Showing papers on "Depletion region published in 2009"

Impedance spectroscopy characterisation of highly efficient silicon solar cells under different light illumination intensities

Abstract: Highly efficient silicon solar cells have been characterised by impedance spectroscopy and current–potential characteristic in the dark and with different illumination intensities. For each illumination the impedance behaviour has been analysed at different applied bias potentials, in the forward and reverse region, comparing the results with the current–potential characteristic. Different cell parameters, as series and parallel resistances, capacitance, diode factor, minority carrier lifetime, acceptor impurities density and depletion layer charge density have been obtained as a function of bias voltage for different light illumination intensities. The effect of light-generated carriers and applied bias in the behaviour of the solar cell under illumination is discussed.

Amplified Electrokinetic Response by Concentration Polarization near Nanofluidic Channel

Abstract: Ion concentration polarization is the fundamental transport phenomenon that occurs near ion-selective membranes, but this important membrane phenomenon has been poorly understood due to theoretical and experimental challenges. Here, we report the first direct measurements of detailed flow and electric potential profiles within and near the depletion region. This work is an important step toward a full characterization of this coupled transport problem. Using microfabricated electrodes integrated with the microfluidic device, we measured and confirmed that the electric field inside an ion depletion region is amplified more than 30-fold compared to outside of the depletion zone due to the highly nonuniform ion concentration distribution along the microchannel. As a result, the electrokinetic motion of both fluid (electroosmosis) and particle (electrophoresis) was significantly amplified. The detailed flow profile within the depletion zone was also measured for the first time by optically tracking photobleac...

Photoelectron spectroscopy study of systematically varied doping concentrations in an organic semiconductor layer using a molecular p-dopant

Abstract: We investigate the doping behavior of the strongly electron accepting molecule 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane coevaporated with the host molecule N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine by photoemission spectroscopy and conductivity measurements. Using interface resolved measurements, we compare the alignment on different substrates and investigate the effects of varying doping concentrations on the Fermi level position. We find that at high doping concentrations the Fermi level gets pinned at the exponentially decaying tail of the highest occupied molecular orbital and compare these results with different dopants and host molecules. The measurement of the doping dependent space charge layer thickness yields information on the amount of free charge carriers and thereby the efficiency of the doping.

Silicon waveguide modulator based on carrier depletion in periodically interleaved PN junctions

Abstract: We present the design and numerical simulation results for a silicon waveguide modulator based on carrier depletion in a linear array of periodically interleaved PN junctions that are oriented perpendicular to the light propagation direction. In this geometry the overlap of the optical waveguide mode with the depletion region is much larger than in designs using a single PN junction aligned parallel to the waveguide propagation direction. Simulations predict that an optimized modulator will have a high modulation efficiency of 0.56 V.cm for a 3V bias, with a 3 dB frequency bandwidth of over 40 GHz. This device has a length of 1.86 mm with a maximum intrinsic loss of 4.3 dB at 0V bias, due to free carrier absorption. (C) 2009 Optical Society of America

Trench process and structure for backside contact solar cells with polysilicon doped regions

Abstract: A solar cell includes polysilicon P-type and N-type doped regions on a backside of a substrate, such as a silicon wafer. A trench structure separates the P-type doped region from the N-type doped region. Each of the P-type and N-type doped regions may be formed over a thin dielectric layer. The trench structure may include a textured surface for increased solar radiation collection. Among other advantages, the resulting structure increases efficiency by providing isolation between adjacent P-type and N-type doped regions, thereby preventing recombination in a space charge region where the doped regions would have touched.

Diameter-dependent electronic transport properties of Au-catalyst/Ge-nanowire Schottky diodes.

Abstract: We present electronic transport measurements in individual Au-catalyst/Ge-nanowire interfaces demonstrating the presence of a Schottky barrier. Surprisingly, the small-bias conductance density increases with decreasing diameter. Theoretical calculations suggest that this effect arises because electron-hole recombination in the depletion region is the dominant charge transport mechanism, with a diameter dependence of both the depletion width and the electron-hole recombination time. The recombination time is dominated by surface contributions and depends linearly on the nanowire diameter.

