Showing papers on "Depletion region published in 2006"

Piezoelectric field effect transistor and nanoforce sensor based on a single ZnO nanowire.

Abstract: Utilizing the coupled piezoelectric and semiconducting dual properties of ZnO, we demonstrate a piezoelectric field effect transistor (PE-FET) that is composed of a ZnO nanowire (NW) (or nanobelt) bridging across two Ohmic contacts, in which the source to drain current is controlled by the bending of the NW. A possible mechanism for the PE-FET is suggested to be associated with the carrier trapping effect and the creation of a charge depletion zone under elastic deformatioin. This PE-FET has been applied as a force/pressure sensor for measuring forces in the nanonewton range and even smaller with the use of smaller NWs. An almost linear relationship between the bending force and the conductance was found at small bending regions, demonstrating the principle of nanowire-based nanoforce and nanopressure sensors.

Graded band gap for dark-current suppression in long-wave infrared W-structured type-II superlattice photodiodes

Abstract: A new W-structured type-II superlattice photodiode design, with graded band gap in the depletion region, is shown to strongly suppress dark currents due to tunneling and generation-recombination processes. The long-wave infrared (LWIR) devices display 19%–29% quantum efficiency and substantially reduced dark currents. The median dynamic impedance-area product of 216Ωcm2 for 33 devices with 10.5μm cutoff at 78K is comparable to that for state-of-the-art HgCdTe-based photodiodes. The sidewall resistivity of ≈70kΩcm for untreated mesas is also considerably higher than previous reports for passivated or unpassivated type-II LWIR photodiodes, apparently indicating self-passivation by the graded band gap.

Operation of the intermediate band solar cell under nonideal space charge region conditions and half filling of the intermediate band

Abstract: A photovoltaic device based on an intermediate electronic band located within the otherwise conventional band gap of a semiconductor, the so-called intermediate band solar cell (IBSC), has been proposed for a better utilization of the solar spectrum. Experimental IBSC devices have been engineered using quantum dot technology, but their practical implementation results in a departure of key underpinning theoretical principles, assumed to describe the operation of the IBSC, away from the ideal. Two principles which are only partially fulfilled are that (i) the intermediate band should be half filled with electrons and (ii) the region containing the quantum dots should not be located fully within the junction depletion region. A model to describe the operation of the devices under these nonidealized conditions is presented and is used to interpret experimental results for IBSCs with ten layers of quantum dots. Values for the electron and hole lifetimes, associated with recombination from the conduction band ...

Interfacial electronic structure of copper phthalocyanine and copper hexadecafluorophthalocyanine studied by photoemission

Abstract: Electronic structures of the heterojunction between copper phthalocyanine (CuPc) and copper hexadecafluorophthalocyanine (F16CuPc) were studied with ultraviolet photoemission spectroscopy. Band bending and an interface dipole were observed at the interface due to the formation of an electron accumulation layer and a depletion layer in F16CuPc and CuPc, respectively. Such an energy level alignment leads to interesting ambipolar characteristics for application of the CuPc∕F16CuPc junction in organic field-effect transistors.

Current-voltage and capacitance-voltage characteristics of Sn/rhodamine-101∕n-Si and Sn/rhodamine-101∕p-Si Schottky barrier diodes

Abstract: The nonpolymeric organic compound rhodamine-101 (Rh101) film on a n-type Si or p-type Si substrate has been formed by means of the evaporation process and the Sn/rhodamine-101/Si contacts have been fabricated. The Sn∕Rh101∕n-Si and Sn∕Rh101∕p-Si contacts have rectifying contact behavior with the barrier height (BH) values of 0.714 and 0.827eV, and with ideality factor values of 2.720 and 2.783 obtained from their forward bias current-voltage (I-V) characteristics at room temperature, respectively. It has been seen that the BH value of 0.827eV obtained for the Sn∕Rh101∕p-Si contact is significantly larger than BH values of the conventional Sn∕p-Si Schottky diodes and metal/interfacial layer/Si contacts. Thus, modification of the interfacial potential barrier for metal/Si diodes has been achieved using a thin interlayer of the Rh101 organic semiconductor; this has been ascribed to the fact that the Rh101 interlayer increases the effective barrier height by influencing the space charge region of Si.

Field effect transistor and manufacturing method thereof

Abstract: A field effect transistor includes a first semiconductor region forming a channel region, a gate electrode insulatively disposed above the first semiconductor region, source and drain electrodes formed to sandwich the first semiconductor region in a channel lengthwise direction, and second semiconductor regions formed between the first semiconductor region and the source and drain electrodes and having impurity concentration higher than the first semiconductor region. The thickness of the second semiconductor region in the channel lengthwise direction is set to a value equal to or less than depletion layer width determined by the impurity concentration so that the second semiconductor region is depleted in a no-voltage application state.

