Carrier Transit Time Research Articles

Photovoltage-Driven Photoconductor Based on Horizontal p-n-p Junction.

The photoconductive gain theory demonstrates that the photoconductive gain is related to the ratio of carrier lifetime to carrier transit time. Theoretically, to achieve higher gain, one can either prolong the carrier lifetime or select materials with high mobility to shorten the transit time. However, the former slows the response speed of the device, while the latter increases the dark current and degrades device sensitivity. To address this challenge, a horizontal p-n-p junction-based photoconductor is proposed in this work. This device utilizes the n-region as the charge transport channel, with the charge transport direction perpendicular to the p-n-p junction. This design offers two advantages: (i) the channel is depleted by the space charge layer generated by the p and n regions, enabling the device to maintain a low dark current. (ii) The photovoltage generated in the p-n junction upon light absorption can compress the space charge layer and expand the conductive path in the n-region, enabling the device to achieve high gain and responsivity without relying on long carrier lifetimes. By adopting this device structure design, a balance between responsivity, dark current, and response speed is achieved, offering a new approach to designing high-performance photodetectors based on both traditional materials and emerging nanomaterials.

Hybrid mode for absorption enhancement in the Ga2O3 nanocavity photodetector with grating electrodes

Gallium oxide (Ga2O3) photodetectors have drawn increased interest for their widespread applications ranging from military to civil. Due to the inherent oxygen vacancy defects, they seriously suffer from trade-offs that make them incompetent for high-responsivity, quick-response detection. Herein, a Ga2O3 nanocavity photodetector assisted with grating electrodes is designed to break the constraint. The proposed structure supports both the plasmonic mode and the Fabry–Perot (F-P) mode. Numerical calculations show that the absorption of 99.8% is realized for ultra-thin Ga2O3 (30 nm), corresponding to a responsivity of 12.35 A/W. Benefiting from optical mechanisms, the external quantum efficiency (EQE) reaches 6040%, which is 466 times higher than that of bare Ga2O3 film. Furthermore, the proposed photodetector achieves a polarization-dependent dichroism ratio of 9.1, enabling polarization photodetection. The grating electrodes also effectively reduce the transit time of the photo-generated carriers. Our work provides a sophisticated platform for developing high-performance Ga2O3 photodetectors with the advantages of simplified fabrication processes and multidimensional detection.

Enhancing carrier collection in CsPbBr3 solar cells through crystal orientation and defect passivation

All-inorganic carbon-based CsPbBr3 perovskite solar cells (PSCs) have gained growing interest for their remarkable stability. However, compared to their organic–inorganic hybrid counterparts, there is still substantial room for improving their performance primarily due to the inferior photogenerated carrier collection efficiency. Here, we employ area-dependent transient photocurrent to assess the carrier transit time in CsPbBr3 PSCs, revealing that an extended carrier transit time relative to the lifetime significantly contributes to their low carrier collection efficiency. To address this challenge, we narrow the gap between carrier transit time and lifetime by introducing a dual-functional additive, serving to facilitate both crystallization orientation and defect passivation. Consequently, we achieve enhanced short-circuit current and efficiency in CsPbBr3 PSCs.

Dependence of Ge/Si Avalanche Photodiode Performance on the Thickness and Doping Concentration of the Multiplication and Absorption Layers

In this article, the performance and design considerations of the planar structure of germanium on silicon avalanche photodiodes are presented. The dependences of the breakdown voltage, gain, bandwidth, responsivity, and quantum efficiency on the reverse bias voltage for different doping concentrations and thicknesses of the absorption and multiplication layers of germanium on the silicon avalanche photodiode were simulated and analyzed. The study revealed that the gain of the avalanche photodiode is directly proportional to the thickness of the multiplication layer. However, a thicker multiplication layer was also associated with a higher breakdown voltage. The bandwidth of the device, on the other hand, was inversely proportional to the product of the absorption layer thickness and the carrier transit time. A thinner absorption layer offers a higher bandwidth, but it may compromise responsivity and quantum efficiency. In this study, the dependence of the photodetectors’ operating characteristics on the doping concentration used for the multiplication and absorption layers is revealed for the first time.