Threshold Voltage Shifts in Organic Thin‐Film Transistors Due to Self‐Assembled Monolayers at the Dielectric Surface

Abstract: Recently, it has been shown by several groups that the electrical characteristics of organic thin-film transistors (OTFTs) can be significantly influenced by depositing self-assembled monolayers (SAMs) at the organic semiconductor/dielectric interface. In this work, the effect of such SAMs on the transfer characteristics and especially on the threshold voltage of OTFTs is investigated by means of two-dimensional drift-diffusion simulations. The impact of the SAM is modeled either by a permanent space charge layer that can result from chemical reactions with the active material, or by a dipole layer representing an array of ordered dipolar molecules. It is demonstrated that, in both model cases, the presence of the SAM significantly changes the transfer characteristics. In particular, it gives rise to a modified, effective gate voltage V eff that results in a rigid shift of the threshold voltage, ΔV th , relative to a SAM-free OTFT. The achievable amount of threshold voltage shift, however, strongly depends on the actual role of the SAM. While for the investigated device dimensions, an organic SAM acting as a dipole layer can realistically shift the threshold voltage only by a few volts, the changes in the threshold voltage can be more than an order of magnitude larger when the SAM leads to charges at the interface. Based on the analysis of the different cases, a route to experimentally discriminate between SAM-induced space charges and interface dipoles is proposed. The developed model allows for qualitative description of the behavior of organic transistors containing reactive interfacial layers; when incorporating rechargeable carrier trap states and a carrier density-dependent mobility, even a quantitative agreement between theory and recent experiments can be achieved.

Evidences for the depletion region induced by the polarization of ferroelectric semiconductors

Abstract: Ferroelectric materials possess spontaneous polarization pointing from negative to positive bound surface charges. When a ferroelectric semiconductor is polarized, the induced electric field can drive free carriers, e.g., electrons in an n-type material, to neutralize surface charges until such field becomes zero. Such diffusion of free carriers induces a depletion region. Polarization switch can move the depletion region to the opposite surface, thus it can be used to manipulate any properties that are affected by such depletion region, such as unidirectional current and photovoltaic current.

Formation of a plasma depletion shell in the equatorial ionosphere

Abstract: [1] An accurate description of the irregularity region defined by a plasma bubble is critically important in understanding the dynamics of the region and its effects on radio scintillation. Here we describe a plasma depletion region as a "depletion shell" and show how two-dimensional optical images from space can be used to define the shape of the depletion shell. Our simple model calculation demonstrates that the space-based optical observation can detect the plasma-depleted magnetic flux tubes only near the F-peak height. The backward C-shape in bubble images from optical observations is the trace of the plasma depletion shell near the F-peak height. The westward tilt of bubbles at the magnetic equator can also be explained by this shell structure. The in situ measurement of the ion velocity at night in the topside shows the decrease of the eastward plasma drift with an increase of latitude. The formation of the plasma depletion shell is consistent with the latitudinal/altitudinal shear in the zonal plasma flow.

Photoadsorption and photodesorption for GaN

Abstract: The effect of an ambient environment on the surface photovoltage and photoluminescence observed for GaN is studied. In air ambient the upward band bending gradually increases under UV illumination and is explained by the photoinduced chemisorption of surface adsorbates. Specifically, the increase in negative surface charge is consistent with the transfer of electrons from surface states or bulk to oxygen species physisorbed at the GaN surface. In contrast, the upward band bending gradually decreases in vacuum under UV illumination and can be explained by the photoinduced desorption of these species. The photoadsorption and photodesorption of negatively charged species cause the surface depletion region to increase and decrease, respectively. This change in depletion region width is consistent with the observed decrease in photoluminescence intensity in air ambient and its significant increase in vacuum for a sample with low free electron concentration.

Method for fabricating semiconductor device

Abstract: A method for fabricating semiconductor device is provided to improve the productivity and reduce the device test time by maintaining the width of the depletion layer although the power supply voltage does not lowered. The mask pattern exposing the channel region is formed on the p-type substrate. The channel ion is injected to the barrier and then the mask pattern is formed to the channel ion injected layer(S201). The gate is formed on the substrate of the channel region(S202). The source and drain of the second conductive type opposed to the first conductivity type is formed on both sides of the channel region(S203). The impurity ion of the first conductivity type or the second conductive type is injected between the channel region and the source and drain(S204).