ZnO tetrapod Schottky photodiodes

Abstract: The fabrication of an ultraviolet photodiode employing a single ZnO tetrapod nanocrystal is reported. This diode structure is prepared by depositing W and Pt electrodes to form Ohmic and Schottky contacts, respectively. Dark current-voltage measurements show rectifying behavior. The properties of the metal-semiconductor interface are studied with above and below band gap illumination. It is found that with increasing UV excitation the device converts from a rectifying to an Ohmic behavior. This effect is attributed to a flattening of the energy bands due to the migration of photogenerated carriers within the space charge region at the metal-semiconductor interface.

Characterization of Zn-doped hematite thin films for photoelectrochemical splitting of water

Abstract: Hydrogen production using the photoelectrochemical (PEC) route promises to be a clean and efficient way of storing solar energy for use in hydrogen-powered fuel cells. Iron oxide (α-Fe 2 O 3 ) is best suited to be used as a photoelectrode in PEC cells for solar hydrogen production due to its favourable bandgap of -2.2 eV, lying in visible region. The present communication describes the PEC study on Zn-doped α-Fe 2 O 3 thin films prepared by spray pyrolytic method at different doping concentrations (0.5, 1.0, 1.5, 5.0 and 10.0 at%). Photocurrent density was found to depend upon the doping concentration of Zn. Maximum photocurrent density of -0.64 mA cm -2 at an applied potential of 0.7 V/SCE was observed for 5.0 at% doping concentration. All the samples of doped/undoped α-Fe 2 O 3 were characterized for phase formation, particle size, nature of charge carrier, bandgap, resistivity and carrier density. The flatband potential of -0.78V/SCE and depletion layer width of 28.7 A were calculated from the Mott-Schottky plot for 5.0 at% doping concentration.

Electron depletion at InAs free surfaces : Doping-induced acceptorlike gap states

Abstract: For almost all $n$-type zinc-blende III-V semiconductor free surfaces a depletion layer is observed InAs is an exception, since $n$-type InAs free surfaces exhibit electron accumulation, due to donorlike surface states which pin the Fermi level far above the conduction band minimum High-resolution electron-energy-loss spectroscopy (HREELS) has been used to investigate the free surfaces of slightly degenerate $(n\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{03em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3})$ and highly degenerate $(n\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{03em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3})$ InAs From HREEL studies of these InAs samples, an electron accumulation and depletion layer were observed, respectively The Fermi-level pinning mechanism at free surfaces is discussed in terms of the position of the bulk Fermi level with respect to the branch-point energy $({E}_{B})$ Depending on the location of the bulk Fermi level, amphoteric defects are incorporated into the surface, which if ionized facilitate the band bending that results in the surface Fermi level becoming pinned close to ${E}_{B}$ Occupied cation-on-anion antisite defects and unoccupied anion-on-cation antisite defects at the surface are considered to be the ionized surface states responsible for the depletion and accumulation layer profiles, respectively

Electrical barriers in the ZnO varistor grain boundaries

Abstract: In this paper a detailed nature of the electrical potential barriers in ZnO based varistors are presented. The excellent nonlinear current–voltage (I–V) characteristic of these varistors is attributed to the potential barriers formed between the successive ZnO grains during the processing cycle. These grain boundaries in the microstructure results in a few device-related parameters such as built-in-potential, barrier height, barrier width, grain carrier density, Fermi level position in the grains, etc. Thermal activation energy is determined from the status of the leakage current of this device. Substantial studies conducted to comprehend possible conduction processes in the grain boundary regions under applied bias (electric field). It is understood that electron transport dominate the grain boundary regions based on electrical potential barrier consideration across the grain boundaries. The free electrons overcome potential barrier at elevated temperatures as well as under bias. These carriers fill trap sites (charged defect states) both in the bulk ZnO and in the grain surfaces (surface states). The formation of space charge region within the successive ZnO grains across the grain boundaries is a result of the equilibrium state of the device. Based on the findings an expression of the Schottky potential barrier width is proposed satisfying relevant expression of the current caused by the application of the electric field. It is observed that the potential barrier width is sharply reduced in the temperature range 320 K ≤ T ≤ 350 K. This reduction in the potential barrier width is likely to be associated with the relaxation process of the trap filled (trapped) carriers, usually monitored by the dielectric response (in the form of differential capacitance) of the device. Thus, the results provide an improved understanding of the nonlinear conduction behavior in ZnO varistors. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

LaMnO3∕SrMnO3 interfaces with coupled charge-spin-orbital modulation

Abstract: The artificial perovskite superlattices composed of LaMnO3 and SrMnO3 have been investigated to elucidate the interface electronic phases created by adjoining the two Mott insulators. Charge transfer at the interface due to chemical potential difference, as observed in p-n junctions of semiconductors, can realize metallic ferromagnet instead of resulting in insulating depletion layer. The interface electronic phases strongly depend on the orbital states at the interface which can be tuned by epitaxial strain.