Investigation of the metal–semiconductor interface by equivalent circuit model in zinc phthalocyanine (ZnPc) based Schottky diodes and its charge transport properties

In this report, the ZnPc-based metal–semiconductor (MS) junction Schottky devices (Al/ZnPc/ITO) were fabricated using an effusion cell coating unit. To study the formation of the MS junction, the surface morphology of the deposited ZnPc thin film was investigated using field-emission scanning electron microscope (FESEM) images. The interfacial properties of the MS junction of the Al/ZnPc/ITO configuration were studied using the ac impedance spectroscopy technique within the frequency range of 50 Hz–10 MHz at room temperature (300 K). The bias-dependent impedance spectroscopy was carried out within the voltage range of ±1 V to establish the equivalent circuit of the MS junction Schottky diodes (SDs). The current vs. voltage (I-V) measurements of the fabricated SDs were also conducted to deduce the diode parameters, namely on/off ratio, photosensitivity, ideality factor, barrier height, and series resistance. The charge transport parameters including dc conductivity, mobility and transit time of the charge carriers were also estimated employing the spatial-charge limited current (SCLC) theory, which illustrates the enhanced carrier’s mobility and consequently, the device performance after light irradiation.

Evidence of Lampert Triangle and Jonscher’s Double Power Law in Doped Poly(3,4-ethylenedioxythiophene) Devices

The Lampert triangle model can be an interesting method for investigating the conduction mechanism in semiconducting polymers. A complete Lampert triangle is observed in PF6− doped poly(3,4-ethylenedioxythiophene) [PEDOT:PF6] devices, fabricated in a stainless steel/PEDOT/Ag structure and synthesized by electro-polymerization at three different doping levels (high-S1, medium-S2, and low-S3). The temperature-dependent current–voltage characteristics show three distinct regimes limited within a triangular region called the Lampert triangle: Ohmic, trap-limited/filling space charge-limited conduction (TFL-SCLC), and trap-free/trap-filled space charge-limited conduction (TF-SCLC). The vertices of the Lampert triangle represents the transition voltages (VT) for different conduction mechanisms, and the carrier density (p0), carrier transit time (tt), and relaxation time (td) for different doping levels are estimated through the analysis of the triangle. The mobility of carriers shows significant temperature dependence (Arrhenius type). The Jonscher’s double power law fit to AC conductivity data shows that the exponent (in the range 0–1) varies with carrier density and has different trends in bulk and interface. The impedance data and Nyquist plots are analyzed with two parallel RQ circuits (R: resistance; Q: constant phase element) connected in series. The values of onset frequency (ωo) in the conductivity plot of the different samples (S1, S2, and S3) give insights into their relative disorder. Raman spectroscopy studies have attempted to corroborate the changes in bond deformation with carrier density in correlation with the structure and disorder.

Broadband long-wave infrared high-absorption of active materials through hybrid plasmonic resonance modes

Broadband high absorption of long-wavelength infrared light for rough submicron active material films is quite challenging to achieve. Unlike conventional infrared detection units, with over three-layer complex structures, a three-layer metamaterial with mercury cadmium telluride (MCT) film sandwiched between an Au cuboid array and Au mirror is studied through theory and simulations. The results show that propagated/localized surface plasmon resonance simultaneously contribute to broadband absorption under the TM wave of the absorber, while the Fabry–Perot (FP) cavity resonance causes absorption of the TE wave. As surface plasmon resonance concentrates most of the TM wave on the MCT film, 74% of the incident light energy is absorbed by the submicron thickness MCT film within the 8–12 μm waveband, which is approximately 10 times than that of the rough same thickness MCT film. In addition, by replacing the Au mirror with Au grating, the FP cavity along the y-axis direction was destroyed, and the absorber exhibited excellent polarization-sensitive and incident angle-insensitive properties. For the corresponding conceived metamaterial photodetector, as carrier transit time across the gap between Au cuboid is much less than that of other paths, the Au cuboids simultaneously act as microelectrodes to collect photocarriers generated in the gap. Thus the light absorption and photocarrier collection efficiency are hopefully improved simultaneously. Finally, the density of the Au cuboids is increased by adding the same arranged cuboids perpendicular to the original direction on the top surface or by replacing the cuboids with crisscross, which results in broadband polarization-insensitive high absorption by the absorber.

Theoretical Study for Carrier Transit Limited Performance of Gate-All-Around Si Nanowire Transistor by Time-Dependent Quantum Transport Simulation

Theoretically, the upper limit performance of nanoscale transistor should be limited by carrier transit time through the channel as the charge in channel cannot follow the gate voltage and the gate capacitance will show strong frequency dependence if the gate voltage varies faster than the carrier transit time. Currently, the operation frequency of nanoscale transistor is limited by the non-ideal factors such as parasitic effects, and it is far below the limit caused by carrier transit time. However, how good the transistor can be if one can minimize these non-ideal factors is still a problem, thus, it would be worth exploring its upper limit performance. In this article, time-dependent quantum transport simulation is performed to explore the carrier transit limited performance of the gate-all-around (GAA) Si nanowire transistor. By using the mode space approach, the 3-D time-dependent quantum transport equation is decoupled to a 2-D confinement problem and a 1-D transport problem, and then self-consistently solved with the Poisson’s equation. The transient characteristics of channel charge are investigated, and it shows the response time of channel charge is in the scale of 0.1 ps. The responses of channel charge and drain current to high-frequency gate voltages are simulated. The carrier transit limited performance including the frequency-dependent gate capacitance and transconductance, 3 dB bandwidth, and cutoff frequency, are calculated and discussed.

