Rectification in AlxGa1-xAs-GaAs N-n heterojunction devices

The previously unsolved problem of rectification at Al_xGa_1-xAs-GaAs N-n heterojunction is found to originate from a vague concept regarding the maximum junction grading width which can sustain rectification. The theoretical current density vs voltage characteristics of this heterojunction system are derived from thermionic emission theory. It is found that, unless the impurity concentration of the AlGaAs layer (prepared by LPE techniques) is less than 10¹⁶ cm^?3, typical 90–200 Å metallurgical grading widths at the N-n heterojunction interface produce either ohmic or poorly rectifying characteristics. These results explain (1) the lack of rectification in most N-n Al_xGa_1-xAs-GaAs heterojunctions reported in the literature and (2) the recent observation of significant rectification in high purity (N)Al_0.3Ga_0.7As-(n)GaAs heterojunctions reported by Chandra and Eastman.     

Photoresponse characteristics of p-Si/n-CuxIn1−xO heterojunction diode prepared by sol-gel spin coating

In the present work, we report on the fabrication and detailed electrical characterization of p-Si/n-Cu_xIn_1−xO heterojunction prepared via the deposition of nanocrystalline Cu_xIn_1−xO thin films on p-type silicon substrate by sol-gel method using spin coating technique. X-ray diffraction and Raman spectroscopy results revealed the polycrystalline nature of Cu_xIn_1−xO thin films consisting diffractions peaks and vibration modes, respectively, corresponding to CuO and In₂O₃. Field-emission scanning electron microscopy showed compact surface morphology while UV–vis absorption spectra exhibited sharp absorption between 300 and 425 nm along with a long tail extending in the visible region. The current-voltage (I-V) characteristics of the fabricated heterojunction demonstrated obvious rectifying behavior in the dark and under illumination. The heterojunction exhibited low reverse leakage current (~10⁻⁷), and upon illumination, the forward current and rectification ratio of the junction was improved while the forward threshold voltage lowered. By fitting the experimental data we have observed that the forward current conduction is dominated by the space charge limited current mechanism.     

ZnxCd1−xS/Cu2S heterojunction solar cells—II: Junction analysis

Cu₂S/Zn_xCd_1?xS heterojunction solar cells have been fabricated by two processes namely, spray pyrolysis and evaporation. The cells have been characterised in terms of the temperature dependence of the current-voltage and capacitance-voltage characteristics. Based on the analysis of the data, an energy band model has been proposed to explain the photovoltaic performance of the heterojunction. The dominant transport mechanism in both cases has been found to be interface recombination. The observed increase in open circuit voltage on increasing ZnS concentration has been understood on the basis of a reduction in the reverse saturation current caused primarily by an increase in the barrier height at the interface. The density of interface states is not observed to be significantly affected by the lattice parameter mismatch between Cu₂S and Zn_xCd_1?xS.     

Electrical characterization of cobalt phthalocyanine/p-silicon heterojunction

We report the fabrication of organic/inorganic heterojunction of cobalt phthalocyanine (CoPc) with p-type silicon (p-Si) using vacuum thermal evaporation. At ambient conditions, the electrical characteristics of the heterojunction are investigated. The optical band gap of CoPc is calculated from absorption spectrum using Tauc׳s law. The electrical characterization of the heterojunction shows rectifying behavior with a rectification ratio (RR) of 316. Different diode parameters are extracted from the current–voltage (I–V) curves, such as ideality factor n, barrier height ϕ, series resistance R_s and shunt resistance R_sh. These parameters are in good agreement with those calculated from the functions of Cheungs and Norde . The conduction of charge carriers through the interface of p-Si/CoPc is also studied. The fabricated heterojunction could be a promising candidate for its potential use in electronic applications.     

The effect of grading on the electrical behavior of Np AlGaAs/GaAs heterojunction diodes

We have studied the effect of compositional grading on the characteristics of N AlGaAs p GaAs heterojunction diodes, and have found that the elimination by grading of the heterojunction spike leads to a factor of 10 difference in the magnitude of current density. The behavior of heterojunction diodes has also been compared to GaAs homojunction diodes. Ideality factors for graded diodes were as good as n = 1.12, while those for the abrupt diodes were n = 1.25−1.30. At low forward-bias voltages the I–V characteristics are dominated by a recombination mechanism which leads to a temperature-independent logarithmic slope with applied bias. We also observe a difference in the built-in voltage between the graded-, abrupt- and homojunction diodes. By comparing the built-in voltage between abrupt heterojunction diodes and homojunction diodes, a value of ΔE_c = 0.64 ΔE_G for the conduction-band-edge discontinuity was obtained in excellent agreement with recent measurements on other device structures. An understanding of the current conduction mechanisms of these pN heterojunction diodes is essential for the understanding of GaAs/AlGaAs heterojunction bipolar transistors.     

Electronic properties of Pb1−xHgxSSi heterojunctions

Two types of heterojunctions have been prepared by electroless deposition of Pb_1?xHg_xS (x = 0–0.33) films with α′ and β′ structures on n-type silicon single crystal substrates. Functional behaviour of the forward characteristics is explained on the basis of band to band tunnelling coupled with the recombination processes. The energy band diagram is given for both types of Pb_1?xHg_xS/Si heterojunctions in agreement with the experimental results.     

