Forward-biased Diode Research Articles

Analytical derivation of and simulation on current voltage characteristics equation of a solar cell

A quantitative approach was employed with the aim of reviewing and elucidating the development of Current Density-Voltage (J-V) characteristics of solar cell with a single dominant current transport from transport and continuity equation involving drift and diffusion terms with the application of Fermi-Dirac statistical distribution. A number of assumptions were made on unidirectional flow of minority- current carrier outside and within the depletion layer. The simulations of saturation current density with forward bias voltage and diode factors show similar curves but with increase in total current densities with a lower diode factor. Temperature increment was however found not to favour the increase of the current density. Suggestions were also given on experimental works on the photovoltaic cell based on the simulations. Keywords: current density, recombination, diffusion, concentration Nigeria Journal of Pure and Applied Physics Vol. 4(1) 2005: 54-59

Large-signal diode modeling-an alternative parameter-extraction technique

An alternative numerical optimization method of large-signal equivalent-circuit diode modeling using dc and small-signal S-parameter measurements is described. In general, there are two ways to extract the equivalent-circuit parameters to model a nonlinear device such as a diode. They are based on numerical optimization or noniterative analytical procedure. The former is usually better in approximating the device response. Nevertheless, it requires arbitrary selection of a voltage-dependent S-parameter set to obtain the voltage-independent parameters. In this alternative numerical optimization method, an arbitrary selection of a voltage-dependent S-parameter set to obtain the voltage-independent parameters is not required. Validation of this parameter-extraction technique is done via modeling a forward-biased tunnel diode and a reverse-biased varactor diode. The models are further verified by implementing them in designing and developing an oscillator and a voltage-controlled oscillator.

Observation of recombination enhanced defect annealing in 4H–SiC

We report observation of recombination enhanced defect annealing in 4H–SiC detected by capacitance transient spectroscopy and low temperature photoluminescence (PL). Intrinsic defect centers, created by 160keV electron irradiation, reduce in concentration after illumination at temperatures much lower than previously reported annealing temperatures of 400 and 800°C. The effect is observed after both external intense above band gap laser excitation, and with recombination in a forward biased pin diode. PL measurements show that several lines, normally detected after electron irradiation, have almost or entirely disappeared by recombination enhanced annealing at room temperature. From capacitance transient measurements, the annealing enhancement is found to be largest for the HS2 hole trap, while the EH1 and EH3 electron traps also anneal out by recombination enhanced reaction but at a lower rate.

Spin accumulation in forward-biased MnAs/GaAs Schottky diodes.

We describe a new means for all-electrical generation of spin polarization in semiconductors. In contrast with spin injection of electrons by tunneling through a reverse-biased Schottky barrier, we observe accumulation at the metal-semiconductor interface of forward-biased ferromagnetic Schottky diodes, which is consistent with a theory of spin-dependent reflection off the interface. Spatiotemporal Kerr microscopy is used to image the electron spin and the resulting dynamic nuclear polarization that arises from the nonequilibrium carrier polarization.

TEM of Dislocations in Forward-Biased 4H-SiC PiN Diodes

TEM of Dislocations in Forward-Biased 4H-SiC PiN Diodes

Investigations into the source of 1/f noise in HgxCd1−xTe diodes

This work is aimed at elucidating the mechanism of 1/f noise in HgxCd1−xTe diodes in order to reduce 1/f noise in high-performance thermal cameras. Currently, to reach the required signal-to-noise ratio, the operating temperature of thermal cameras is 77 K. If 1/f noise could be reduced, this operating temperature could be raised without reducing the signal-to-noise ratio, allowing cheaper, smaller, and lighter cooling systems to be used. There are still some fundamental aspects of 1/f noise that are not fully understood. One aspect in particular is whether the noise is caused by mobility or number density fluctuations because both could give the conductivity fluctuations observed. This paper explores the noise coupling and current coupling between diodes on an array. It is shown that 1/f noise is present on the photons emitted from a forward-biased diode, suggesting that the rate of radiative recombination fluctuates. The results suggest that these fluctuations are due to carrier number-density fluctuations. From correlation measurements, it is shown that at least 32% of the 1/f noise in the forward-biased diode is correlated with carrier density fluctuations in HgxCd1−xTe diodes.

