Forward Bias Conditions Research Articles

Investigation of electrical conductivity in Schottky-barrier devices based on nickel phthalocyanine thin films

Evaporated multilayer sandwich structure of Au/NiPc/Al was fabricated by thermal evaporation technique. The electrical conductivity of Au/NiPc/Al device has been measured both as prepared and after annealing at 623 K for 2 h. Under forward bias conditions, ohmic and SCLC conductions were identified at low and higher voltages respectively. After annealing, a strong rectifying effect was observed. The potential barrier height and the depletion region width for annealed sample were calculated as 0.96 eV and 70 nm, respectively. Hole and trap parameters for as prepared and after annealing devices, also, were evaluated.

Carrier capture and escape processes in (In,Ga)N single‐quantum‐well diode under forward bias condition by photoluminescence spectroscopy

AbstractCarrier capture and escape processes in the super‐bright green (In,Ga)N single‐quantum‐well (SQW) light‐emitting diode (LED) has been studied by photoluminescence (PL) spectroscopy under reverse and forward bias conditions. The PL spectra were measured at 20 K under excitation photon energies above and below the bandgap energy of GaN barrier layers. The PL spectra under both excitation conditions show green emission from the (In,Ga)N SQW layer. The wavelength‐integrated PL intensity changes drastically depending on the applied bias voltage. For the excitation below the bandgap energy of GaN (direct excitation), the PL intensity increases with increasing the forward bias voltage up to +2 V and significant reduction of the PL intensity is observed with further increase of the forward bias voltage. On the other hand, for the excitation above the bandgap energy of GaN (indirect excitation), the PL intensity rapidly increases up to +2 V, decreases once, increases again with the maximum value at +3.25 V, and drastically decreases again. These differences of the PL intensity variation reflect carrier escape and capture processes. That is, for the direct excitation condition, the PL intensity variation indicates the effect of the electric field on the radiative recombination and the carrier escape processes. In contrast, for the indirect excitation condition, it indicates the carrier transfer and capture processes. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron Resonant Tunneling Through InAs ∕ GaAs Quantum Dots Embedded in a Schottky Diode with an AlAs Insertion Layer

Molecular beam epitaxy was employed to manufacture self-assembled quantum dot Schottky resonant tunneling diodes. By virtue of a thin insertion barrier, the thermal current was effectively reduced and electron resonant tunneling through quantum dots under both forward and reverse biased conditions was observed at relatively high temperature of . The ground states of quantum dots were found to be at below the conduction band of matrix. The theoretical computations were in conformity with experimental data.

A-SiC:H/a-Si:H tandem structure analysis for RGB color recognition in LSP devices

A device structure based on a a-SiC:H/a-Si:H pin/pin tandem structure is proposed for a proper color separation process that takes advantage of the different light spectrum filtering properties of the a-Si:H and a-SiC:H absorbers. We have analyzed a tandem pin–pin device with the following structure deposited using the plasma enhanced chemical vapor deposition (PECVD) technique: ITO/p-type a-SiC:H (20nm)/a-SiC:H (200nm)/n-type a-SiC:H (20nm)/p-type a-SiC:H (20nm)/a-Si:H (1500nm)/n-type a-Si:H (50nm)/ITO. Carbon concentration in the top a-SiC:H cell produces an optical gap of about 2.0eV. A numerical simulation of the internal electrical configuration of the device under different wavelength radiation supports our analysis of the device operation. The bias dependence of the device output is explained by taking into account, from the point of view of the color sensor applications, considerations about an asymmetric reaction of the internal electric fields to the externally imposed forward bias and a self-biasing effect of the sub-cells under certain unbalanced light generation of carriers. The simulation shows that this structure permits a good recognition of blue color under reverse bias and red color under forward bias conditions. The acquisition of a satisfactory RGB image mapping by controlling the applied bias is possible but remains problematic due to the poor separation of the green component at any value of the applied bias. A simple algorithm is proposed to deduce the green component of the light by combining the previously acquired information about the total intensity of the incident radiation and about the blue and red components.

Organic Multifunction Diodes Operable for Emission and Photodetection Modes

We have proposed multifunction diodes (MFDs) with both emission and photodetection functions. The basic structure is similar to a conventional organic light emitting diode. To fabricate MFDs, a special organic material of pyrazoline derivative was used. For organic light emitting operation, the highest luminance was 9,270 cd/m2 under forward bias condition. For organic photodiode operation, the photocurrent density was 71 µA/cm2 and the photoconductivity to dark conductivity was 104 under reverse bias condition and xenon lamp irradiation. A prototype 4×4 matrix was demonstrated and we achieved a clear emission pattern of the organic light-emitting diodes and also a clear photodetection of organic photodiode.