Enhanced Light–Tellurium Interaction through Evanescent Wave Coupling for High Speed Mid‐Infrared Photodetection

AbstractMid‐infrared (MIR) waveguide‐integrated photodetector is essential for various applications in the fields of sensing and optical communications. However, it is challenging to integrate traditional MIR photoactive materials such as HgCdTe or III−V compounds with complementary metal‐oxide‐semiconductor (CMOS)‐compatible silicon platform due to the lattice mismatch. Tellurium (Te), a novel van der Waals (vdW) material with a narrow bandgap, high carrier mobility, and great air stability, is a promising candidate for high‐performance MIR detection. Here, high‐quality Te nanosheets are synthesized using a hydrothermal method and their carrier dynamics are characterized by transient reflection spectrum. The effect of mobility anisotropy on response speed is investigated intuitively by a free space phototransistor. Combining the strong evanescent wave of a waveguide architecture with the synergy effect between the carrier collection path and the highest mobility crystal orientation in Te, an integrated Te photodetector with enhanced light–matter interaction and reduced carrier transit time is achieved. For the first time, the MIR waveguide‐integrated Te photodetector with a responsivity of 2.3 A W−1 and a bandwidth of 4 GHz at 2015 nm is realized, which is the highest speed MIR photodetector based on narrow bandgap vdW materials to date.

Ultrafast intrinsic optical-to-electrical conversion dynamics in a graphene photodetector

Optical-to-electrical (O-E) conversion in graphene is a central phenomenon for realizing anticipated ultrafast and low-power-consumption information technologies. However, revealing its mechanism and intrinsic time scale require uncharted terahertz (THz) electronics and device architectures. Here, we succeeded in resolving O-E conversion processes in high-quality graphene by on-chip electrical readout of ultrafast photothermoelectric current. By suppressing the RC time constant using a resistive zinc oxide top gate, we constructed a gate-tunable graphene photodetector with a bandwidth of up to 220 GHz. By measuring nonlocal photocurrent dynamics, we found that the photocurrent extraction from the electrode is instantaneous without a measurable carrier transit time across several-micrometer-long graphene, following the Shockley-Ramo theorem. The time for photocurrent generation is exceptionally tunable from immediate to > 4 ps, and its origin is identified as Fermi-level-dependent intraband carrier-carrier scattering. Our results bridge the gap between ultrafast optical science and device engineering, accelerating ultrafast graphene optoelectronic applications.

Plasmonic internal-photoemission-based Si photodetector design suitable for optical communication.

We propose a high-performance plasmonic photodetector based on the internal photoemission (IPE) process for the C-band communication wavelength. This photodetector takes advantage of an embedded nanohole array in Schottky metal. Owing to localized surface plasmon resonance, the absorption of the active metal layer increases, which results in the generation of more hot carriers and subsequently compensates for the low efficiency of IPE-based photodetectors. Simulations show that for the proposed photodetector with 2-nm-thick Au, Cu, and Ag Schottky contacts, the absorptance dramatically enhances to 95.1%, 93.2%, and 98.2%, respectively, at the wavelength of 1.55 µm. For the detector based on Au, the highest external quantum efficiency of 25.3% and responsivity of 0.32 A/W are achieved at a reverse bias voltage of 1 V. Furthermore, the 3 dB bandwidth can exceed 369 GHz owing to the low capacitance of the structure and the fast transit time of carriers from the thin p-Si layer. Finally, by studying the current-voltage characteristics of the photodetector, it is shown that under the reverse bias voltage of 1 V, the dark current is 665 nA at room temperature, and by reducing the temperature to 200 K, it improves three orders of magnitude and decreases to 810 pA.