Transport in carbon nanotube p-i-n diodes

Abstract: Single-walled carbon nanotube diodes are fabricated in a split-gate geometry with electron (n) and hole (p) regions separated by a central region. With the central region gated p or n type the diodes “leak” at low voltages, likely due to tunneling across the smaller depletion region. With the central region intrinsic, nearly ideal diode behavior is observed. Comparison to theory for a one-dimensional diode yields the band gap of the tube and the transmission coefficient through the junction. In reverse bias, the breakdown voltage depends weakly on temperature and nanotube diameter. Comparisons are made to predictions for Zener tunneling and avalanche breakdown.

Modeling of the conduction in a WO3 thin film as ozone sensor

Abstract: In this paper we propose a model for ozone detection in atmospheric conditions. The sensitive layer material used in this study is tungsten oxide. The interaction between the semiconductor surface and the gases is approached by means of the adsorption theory described by Wolkenstein in order to determine the equilibrium state of the grains. The layer conductivity is then determined by computing the current flowing between the grains (in the spherical assumption) across the depletion layer induced by the adsorbed molecules and the semiconductor interaction. This calculation is performed using the “drift diffusion” equation set. We have first analyzed the oxygen adsorption effect, then the ozone adsorption one and finally, the combined action of the two mixed gases on the sensor layer. This model takes into account the fundamental mechanisms implied in the gas detection and the results obtained are in good agreement with the experimental results.

Hole current impedance and electron current enhancement by back-contact barriers in CdTe thin film solar cells

Abstract: The combined effects of a significant back-contact barrier and a low absorber carrier density frequently alter the current-voltage (J-V) characteristics of CdTe solar cells. This combination leads to two competing mechanisms that can alter the J-V characteristics in two different ways. One is a majority-carrier (hole) limitation on current in forward bias that reduces the fill factor and efficiency of the solar cell. The second is a high minority-carrier (electron) contribution to the forward diode current that results in a reduced open-circuit voltage. CdTe solar cells are particularly prone to the latter, since the combination of a wide depletion region and impedance of light-generated holes at the back contact increases the electron injection at the front diode. The overlap of front and back space-charge regions will generally enhance the electron current, but is not a requirement for substantially increased forward current. The simulated J-V curves, illustrating the two major effects, are in good agre...

No intrinsic depletion layer on a polystyrene thin film at a water interface.

Abstract: Using neutron reflectivity, we found that there is no intrinsic depletion layer at a deuterated polystyrene (dPS) film and deuterium oxide (D2O) interface. A spun-cast film is susceptible to contamination on its surface from its surroundings during sample preparation. A contamination layer of hydrogenated organic material will be detected as a reduced scattering length density layer at the interface. We demonstrate that, by careful treatment of the film, contamination would be the primary cause of the reduced scattering length density layer at the interface.

Application of junction capacitance measurements to the characterization of solar cells

Abstract: The quasi-static capacitance-voltage ( C-V) technique measures the dependence of junction capacitance on the bias voltage by applying a slow, reverse-bias voltage ramp to the solar cell in the dark, using simple circuitry. The resulting C-V curves contain information on the junction area and base dopant concentration, as well as their built-in potential. However, in the case of solar cells made on low to medium resistivity substrates and having thick emitters, the emitter dopant profile has to be taken into account. A simple method can then be used to model the complete C-V curves, which, if the base doping is known, permits one to estimate the emitter doping profile. To illustrate the method experimentally, several silicon solar cells with different base resistivities have been measured. They comprise a wide range of areas, surface faceting conditions and emitter doping profiles. The analysis of the quasi-static capacitance characteristics of the flat surface cells resulted in good agreement with independent data for the wafer resistivity and the emitter doping profile. The capacitance in the case of textured surfaces is a function of the effective junction area, which is otherwise difficult to measure, and is essential to understand the emitter and space charge region recombination currents. The results indicate that the effective area of the junction is not as large as the area of the textured surface.