Band-edge electroluminescence from N+-implanted bulk ZnO

N + ion implantation at moderate doses (1013–1014cm−2) into nominally undoped (n∼1017cm−3) bulk single-crystal ZnO substrates followed by annealing in the range 600–950°C was used to fabricate diodes that show visible luminescence at 300K and band-edge electroluminescence at 120K (∼390nm) under forward bias conditions. The current-voltage behavior of the diodes are characteristic of metal-insulator-semiconductor devices and suggest the implantation creates a more resistive region in the n-ZnO in which holes are created by impact ionization during biasing, similar to the case of electroluminescence in ZnO varistors. The series resistance is only 25Ω due to the use of the conducting ZnO substrate.

Interplay of external and internal field effects on radiative recombination efficiency in InGaN quantum well diodes

Electroluminescence (EL) and photoluminescence (PL) properties have been investigated of the high-brightness green InGaN single quantum well (SQW) diode over a wide temperature range (T = 15–300 K) and as a function of injection current level. When the necessary forward bias conditions to get a certain current level are different, it is found that the anomalous temperature-dependent EL efficiency varies quite differently. That is, when the current is low and thus the forward driving voltage is small, the EL quenching observed below 100 K for high injection current levels is less significant or even absent due to the efficient carrier capture. This finding is consistent with decrease of the PL efficiency with increasing the bias over +2.5 V. These results indicate that the EL efficiency is significantly influenced by the interplay of internal and external field effects on the carrier capture and escape processes in addition to the localization phenomena caused by In fluctuations in the SQW layer. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Nonequilibrium radiation of long-wavelength InAs∕GaSb superlattice photodiodes

The emission behavior of binary-binary type-II InAs∕GaSb superlattice photodiodes has been studied in the spectral range between 8 and 13μm. With a radiometric calibration of the experimental setup the internal and external quantum efficiencies have been determined in the temperature range between 80 and 300K for both the negative and positive luminescences. The negative luminescence efficiency approaches values as high as 35% without antireflection coating. An analytic description of the temperature dependence of the internal quantum efficiency around a zero-bias voltage allows for the determination of the coefficient for electron-hole-electron Auger recombination Γn=1×1024cm6s−1. For an n-type material, the minority-carrier lifetime is provided as a function of band gap and temperature, explaining the strong decrease of the minority-carrier lifetime in the case of an n-type residual background exceeding 1×1016cm−3. Furthermore, an analytic expression of the quantum efficiency for the radiation upon forward-bias conditions is given.

Photoluminescence from n-type porous silicon layer enhanced by a forward-biased np-junction

A new approach for the fabrication of n-type porous silicon layer is proposed. A hole-rich p-layer is arranged underneath the n-layer, and the np-junction is under forward biased condition in the etching process. Therefore sufficient holes can drift straight-upward and pass across the np-junction from p-region to n-region to participate in electrochemical reaction during the etching process with an unfailing supply. Illumination is an optional hole-supplier in this approach, so the problem of illumination-depth limitation can be overcome. Strong visible photoluminescence emissions are demonstrated on the hole-poor n-type porous layer at about 650 nm.

On the Hydrogen Sensing Properties of a Pt-Oxide-In[sub 0.5]Al[sub 0.5]P Schottky Diode

A new Pt-InAlP metal oxide semiconductor Schottky diode hydrogen sensor with highly sensitive hydrogen detection among wide operating temperature regime is demonstrated and studied. Experimentally, the studied hydrogen sensor can be operated systematically either under forward or reverse bias conditions. At 30°C, the relatively hydrogen detection ratio S r value is increased from 108.8% (107.4%) to 943.1% (1337.3%), under the forward (reverse) bias of 0.3 V, when the hydrogen concentration is increased from 4.3 to 9970 ppm H 2 /air. Note that even an extremely low hydrogen concentration of 4.3 ppm H 2 /air can be effectively detected at the temperature of 30-250°C.