Backside contact solar cell with formed polysilicon doped regions

Abstract: A solar cell includes abutting P-type and N-type doped regions in a contiguous portion of a polysilicon layer. The polysilicon layer may be formed on a thin dielectric layer, which is formed on a backside of a solar cell substrate (e.g., silicon wafer). The polysilicon layer has a relatively large average grain size to reduce or eliminate recombination in a space charge region between the P-type and N-type doped regions, thereby increasing efficiency.

Optimization of InGaN–GaN MQW Photodetector Structures for High-Responsivity Performance

Abstract: InGaN-GaN multiple-quantum-well (MQW)-based photodetectors, with a detection edge at 450 nm and a high responsivity, have been fabricated and characterized We show that the performance of MQW-based photodetectors strongly depends on a proper device design, ie, number of QWs, and barrier and blocking layer thickness and doping level Namely, the responsivity can be varied in the ~ 1 to ~ 100 mA/W range in similar structures and with the same number of QWs These results support a model where the photocurrent increase is due to the improvement of collection efficiency caused by a change in transport mechanism for carriers photogenerated in the QWs The transport mechanism depends on the location of the QWs in relation to the depletion region

Geiger-mode photodiode with integrated and adjustable quenching resistor, photodiode array, and manufacturing method thereof

Abstract: An embodiment of a Geiger-mode avalanche photodiode includes a body of semiconductor material having a first conductivity type, a first surface and a second surface; a trench extending through the body from the first surface and surrounding an active region; a lateral-isolation region within the trench, formed by a conductive region and an insulating region of dielectric material, the insulating region surrounding the conductive region; an anode region having a second conductivity type, extending within the active region and facing the first surface. The active region forms a cathode region extending between the anode region and the second surface, and defines a quenching resistor. The photodiode has a contact region of conductive material, overlying the first surface and in contact with the conductive region for connection thereof to a circuit biasing the conductive region, thereby a depletion region is formed in the active region around the insulating region.

Study of the Transit-Time Limitations of the Impulse Response in Mid-Wave Infrared HgCdTe Avalanche Photodiodes

Abstract: The impulse response in frontside-illuminated mid-wave infrared HgCdTe electron avalanche photodiodes (APDs) has been measured with localized photoexcitation at varying positions in the depletion layer. Gain measurements have shown an exponential gain, with a maximum value of M = 5000 for the diffusion current at a reverse bias of V b = 12 V. When the light was injected in the depletion layer, the gain was reduced as the injection approached the N+ edge of the junction. The impulse response was limited by the diode series resistance–capacitance product, RC, due to the large capacitance of the diode metallization. Hence, the fall time is given by the RC constant, estimated as RC = 270 ps, and the rise time is due to the charging of the diode capacitance via the transit and multiplication of carriers in the depletion layer. The latter varies between t 10–90 = 20 ps (at intermediate gains M < 500) and t 10–90 = 70 ps (at M = 3500). The corresponding RC-limited bandwidth is BW = 600 MHz, which yields a new absolute record in gain–bandwidth product of GBW = 2.1 THz. The increase in rise time at high gains indicates the existence of a limit in the transit-time-limited gain–bandwidth product, GBW = 19 THz. The impulse response was modeled using a one-dimensional deterministic model, which allowed a quantitative analysis of the data in terms of the average velocity of electrons and holes. The fitting of the data yielded a saturation of the electron and hole velocity of v e = 2.3 × 107 cm/s and v h = 1.0 × 107 cm/s at electric fields E > 1.5 kV/cm. The increase in rise time at high bias is consistent with the results of Monte Carlo simulations and can be partly explained by a reduction of the electron saturation velocity due to frequent impact ionization. Finally, the model was used to predict the bandwidth in diodes with shorter RC = 5 ps, giving BW = 16 GHz and BW = 21 GHz for x j = 4 μm and x j = 2 μm, respectively, for a gain of M = 100.

