying dispersive charge transport in an organic semiconductor

We show that admittance spectroscopy (AS) is a better technique than time of flight (TOF) to study the charge transport properties in dispersive materials. The hole transport properties of N,N′-diphenyl-N,N′-bis(1-naphthyl)(1,1′-biphenyl)-4,4′-diamine (NPB) doped with different traps were evaluated by AS and TOF techniques. It was found that both techniques can show clear signals for measuring the mobility of NPB doped with shallow traps. When NPB was doped with deep traps, the AS signals were still clear for mobility extraction. In sharp contrast, the TOF transients become featureless and the carrier transit time cannot be determined. The validity of AS in mobility determination was demonstrated by comparing the extracted AS to TOF mobilities. Generally, the hole mobilities extracted by these two techniques were in excellent agreement. In addition, we will demonstrate that AS can be employed to measure carrier dispersion.

Bipolar Transistor Excess Phase Modeling in Verilog-A

The collector current Ic of a bipolar transistor does not instantaneously respond to changes in applied base-emitter voltage Vbe ; its response exhibits a time lag because of the finite transit time of carriers through the transistor, which is manifest as a phase lag (or ldquoexcess phaserdquo) in the frequency domain. In this paper we present an excess phase model that has a constant magnitude response, in contrast to previous models which introduce a change in magnitude as well as in phase, and detail how our model can be implemented in Verilog-A. In addition, we show how a bias dependence of the time lag can be added to the Verilog-A implementation of the Weil-McNamee excess phase model without introducing undesired behavior.