Electrical Properties of Anodically Oxidized Nb2O5 and Si-Doped Nb2O5 Films

The basic electrical properties of Nb2O5 and Si-doped Nb2O5 (Nb2O5–Si) thin films, which were obtained by anodically oxidizing Nb in an H3PO4 solution, were investigated using their current–voltage (I–V) and capacitance–voltage (C–V) characteristics with the aim of determining their electrical conduction mechanism. In the forward bias condition, Poole–Frenkel conduction is plausible for both Nb2O5 and Nb2O5–Si (Si-doping less than 19 at. %). For the reverse bias condition, a Schottky-barrier is formed at the Au/Nb2O5 and/or Au/Nb2O5–Si interface (Si-doping less than 5 at. %) because of the difference in the work function. The low frequency capacitance (1 kHz) increased with forward bias due to the lifetime of the carriers (electrons, holes, or both) injected into the Nb2O5 and/or Nb2O5–Si films. A schematic energy band diagram for Au/Nb2O5/Nb and/or Au/Nb2O5–Si/Nb–Si (Si doping less than 5 at. %) capacitor structures is hypothesized in order to explain the I–V and C–V characteristics.

Imaging of thickness and compositional fluctuations in InGaN∕GaN quantum wells by scanning capacitance microscopy

We have used scanning capacitance microscopy (SCM) and atomic force microscopy (AFM) to characterize structural and electronic properties of InxGa1−xN∕GaN quantum-well structures at the nanoscale. Macroscopic capacitance-voltage measurements combined with numerical simulations indicate that either electron or hole accumulation in the quantum-well layer can be induced by application of forward or reverse bias, respectively. Under reverse bias conditions (hole accumulation), features corresponding to monolayer fluctuations in InxGa1−xN quantum-well thickness are clearly evident. Under forward bias conditions (electron accumulation), samples exhibiting high luminescence efficiencies are found to contain regions of increased carrier accumulation within the quantum well, which on the basis of spatially resolved spectroscopy, bias-dependent imaging, and numerical simulations are attributed to nanoscale In-rich clusters in the quantum well. No such features are observed in samples exhibiting lower luminescence efficiencies. Together, these studies demonstrate the ability to image, and distinguish, nanoscale variations in subsurface electronic properties arising from either monolayer thickness fluctuations or compositional inhomogeneities in InxGa1−xN∕GaN quantum-well structures.

Wavelength Shift of Gallium Nitride Light Emitting Diode With p-Down Structure

Both the p-gallium nitride (GaN) on an n-light-emitting diode (LED) structure and the n-GaN on p-LED structure were fabricated by metal-organic vapor phase epitaxy on c axis sapphire substrate. The output spectra with different applied bias voltages were studied on these structures. The small amount of the wavelength shift under a forward bias condition in the proposed p-down LED structure was observed but not in the n-down LED structure. This phenomenon was characterized by the effective electric field in the well created from the built-in electric field and the applied electric field. This proposed p-down LED shows an opportunity in the application of wavelength shift-free optoelectronic devices.

Generation of short electrical pulses based on bipolar transistorsny

Abstract. A system for the generation of short electrical pulses based on the minority carrier charge storage and the step recovery effect of bipolar transistors is presented. Electrical pulses of about 90 ps up to 800 ps duration are generated with a maximum amplitude of approximately 7V at 50Ω. The bipolar transistor is driven into saturation and the base-collector and base-emitter junctions become forward biased. The resulting fast switch-off edge of the transistor’s output signal is the basis for the pulse generation. The fast switching of the transistor occurs as a result of the minority carriers that have been injected and stored across the base-collector junction under forward bias conditions. If the saturated transistor is suddenly reverse biased the pn-junction will appear as a low impedance until the stored charge is depleted. Then the impedance will suddenly increase to its normal high value and the flow of current through the junction will turn to zero, abruptly. A differentiation of the output signal of the transistor results in two short pulses with opposite polarities. The differentiating circuit is implemented by a transmission line network, which mainly acts as a high pass filter. Both the transistor technology (pnp or npn) and the phase of the transfer function of the differentating circuit influence the polarity of the output pulses. The pulse duration depends on the transistor parameters as well as on the transfer function of the pulse shaping network. This way of generating short electrical pulses is a new alternative for conventional comb generators based on steprecovery diodes (SRD). Due to the three-terminal structure of the transistor the isolation problem between the input and the output signal of the transistor network is drastically simplified. Furthermore the transistor is an active element in contrast to a SRD, so that its current gain can be used to minimize the power of the driving signal.

Application of the thermionic field emission model in the study of a Schottky barrier of Ni on p-GaN from current–voltage measurements

Barrier height values of Ni contacts to Mg-doped p-type GaN (p-GaN) were obtained from current–voltage measurements in this study. The induced deep level defect band through high Mg doping led to a reduction of the depletion layer width in the p-GaN near the interface and an increase in the probability of thermionic field emission. It also resulted in an increase in current flow under forward bias condition, which was not analyzed using the thermionic emission model. Further, the calculated barrier height value of Ni contacts to p-GaN using the thermionic field emission model is in good agreement with the value of 1.9eV obtained from x-ray photoelectron spectroscopy measurements.