Material and doping transitions in single GaAs-based nanowires probed by Kelvin probe force microscopy

Abstract: We demonstrate the potential of Kelvin probe force microscopy for simultaneously probing the topography and the work function of individual nanowires. Our technique allows us to visualize both the material and the doping contrast in single GaAs-based nanowires without the need to electrically contact the nanowires. In a GaAs/GaP heterostructure nanowire, a core-shell structure is found. This is attributed to a thermally activated radial overgrowth of GaAs, while in the GaP region the vertical nanowire growth dominates. In partially p-doped GaAs nanowires the doping transitions can be localized and the width of the depletion layer is estimated.

Ferroelectric switching dynamics in VDF-TrFE copolymer thin films spin coated on Si substrate

Abstract: Simultaneous measurements of the charge Q and the capacitance C were performed for an MFS capacitor with Au-(vinylidene fluoride-trifluoroethylene copolymer)-(n-Si) structure using a double-frequency voltage consisting of a low-frequency high voltage and a high-frequency sinusoidal low voltage The use of a triangular high voltage yields asymmetrical Q-V and C-V hysteresis loops that support the full ferroelectric polarization reversal occurring in close relation to charge compensation in the n-Si layer The use of a rectangular high voltage reveals the details of asymmetric switching dynamics Polarization switching toward the positive side starts with the loss of the depletion layer and progresses rapidly owing to the accumulation of majority carriers to be completed at a time analogous to that for the case of a metal-ferroelectric-metal capacitor On the other hand, that toward the negative side is markedly impeded by depletion layer formation but is eventually completed via a constant-current process

Revisiting the origin of open circuit voltage in nanocrystalline-TiO2/polymer heterojunction solar cells

Abstract: Recent reports suggest that the open circuit voltage, VOC, in polymeric heterojunction solar cells is determined by the energy difference between the highest occupied molecular orbital of the electron donor and the lowest unoccupied molecular orbital of the electron acceptor. Here we show that in solar cells formed from nanocrystalline titanium dioxide (nc-TiO2) and poly(3-hexylthiophene) (P3HT), VOC may arise from a depletion region at the nc-TiO2/P3HT interface, which only forms in the presence of oxygen. Under illumination in vacuum, photovoltaic behavior is swamped by a significant increase in photoconduction enabled by reduced electron trapping in the absence of oxygen.

Alpha particle detection with GaN Schottky diodes

Abstract: Ni/GaN Schottky diode radiation detectors were fabricated on 3-μm-thick unintentionally doped n-GaN films grown by molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD) and on 12-μm-thick undoped n-GaN layers prepared by epitaxial lateral overgrowth (ELOG). The reverse current of all detector structures was <10−9 A for bias voltages necessary for detector operation, with the level of background donor doping of <1015 cm−3. With this doping level the space charge region of the Schottky diode could be extended to the entire thickness of the films. The charge collection efficiency of the detectors was close to 100% for MOCVD and ELOG detectors for α-particles with range comparable to the thickness of the layer. Electrical properties and deep trap spectra were also studied. The collection efficiency decreased when the concentra-tion of deep electron traps, particularly Ec-0.6 eV traps, increased in MBE grown films.

Edge Termination With Improved Breakdown Voltage

Abstract: A MOSFET switch which has a low surface electric field at an edge termination area, and also has increased breakdown voltage. The MOSFET switch has a new edge termination structure employing an N-P-N sandwich structure. The MOSFET switch also has a polysilicon field plate configuration operative to enhance any spreading of any depletion layer located at an edge of a main PN junction of the N-P-N sandwich structure.

Comprehensive study on the deep depletion capacitance-voltage behavior for metal-oxide-semiconductor capacitor with ultrathin oxides

Abstract: The deep depletion behaviors at the structure of Si/SiO2 with various equivalent oxide thicknesses (EOTs) are comprehensively studied by magnified capacitance versus gate voltage (C-V) curves of metal-oxide-semiconductor (P-substrate) capacitors in this work. According to the correlation between inversion tunneling current and deep depletion, it was found that the initiation voltage of deep depletion phenomenon increases with EOT (2.8–3.1 nm). After the constant voltage stress, the early occurrence of initiation voltage of deep depletion is observed after oxide breakdown. In addition, the uniform area ratio concept is proposed for the electrical characterization of deep depletion via local depletion capacitance model. It was novel for the evaluation of interfacial property between dielectric and Si substrate.