Carrier transport across heterojunction interfaces

A general theory is presented to describe the carrier transport across heterojunction interfaces. In matching the boundary conditions at the interface, the conservation of total energy and perpendicular momentum is assumed and the difference of effective masses on two sides of the junction is taken into account. The quantum mechanical transmission coefficient is calculated by a combined numerical and WKB method. Application of the present model to an Al_xGa_1?xAsGaAs N-n heterojunction is performed and it gives rise to rectifying characteristics together with non-saturated reverse current. Comparison with the classical thermionic emission model is made to show the significance of tunneling and effect of quantum mechanical reflection.     

Electrical and photoelectric properties of anisotype n-TiN/p-Si heterojunctions

Photosensitive n-TiN/p-Si heterojunctions are fabricated by the reactive magnetron sputtering of a thin titanium-nitride film with n-type conductivity onto polished polycrystalline p-Si wafers. The I–V characteristics of the heterostructures are measured at different temperatures. The temperature dependences of the potential-barrier height and series resistance of the n-TiN/p-Si heterojunction are studied. The dominant mechanisms of current transport through the heterojunction in the cases of forward and reverse bias are established. The heterostructures generate the open-circuit voltage V _oc = 0.4 V and the short-circuit current I _sc = 1.36 mA/cm² under illumination with a power density of 80 mW/cm².     

Pentacene/n-Si heterojunction diodes and photovoltaic devices investigated by I-V and C-V measurements

The forward and reverse current density-voltage (J-V) and capacitance-voltage (C-V) characteristics of pentacene/n⁻-silicon heterojunction diodes were investigated to clarify the carrier conduction mechanism at the organic/inorganic heterojunction. Current rectification characteristics of the pentacene/n⁻-Si junctions can be explained by a Schottky diode model with an interfacial layer. The diode parameters such as Schottky barrier height and ideality factor were estimated to be 0.79-1.0 eV and 2.4-2.7, respectively. The C-V analysis suggests that the depletion layer appears selectively in the n⁻-Si layer with a thickness of 1.47 μm from the junction with zero bias and the diffusion potential was estimated at 0.30 eV at the open-circuit condition. The present heterojunction allows the photovoltaic operation with power conversion efficiencies up to 0.044% with a simulated solar light exposure of 100 mW/cm².     

Temperature dependent current-voltage characteristics of n-MgxZn1−xO/p-GaN junction diodes

This study investigates the temperature dependence of the current-voltage (I-V) characteristics of n-Mg_xZn_1−xO/p-GaN junction diodes. The n-Mg_xZn_1−xO films were deposited on p-GaN using a radio-frequency (rf) magnetron sputtering system followed by annealing at 500, 600, 700, and 800 °C in nitrogen ambient for 60 s, respectively. The n-Mg_xZn_1−xO/p-GaN diode at a substrate temperature of 25 °C had the lowest leakage current in reverse bias. However, the leakage current of the diodes increased with an increase in annealing temperatures. The temperature sensitivity coefficients of the I-V characterizations were obtained at different substrate temperatures (25, 50, 75 100, and 125 °C) providing extracted values of 26.4, 27.2, 17.9, and 0.0 mV/°C in forward bias and 168.8, 143.4, 84.6, and 6.4 mV/°C in reverse bias, respectively. The n-Mg_xZn_1−xO/p-GaN junction diode fabricated with Mg_xZn_1−xO annealed at 800 °C demonstrated the lowest temperature dependence. Based on these findings, the n-Mg_xZn_1−xO/p-GaN junction diode is feasible for GaN-based heterojunction bipolar transistors (HBTs).     

Effects of arsenic mole fraction x on the gain characteristics of type-II InP/GaAsxSb1−x DHBTs

The static gain characteristics of N–p–N InP/GaAs_xSb_1?x/InP double heterojunction bipolar transistors (DHBTs) were studied as a function of the base arsenic (As) mole fraction x. Compared to the devices with a lattice-matched base (x = 0.51), a current gain improvement largely arising from a base current reduction is observed in DHBTs with higher As base mole fractions (and consequently a reduced type-II conduction band discontinuity ΔE_C at the emitter-base heterojunction). Both the surface periphery and the intrinsic recombination currents decrease markedly as the base arsenic concentration increases. The present work therefore unambiguously demonstrates that the emitter type-II conduction band discontinuity between the InP emitter and the GaAs_xSb_1?x base enhances the undesirable emitter size effects (ESEs) and increases intrinsic recombination currents directly under the emitter contact.     