Performance of metal-semiconductor-metal structured diamond deep-ultraviolet photodetector with a large active area

In this work, the uniformity is significantly improved of the photoresist film spinning-coated on the small-size diamond wafer by inlaying the diamond wafer into a 1-inch polytetrafluoroethylene substrate; consequently, the utilizable surface area of the diamond wafer is remarkably increased. As a result, the interdigital electrodes of 2.5 mm × 2.5 mm in dimension are prepared on the single crystal diamond (5 mm × 5 mm × 0.5 mm) and a metal–semiconductor–metal structured diamond deep-ultraviolet photodetector with a large active area of 3.093 mm2 has been fabricated. Compared to the maximum values of the interdigital-typed intrinsic diamond deep-ultraviolet photodetectors, the active area is increased by more than six times, and the photocurrent reaches the order of milliampere, which is about two orders of magnitude larger. Meanwhile, the responsivity and external quantum efficiency are 56.3 A W−1 and 328, respectively, at 50 V bias under 3.125 μW mm−2 213 nm illumination, and the corresponding mobility-lifetime product of the diamond wafer is 1.11 × 10−5 cm2 V−1. As the voltage continued to increase, which still maintained an upward trend and did not appear saturated; the corresponding responsivity is up to 275.9 A W−1 at 120 V. In addition, the ultraviolet-visible light discrimination ratio is 1.4 × 104 at 10 V, and the carrier transit time between interdigital electrodes is measured to be only about 1 ns (excited by a 213 nm pulse laser), which shows that the photodetector has an ultrafast response speed.

Influence of the environment on the morphology, optical and electrical characteristics of the PEDOT:PSS polymer

The paper presents the results of studying the influence of the environment on the surface structure, optical and electrical characteristics of the PEDOT:PSS film. The studies have been carried out in three different types of atmosphere under the same conditions of thermal annealing of the films. It has been established that the modification of the surface of a polymer film with ethanol and isopropanol in vacuum, in a nitrogen atmosphere, and in the air atmosphere leads to a change in the surface morphology and the optical electric transport properties of the polymer. Depending on the environment, when the PEDOT:PSS film is modified with a certain concentration of ethyl and isopropyl alcohols, the absorption spectra show a shift in the absorption maximum responsible for PEDOT to the short-wavelength region of the spectrum, as well as a decrease in the absorption peak of the part of the spectrum responsible for the absorption of the aromatic fragment PSS. It is identified that the structural features of the PEDOT:PSS surface morphology affect the electrical transport parameters of the films: Rh is the resistance of the PEDOT:PSS polymer film, Rext is the charge carrier transfer resistance at the PEDOT:PSS/electrode interface, keff is the effective charge carrier extraction rate and τeff is the effective transit time of charge carriers. The optimal technological parameters for film production have been determined, at which the electrical transport properties of PEDOT:PSS polymer film are increased.

Influence of the environment on the morphology, optical and electrical characteristics of the PEDOT:PSS polymer

The paper presents the results of studying the influence of the environment on the surface structure, optical and electrical characteristics of the PEDOT:PSS film. The studies have been carried out in three different types of atmosphere under the same conditions of thermal annealing of the films. It has been established that the modification of the surface of a polymer film with ethanol and isopropanol in vacuum, in a nitrogen atmosphere, and in the air atmosphere leads to a change in the surface morphology and the optical electric transport properties of the polymer. Depending on the environment, when the PEDOT:PSS film is modified with a certain concentration of ethyl and isopropyl alcohols, the absorption spectra show a shift in the absorption maximum responsible for PEDOT to the short-wavelength region of the spectrum, as well as a decrease in the absorption peak of the part of the spectrum responsible for the absorption of the aromatic fragment PSS. It is identified that the structural features of the PEDOT:PSS surface morphology affect the electrical transport parameters of the films: Rh is the resistance of the PEDOT:PSS polymer film, Rext is the charge carrier transfer resistance at the PEDOT:PSS/electrode interface, keff is the effective charge carrier extraction rate and τeff is the effective transit time of charge carriers. The optimal technological parameters for film production have been determined, at which the electrical transport properties of PEDOT:PSS polymer films are increased.

A Modified Equivalent Circuit Model for High-Speed InGaAs/InAlAs Avalanche Photodiodes

In this paper, we establish a modified equivalent circuit model suitable for all bias conditions for high-speed InGaAs/InAlAs avalanche photodiodes (APDs). The model includes three mechanisms: the carrier transit time, the avalanche buildup time, and the RC time constant. The physical meaning of each component in it is elaborated in detail. Component parameters in it of the demonstrated APD are extracted by fitting the RF reflection parameters S22 and transmission parameters S21 measured at different bias voltages. To verify the accuracy and regularity of these parameters, some relevant finite element simulations and theoretical analyses are carried out. Values and regularities of these parameters are consistent with the results of finite element simulations and theoretical analyses. With the help of this model and these accurate parameters, impedance distribution and bandwidth limiting factors of the demonstrated APD are analyzed thoroughly. According to the above analysis results, the performance optimization scheme for the demonstrated APD is obtained.