Transient drain current characteristics of ZnO nanowire field effect transistors

Abstract: We investigated the characteristics of the time-dependent drain current of ZnO nanowire field effect transistors (FETs). The drain current of ZnO nanowire FETs in ambient air decreases from an initial current level in the microampere range and saturates to the 1–100 nA range in tens of seconds. This transient phenomenon is ascribed to electrically interactive adsorption of oxygen ions to the nanowire surface. Exposure to ambient air during positive gate biasing reduces the conduction channel width by extending the depletion region, resulting in a higher resistivity with conduction only through the narrower nanowire core.

Temperature control device for optoelectronic devices

Abstract: Current may be passed through an n-doped semiconductor region, a recessed metal semiconductor alloy portion, and a p-doped semiconductor region so that the diffusion of majority charge carriers in the doped semiconductor regions transfers heat from or into the semiconductor waveguide through Peltier-Seebeck effect. Further, a temperature control device may be configured to include a metal semiconductor alloy region located in proximity to an optoelectronic device, a first semiconductor region having a p-type doping, and a second semiconductor region having an n-type doping. The temperature of the optoelectronic device may thus be controlled to stabilize the performance of the optoelectronic device.

Quantum Efficiency Analysis of InAs–GaSb Type-II Superlattice Photodiodes

Abstract: We compare the experimentally measured and theoretically calculated quantum efficiency (QE), where an analytical drift-diffusion photocurrent model is used, of n+ -on-p InAs-GaSb superlattice (SL) photodiodes. With inputs of the transport parameters obtained by the electron-beam-induced current technique and absorption coefficient spectra calculated by the eight-band kldrp method for the p-SL, n+ -SL, and depletion region, taking into account the band filling effect, we show that the drift-diffusion photocurrent model is a good approximation for the InAs-GaSb type-II SL photodiodes, which implies that the SL depletion region in InAs-GaSb SL photodiodes is as effective as that in bulk semiconductor photodiodes in terms of collecting the photo-excited electron-hole pairs. Using this theoretical model, we also find that the high doping density in n-type SL degrades the QE by reducing the absorption coefficient. As a result, the n-type doping density is suggested to be below 1times1017cm-3 in order to optimize the QE for the studied InAs-GaSb SL photodiodes.

Au-TiBx-n-6H-SiC Schottky barrier diodes: Specific features of charge transport in rectifying and nonrectifying contacts

Abstract: Mechanism of charge transport in a diode of a silicon carbide’s Schottky barrier formed by a quasi-amorphous interstitial phase TiB x on the surface of n-6H-SiC (0001) single crystals with an uncompensated donor (nitrogen) concentration of ∼1018 cm−3 and dislocation density of ∼(106–108) cm−2 has been studied. It is demonstrated that, at temperatures T ≲ 400 K, the charge transport is governed by the tunneling current along dislocations intersecting the space charge region. At T > 400 K, the mechanism of charge transport changes to a thermionic mechanism with a barrier height of ∼0.64 eV and ideality factor close to 1.3.

On the formation of stationary destructive cathode-side filaments in p + -n − -n + diodes

Abstract: Analyzing the dynamics of current filaments is essential for a correct understanding of SOA limitations. Current filaments can occur during the reverse-recovery period of p+-n−-n+ diodes. In this work, we apply the results from an analysis of the plasma-front dynamics for the one-dimensional case to conditions under which current filaments appear in the depletion layers due to dynamic avalanche. We show that the anode-side plasma front velocity is higher in the vicinity of the filament than far away from the filament center, favoring the evolution of a lateral traveling anode-side filament. Furthermore, we find that the cathode-side plasma front changes its vertical propagation direction when a dynamic avalanche in the cathode-side depletion layer causes current crowding. As a result, the cathode-side depletion layer in the vicinity of the filament decreases, favoring the formation of a standing cathode-side filament that may cause final destruction of the device. The analytical results are in good agreement with numerical simulations and results of previously published work.