A direct parameter-extraction method for GaInP/GaAs heterojunction bipolar transistors small-signal model

An accurate and broad-band method for hetero junction bipolar transistors (HBT) small-signal model parameters-extraction is presented in this paper. An equivalent circuit for the HBT under a forward bias condition is proposed for extraction of access resistance and parasitic inductance. This method differs from previous ones by extracting the equivalent circuit parameters without using special test structure or global numerical optimization techniques. The main advantage of this method is that a unique and physically meaningful set of intrinsic parameters is extracted from im pedance and admittance representation of the measuredS-parameters in the frequency range of 1–12 GHz under different bias conditions. The method yields a deviation of less than 5% between measured and modeledS-parameters.

High efficiency visible electroluminescence from silicon nanocrystals embedded in silicon nitride using a transparent doping layer

We have fabricated light-emitting diodes with a transparent doping layer on silicon nanocrystals (nc-Si) embeded in silicon nitride matrix formed by plasma-enhanced chemical vapor deposition. Under forward biased condition, orange electroluminescence (EL) with its peak wavelength at about 600 nm was observed at room temperature. The peak position of the EL is very similar to that of the photoluminescence (PL) and the emitted EL intensity is proportional to the current density passing through the device. We suggest that the observed EL is originated from electron-hole pair recombination in nc-Si. By using indium tin oxide and n-type SiC layer combination as a transparent doping layer, we obtained high external quantum efficiency greater than 1.6%.

Visible and Infrared Emission from Er-doped III-N Light Emitting Diodes

AbstractWe report on the visible and infrared emission characteristics of Er-doped III-N lightemitting diodes (LEDs). Quantum well-like device structures were grown through a combination of metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) on cplane sapphire substrates. The dual stage growth process was used to take advantage of the high quality of AlGaN layers produced by MOCVD and in situ doping of Er during MBE growth. The multilayer structures were processed into devices and LEDs with different sizes and geometric shapes were produced. Electroluminescence (EL) was observed under either forward or reverse bias conditions. Visible and infrared spectra displayed narrow emission lines representative of the Er3+ system. The temperature dependence of the spectra, which were measured from 100K to 300K, showed a stability in the visible emission intensity but a sharp decrease in the infrared intensity at room temperature. Based on light output vs current measurements, estimates of the excitation cross-section were obtained for visible EL emission.

Transient and ac electrical transport under forward and reverse bias conditions in aluminum/porous silicon∕p-cSi structures

Dielectric impedance measurements as well as transient current–voltage (I–V) characteristics under conditions of forward and reverse bias are reported in aluminum/porous silicon (PS)∕p-cSi structures at different temperatures. Under reverse bias conditions, the electrical conduction of the structures can be modeled by a simple equivalent circuit of two parallel RC networks in series combination, representing a bulk and a junction region. The bulk conduction is ohmic. From the detailed analysis of the PS/cSi junction, the carrier concentration NA and the depletion layer width W are evaluated at each temperature. The elapsed time t0 from the onset of the square pulse, at which the transition from the dominant bulk resistance to junction conduction takes place, is a function of temperature. As the temperature decreases, conduction due to bulk resistance dominates over junction conduction. On the other, hand forward conduction is dominated from the bulk. Two conduction mechanisms are present. As the time proceeds, Poole–Frenkel conduction follows an ohmic conduction. The time after the onset of the square voltage pulse, at which this turnover takes place, is a function of temperature.

Quantum modelling of I–V characteristics for 4H–SiC Schottky barrier diodes

The prediction of reverse bias I–V characteristics in 4H–SiC Schottky barrier diodes over a wide range of temperatures and bias is not possible using established modelling techniques. This paper reports on the development of an established general model for both reverse and forward characteristics that is applied to 4H–SiC Schottky barrier diodes. This model is based on the self-consistent evaluation of the quantum-mechanical probabilities of carriers to traverse the metal–semiconductor barrier. These are integrated over all possible energies to describe the transport process not only at energies below the barrier but also at energies equal and superior to the barrier height. In this manner the model naturally covers the entire range of the traditional theories of tunnelling, thermionic-field and thermionic emission. The model relies on a single set of parameters, extracted from the experimental forward characteristics of each device using a modified Norde method, to predict both forward and reverse characteristics. The model is compared to measured I–V characteristics, in both forward and reverse bias conditions, for 4H–SiC Schottky barrier diodes fabricated using two different edge termination technologies. Comparisons with models previously described in the literature show a superior fit to experimental data, without the need for additional fitting parameters.