Insights into the charge carrier dynamics in perovskite/Si tandem solar cells using transient photocurrent spectroscopy

Direct bandgap perovskite and indirect bandgap Si, which form the two active layers in a tandem solar cell configuration, have different optoelectronic properties and thicknesses. The charge-carrier dynamics of the two-terminal perovskite-on-Si tandem solar cell in response to a supercontinuum light pulse is studied using transient photocurrent (TPC) measurements. Spectral dependence of TPC lifetime is observed and can be classified into two distinct timescales based on their respective carrier generation regions. The faster timescale (∼500 ns) corresponding to the spectral window (300–750 nm) represents the top-perovskite sub-cell, while the slower timescale regime of ∼25 μs corresponds to the bottom-Si sub-cell (>700 nm). Additionally, under light-bias conditions, the transient carrier dynamics of the perovskite sub-cell is observed to be coupled with that of the Si sub-cell. A sharp crossover from the fast-response to a slow-response of the device as a function of the light-bias intensity is observed. These results along with a model based on transfer matrix formulation highlight the role of charge-carrier dynamics in accessing higher efficiencies in tandem solar cells. The carrier transit times and lifetimes in addition to their optical properties need to be taken into account for optimizing the performance.

Boosting the External Quantum Efficiency of AlGaN-Based Metal–Semiconductor–Metal Ultraviolet Photodiodes by Electrode Geometry Variation

Fast aluminum-gallium-nitride–based metal–semiconductor–metal ultraviolet photodiodes were successfully designed, fabricated, and characterized using conventional photolithography techniques. Various electrode geometries were fabricated to investigate the influence of metal contact shapes on the device performance indices with emphasis on the response speed and bias-voltage–independent efficiency. Based on the independently measured Hall mobility and electric field, we evaluated the carrier transit time of the devices as 1.31 ps with bias-voltage–independent external quantum efficiency of 1198% in photoconductive mode at 19.5 V and 70% at 60 V for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n$ </tex-math></inline-formula> -doped and intrinsic devices, respectively.

Master-equation-based approach to stochastic processes in few-electron systems and advanced considerations for practical applications

This paper revisits the master-equation-based approach to physical parameters to characterize transport in three-dimensional and low-dimensional few-electron systems. Advanced expressions of the electron density are theoretically derived at equilibrium for a system having traps. It is revealed that electron density at equilibrium is slightly higher than that without any interface traps as a result of the influence of dynamic trapping/detrapping processes. The capture time constant of electrons applicable to practical systems having traps, such as silicon-related materials, is also theoretically derived. The theoretical model is examined by numerical calculations and experimental results. In wire-type metal–oxide–semiconductor devices, the capture-time constant model roughly reproduces its inverse-temperature dependence. The effective activation energy of the capture time constant is not significantly influenced by that of the emission time constant. In the conductive filaments of silicon oxide film created by electrical stress, the capture-time constant model basically reproduces its inverse-temperature dependence. The effective activation energy of the capture time constant is not significantly influenced by that of the emission time constant but is influenced by the cross-sectional area of the filament and the electron density in the filament. The capture-time constant model semi-quantitatively reproduces the experimentally observed bias dependence of the silicon oxide film. Numerical calculation results suggest that the carrier transit time assumed in the model depends on the physical properties of the materials used. Given the goal of this study, the theoretical approach basically produces successful results.

Special Issue on the 40th Anniversary of University of Macau

Special Issue on the 40th Anniversary of University of Macau

Analysis of AM-to-PM conversion in MUTC photodiodes based on an equivalent circuit model.

High-speed, high power-handling photodiodes with sufficiently low amplitude-to-phase (AM-to-PM) conversion coefficients are critical components in the systems that generate ultra-stable microwave signals. This paper reports the AM-to-PM conversion in modified uni-traveling carrier photodiodes (MUTC-PDs) with 20 µm and 40 µm diameters. The contributions of AM-to-PM conversions from the carrier transit-time and impedance were quantified systematically based on a photocurrent-dependent nonlinear equivalent circuit model. It is found that the AM-to-PM conversion in 40 µm PD is dominated by the nonlinear impedance, while for 20 µm PD, the transit-time impacts the AM-to-PM conversion more significantly. These results imply that, for large PDs, the nonlinearity of the PDs' photocurrent-dependent impedance is the critical reason causing AM-to-PM conversion.