Verifying the diode–capacitor circuit voltage decay

The voltage on a capacitor discharging through a forward biased diode is calculated from basic equations and is found to be in good agreement with experimental measurements. In contrast to the exponential time decay for a RC circuit, the nonlinear characteristics of the diode result in a nonexponential decay for the diode–capacitor circuit. For a silicon diode the decay is predominantly a logarithmic function of time.

Spin-dependent recombination electron paramagnetic resonance spectroscopy of defects in irradiated silicon detectors

Spin-dependent recombination (SDR) electron paramagnetic resonance (EPR) spectroscopy is applied for investigation of paramagnetic recombination centers in irradiated silicon p−n junction detectors (diodes) formed on float-zone (FZ) silicon wafers. The main radiation defects, associated with SRD-EPR spectra arising from excited triplet states, are assigned to complexes of two substitutional carbon atoms and one interstitial silicon atom (CS+SiI+CS) and to oxygen + vacancy (O+V) complexes (A-centers). In spite of the low concentration of oxygen in FZ silicon the A-centers are found to play an important role in the recombination process in the diodes. At temperatures T<100 K the SDR-EPR spectra are well observable by measurements of the microwave conductivity or by detecting a dc forward current IF below a forward-blocking voltage UFBL. At UFBL the IF has a steep jump followed by a decrease of the voltage over the diode and a negative resistance region with oscillations of the current. The SDR-EPR spectrum arising from the (CS+SiI+CS) complexes was found to decrease strongly in the forward biased diodes. This effect can be attributed to coexistence of two different configurations of the carbon related defect in the bulk n-type region of the diodes.

Study of the potential distribution in a forward-biased silicon diode using electrostatic force microscopy

Electrostatic force microscopy was used to study the potential distribution in a forward-biased epitaxial-diffused n +-n-p-p + silicon diode. Distributions of potential and capacitance were determined across the cleaved surface, which intersected the layers in the diode structure. Variations in the surface potential and capacitance were preliminarily measured with a submicrometer spatial resolution and were used to determine the position and width of the n-p junction; the distribution of applied forward bias in the diode was also assessed. It is shown that an additional potential barrier for injected charge carriers may exist in the vicinity of the n +-n junction in the diode under consideration. For an injection-current density exceeding 100 mA/cm2, the voltage drop across this barrier becomes comparable with the voltage variations across the operating n-p junction.

Recombination-enhanced extension of stacking faults in 4H-SiC p-i-n diodes under forward bias

The extension of stacking faults in a forward-biased 4H-SiC PiN diodes by the recombination-enhanced motion of leading partial dislocations has been investigated by the technique of optical emission microscopy. From the temperature dependence of the measured velocity of the partials, an activation energy of 0.27 eV is obtained. Based on this and analysis of the emission spectra, a radiative recombination level of 2.8 eV for the stacking fault, and two energy levels for the partial dislocation, a radiative one at 1.8 eV and a nonradiative at 2.2 eV, have been determined.

Electroluminescence from a forward-biased Schottky barrier diode on modulation Si δ-doped GaAs/InGaAs/AlGaAs heterostructure

Electroluminescence (EL) from a forward-biased Schottky barrier diode on modulation Si δ-doped pseudomorphic GaAs/InGaAs/AlGaAs heterostructure with high mobility electron gas is investigated in this work. It has been found that the EL from the InGaAs quantum well can be observed at temperatures up to 90 K. The EL line shape depends on the current density, which reflects the filling of the InGaAs channel with electrons. The total integrated EL intensity depends linearly on the current density. We propose that hole diffusion from an inversion layer at the Schottky barrier is responsible for the observed optical recombination with electrons in the InGaAs quantum well.