Study of hole concentration of 1,4-bis[N-(1-naphthyl)-N-'-phenylamino]-4,4(') diamine doped with tungsten oxide by admittance spectroscopy

Abstract: The effect of tungsten oxide (WO3) incorporation into 1,4-bis[N-(1-naphthyl)-N′-phenylamino]-4,4′ diamine (NPB) layer is investigated in NPB-tris(8-hydroxyquinoline)aluminium heterojunction organic light-emitting diodes. The admittance spectroscopy studies show that increasing the WO3 volume percentage from 0% to 16% can increase the hole concentration of the NBP layer from 1.97×1014to1.90×1017cm−3 and decrease the activation energy of the resistance of the NPB layer from 0.354to0.176eV. Thus, this incorporation reduces the Ohmic loss and increases the band bending in the NBP layer near the interface, resulting in an improved hole injection via tunneling through a narrow depletion region.

Parameter determination from resistance-voltage curve for long-wavelength HgCdTe photodiode

Abstract: A data-processing approach has been developed to obtain device parameters from resistance-voltage (R-V) curves measured on long-wavelength HgCdTe n-on-p photodiodes. The physical model used for R-V curve fitting includes the dark current mechanisms induced by diffusion, generation recombination, trap-assisted tunneling, and band-to-band tunneling. Moreover, the series resistance effect is also taken into account. Six parameters, which include the dopant density Nd in the n region, the ratio of mobility to lifetime of electrons μn∕τn in the p region, the effective lifetime τ0 in the depletion region, the relative energy position of trap level Et∕Eg and its density Nt in the depletion region, and the series resistance Rs, can be extracted from measured R-V curves. The fitting procedure has been presented in detail and the error ranges of the extracted parameters have been discussed. By fitting to the R-V characteristics of three long-wavelength devices with different Cd compositions, the applicability of ou...

A subthreshold surface potential model for short-channel MOSFET taking into account the varying depth of channel depletion layer due to source and drain junctions

Abstract: An analytical subthreshold surface potential model for short-channel MOSFET is presented. In this model, the effect of varying depth of the channel depletion layer on the surface potential has been considered. The effect of the depletion layers around the source and drain junctions on the surface potential, which is very important for short channel devices is included in this model. With this, the drawback of the existing models that assume a constant channel depletion layer thickness is removed resulting in a more accurate prediction of the surface potential. A pseudo-two-dimensional method is adopted to retain the accuracy of two-dimensional analysis yet resulting in a simpler manageable one-dimensional analytical expression. The subthreshold drain current is also evaluated utilizing this surface potential model.

A closed-form model of the drain-voltage dependence of the off-state channel electric field in a HEMT with a field plate

Abstract: The channel-field distribution under the field plate in a high-electron mobility transistor (HEMT) in the off-state is modeled in terms of drain-voltage and physical parameters. Depending upon the drain-voltage and device structure, this distribution can have up to three peaks-one each at the two ends of the field plate and at the drain. It is shown that the complete distribution can be approximated as superposition of triangular distributions, which are analogous to that in the depletion layer of a p-n junction; consequently, the peaks increase as square root of the drain-voltage. The model fits into two-dimensional simulation results and allows estimation of the minimum drain-gate separation, the electric-field reduction, the breakdown-voltage improvement, and critical field for the onset of a parasitic phenomenon in a HEMT with a field plate

Probing semiconductor gap states with resonant tunneling.

Abstract: Tunneling transport through the depletion layer under a GaAs ${110}$ surface is studied with a low temperature scanning tunneling microscope (STM). The observed negative differential conductivity is due to a resonant enhancement of the tunneling probability through the depletion layer mediated by individual shallow acceptors. The STM experiment probes, for appropriate bias voltages, evanescent states in the GaAs band gap. Energetically and spatially resolved spectra show that the pronounced anisotropic contrast pattern of shallow acceptors occurs exclusively for this specific transport channel. Our findings suggest that the complex band structure causes the observed anisotropies connected with the zinc blende symmetry.

Potential profiling of the nanometer-scale charge-depletion layer in n-ZnO∕p-NiO junction using photoemission spectroscopy

Abstract: The authors have performed a depth-profile analysis of an all-oxide p-n junction diode n-ZnO∕p-NiO using photoemission spectroscopy combined with Ar-ion sputtering. Systematic core-level shifts were observed during the gradual removal of the ZnO overlayer, and were interpreted using a model based on charge conservation. Spatial profile of the potential around the interface was deduced, including the charge-depletion width of 2.3nm extending on the ZnO side and the built-in potential of 0.54eV.