Impedance spectroscopic analysis of forward biased metal oxide semiconductor tunnel diodes (MOSTD)

We present impedance spectroscopic analyses made on forward biased minority carrier type metal oxide semiconductor tunnel diodes (MOSTD). Bias dependence of the spectra, i.e. the real and imaginary parts and the Cole–Cole plots, shows the transition of characteristic minority carrier dynamics. At moderate bias levels, interface recombination dominates and the spectra follow the Debye type model with a single time constant determined primarily by the tunneling resistance and by the oxide capacitance. At higher bias levels, a high level of minority carrier injection leads to conductivity modulation in the bulk and the spectra deviate significantly from the prototype model as indicated by an anomalous inductive reactance. The results point to the important role of interfacial ultrathin oxides, i.e. increasing the minority carrier injection ratio, in Schottky diodes.

Optimisation of Er centers in Si for reverse biased light emitting diodes

Depending on the doping and annealing conditions, as well as the oxygen content, erbium produces a big variety of different centres in silicon. These centres can be distinguished by their fine structure patterns seen both in luminescence and in excitation spectroscopy. Centres with a well-defined geometry of their constituents exhibit very sharp line spectra. Such centres can be excited via recombination of electron-hole pairs, produced in a forward biased diode, with surprisingly high quantum efficiency below 100 K — but they are strongly quenched at higher temperatures. Another type of spectrum — with a much higher line width (∼20 nm) and weak thermal quenching up to 370 K is obtained after annealing at temperatures above 950°C. This type of spectrum is identified as being due to Er in SiO 2− δ precipitates. Such centres have much smaller quantum efficiencies for forward bias excitation than for reverse bias. We discuss properties that are important for the realisation of LED structures based on this type of centres for room temperature (RT) operation.

Electrical characterization and carrier transport mechanisms of GaAs p/i/n devices for photovoltaic applications

GaAs p/i/n diodes made by Metal-Organic Vapour Phase Epitaxy were examined by electrical measurements for evaluating the optimum i-region for use as solar cells. Four series of samples were prepared and studied each one with a different i-region width. The performance of the devices was examined by means of Admittance spectroscopy as well as classical current–voltage and capacitance–voltage characterization, allowing the calculation of the minority carriers lifetime ( τ eff) and the diodes ideality factors. The values of the τ eff were found to lie between 8.7 ps and 0.14 ns for i-region widths between 0 and 0.8 μm. These results were used to model the multilayer structure with the two-diode representation and explain the conductance mechanisms inside the diodes. This modeling showed that the recombination/generation currents were dominating in forward biased diodes and the ohmic loss current in reverse bias.

Temperature-dependence of steady-state characteristics of SCR-type ESD protection circuits

Experimental analysis of the temperature-dependent I– V characteristics of various SCR (Silicon-Controlled Rectifier) electrostatic discharges (ESD) protection circuits have been carried out. These circuits include diode-chain-triggering SCR (DCTSCR), low-voltage zener diode trigger SCR (ZDSCR), low-voltage trigger SCR (LVTSCR) and gate-coupled low-voltage trigger SCR (GCSCR) circuits. The ZDSCR uses the zener breakdown mechanism of a reverse-biased p +–n + diode as a trigger mechanism, the DCTSCR uses the current flowing through forward-biased diode chain as a trigger mechanism, the LVTSCR uses the grounded-gate MOSFET breakdown current as the trigger mechanism and the steady-state I– V characteristics of GCSCR also uses the avalanche breakdown as a triggering mechanism. The trigger voltage can decrease or increase with increasing temperature depending upon the triggering mechanism used in the circuit, however the holding voltages of these SCRs decrease with increasing temperature.

Near-infrared to visible up-conversion in a forward-biased Schottky diode with a p-doped channel

Near-infrared radiation of wavelength 1.5 μm is up-converted to a visible wavelength of 818 nm by internal photoemission in a Schottky diode with a modulation p-doped channel. The near-infrared light incident upon the metal–semiconductor interface excites electrons from the metal into the semiconductor. The electrons then drift into the quantum well where they recombine radiatively, producing luminescence at the shorter wavelength of 818 nm. The intensity of the luminescence is strongly dependent on bias, and turns on at a forward bias of −0.7 V.

The origin of current gain under illumination in amorphous silicon n-i-p-i-n structures

Detailed computer modeling in conjunction with experiments has been used to explain the current gain sometimes observed in amorphous silicon n-i-p-i-n structures under illumination and to specify under what conditions such a gain is possible. Calculations reveal that the origin of the current gain at a given applied voltage, is excess injection of electrons from the contacts as a result of the lowering of the i-layer/n-layer barrier in the forward biased diode on account of photogenerated hole trapping. However to actually obtain this gain, the thickness and the density of states of the partitioning p layer must be small enough, so that all the excess injected electrons do not recombine in the p layer before being collected in the reverse biased diode. We also find that under conditions for which the gain varies linearly with voltage, it is almost independent of the incident light flux. On the other hand, when the barrier height at the p layer (Ec−EF0)max, where Ec is the conduction band edge and EF0 the thermodynamic equilibrium Fermi level, approaches its maximum value corresponding to a thick or highly doped p layer, there is no gain. This is the condition to be fulfilled for a color sensitive device.

Radio-frequency impedance measurements using a tunnel-diode oscillator technique

A resonant method based on a tunnel-diode oscillator for precision measurements of relative impedance changes in materials is described. The system consists of an effective self-resonant LC tank circuit driven by a forward-biased tunnel diode operating in its negative resistance region. Samples under investigation are placed in the core of an inductive coil and impedance changes are determined directly from the measured shift in resonance frequency. A customized low temperature insert is used to integrate this experiment with a commercial Model 6000 Physical Property Measurement System (Quantum Design). Test measurements on a manganese-based perovskite sample exhibiting colossal magnetoresistance indicate that this method is well suited to study the magnetoimpedance in these materials.

Electrolyte Transistors: Ionic Reaction−Diffusion Systems with Amplifying Properties

The problem of constructing chemoelectric amplifying devices based on acid−base reactions is discussed. It is shown that the construction of a direct analogue of the semiconductor transistor (that is, when the p- and n-doped semiconductors are replaced by ion exchange membranes in the H+ and OH- forms, respectively) is technically not feasible. The new amplifying device reported here is based exclusively on mobile ions migrating in a hydrogel arrangement that contains no fixed charges. The polarization curve of an acid−base interface in this medium has a diode characteristic. When two of such “electrolyte diodes” are connected appropriately, a transistor action can be observed: the electric current flowing in one (reverse biased) diode is affected not only by the voltage applied there but also by the current flowing through the other (forward biased) diode. The common emitter current gain of an acid−base−acid transistor studied here was between 4 and 5. The active mode characteristics of this device can be explained by the appearance of an acidic tunnel across the alkaline middle section. Although the acid−base−acid transistor is a stable amplifying device, the base−acid−base transistor is unstable in its active mode and displays complex oscillations. The underlying bistable behavior was also demonstrated with this transistor when its acidic feedstream was contaminated with KCl.

Diode-based microfabricated hot-plate sensor

In recent years, different types of microfabricated silicon ‘hot plates’ have been used as substrates for a variety of gas sensors and in some cases as gas thermal conductivity probes. The temperature-sensing elements in all reported devices have been resistors with varying types of thermal coefficients of resistance. The major difficulty with this approach is the reproducibility of the temperature coefficient of resistance from batch to batch. It is well known that the forward-biased voltage of a constant-current diode is directly proportional to the absolute temperature. While the constant of proportionality depends on the fabrication process, this parameter tends to be reproducible and easily controlled. Utilizing the characteristics of a forward-biased diode also eliminates the need to use materials such as platinum, which is both expensive and catalytic, as the resistive element. In this paper we report on the design and construction of a microfabricated silicon ‘hot plate’ that employs a diode-based temperature-measurement system. The noise level of the measurement corresponds to a temperature difference in the 100 μK range in the vicinity of room temperature. The simplicity and utility of the structure described herein makes it an attractive element for various types of thermal measurements, particularly of the thermal conductivity of pure and mixed gases.