Photovoltaic properties and anomalous effects in the ZnSe-GaAs heterojunction

The photoelectric properties of the nZnSe-pGaAs heterojunction obtained by deposition of ZnSe on GaAs in the MOCVD process have been investigated. The current-voltage characteristics of the illuminated solar cells are shifted to lower voltages than those expected from the superposition principle. The dependence of quantum efficiency and C–V curves on light intensity and wavelength has been observed. A heterojunction model is proposed for both light and dark conditions. We assumed that near the interface a compensated ZnSe layer exists with negative charged deep centers. As a result electric current in the heterojunction is limited by a barrier in the conduction band. Variation of charge in the compensated layer is connected with electron transfer from deep centers to the conduction band. This process explains the anomalous effects for such a heterojunction. The characteristics calculated by this model are close to the experimental curves.     

Amorphous-crystalline silicon heterojunction: Theoretical studies of the dark current and spectral response

All the terms contributing to the dark current for an n-amorphous/p-crystalline silicon heterojunction have been evaluated. The abrupt heterojunction model with the presence of interface states has been adopted and flat quasi-Fermi levels have been assumed across both depletion regions. The gap density of states in amorphous silicon has been approximated by a V-distribution. The dominant current mechanisms at forward and reverse biases have been determined and the zero-bias spectral response has been calculated.     

Ultra-violet photo-response characteristics of p-Si/i-SiO2/n-ZnO heterojunctions based on hydrothermal ZnO nanorods

Hydrothermal zinc oxide (ZnO) nanorod (NR)-based p-Si/n-ZnO and p-Si/i-SiO₂/n-ZnO heterojunctions were fabricated, and the effects of interfacial native SiO₂ (~4 nm) on the I-V characteristics of heterojunctions under dark and ultra-violet illumination conditions were investigated. First, the structural and optical properties of ZnO seed crystals grown by sol-gel method and hydrothermal ZnO NRs on two different substrates of p-Si and p-Si/i-SiO₂ were examined, and more improved optical and crystalline quality was obtained as revealed by photoluminescence and X-ray diffraction. The p-i-n heterojunctions showed ~3 times greater forward-bias currents and enhanced rectifying property than those of p-n junctions, which is attributed to the role of native SiO₂ in carrier confinement by promoting the electron-hole recombination current through the deep level states of ZnO crystal. The measured ratios of photocurrent to dark current of the p-i-n structure were also greater under reverse bias (92–260) and forward bias (2.3–7.1) conditions than those (28–225 for reverse bias, 1.6–6.8 for forward bias) of p-n structure, and the improved photosensitivity of the p-i-n structure under reverse bias is due to lower density of recombination centers in the ZnO NR crystals. Fabricated ZnO NR heterojunction showed repeatable and fast photo-response transients under forward bias condition of which response and recovery times were 7.2 and 3.5 s for p-i-n and 4.3 and 1.7 s for p-n structures, respectively.     

Cd1-x Zn x Se thin film electrodes: an electrochemical photovoltaic study

The interface between an n?Cd_1?x Zn _x Se semiconductor and an electrolyte redox couple is investigated through the capacitance–voltage, current–voltage and photovoltaic characteristics. A brief discussion is made on the properties of a semiconductor/electrolyte Schottky barrier with reference to the experiments performed. The observed results of the capacitance–voltage measurements in the dark are compared with the photovoltage measurements. The dependence of the dark current on the dark voltage for both forward and reverse bias is examined and explained. It is probable that the current–voltage characteristics are determined by the electrochemical kinetics in addition to the diode rectifier theory. The measurements of photovoltaic properties show a significant improvement in the cell performance after addition of Zn to CdSe (optimum at x = 0.3).     

A model for laterally graded Cu2S/ZnxCd1-xS photovoltaic heterojunctions

A model is presented for the Cu₂S/Zn_xCd_1-xS heterojunction that is based on the existence of a laterally graded zinc rich region just below the Cu₂S-ZnCdS interface. Integrals for short circuit current density and current density versus applied voltage are presented. The current-voltage characteristics, in particular the increase in open circuit voltage and decrease in short circuit current density that occur with increasing zinc content, follow trends that have been seen experimentally for junctions formed by ion-exchange. Supported by the Solar Energy Research Institute.     

Evaluation of lateral barrier height of inhomogeneous photolithography-fabricated Au/n-GaAs Schottky barrier diodes from 80 K to 320 K

In order to evaluate current conduction mechanism in the Au/n-GaAs Schottky barrier diode (SBD) some electrical parameters such as the zero-bias barrier height (BH) Φ_bo(I–V) and ideality factor (n) were obtained from the forward bias current–voltage (I–V) characteristics in wide temperature range of 80–320 K by steps of 10 K. By using the thermionic emission (TE) theory, the Φ_bo(I–V) and n were found to depend strongly on temperature, and the n decreases with increasing temperature while the Φ_bo(I–V) increases. The values of Φ_bo and n ranged from 0.600 eV and 1.51(80 K) to 0.816 eV and 1.087 (320 K), respectively. Such behavior of Φ_bo and n is attributed to Schottky barrier inhomogeneities by assuming a Gaussian distribution (GD) of BHs at Au/n-GaAs interface. In the calculations, the electrical parameters of the experimental forward bias I–V characteristics of the Au/n-GaAs SBD with the homogeneity in the 80–320 K range have been explained by means of the TE, considering GD of BH with linear bias dependence.     

Electrical Properties of <Emphasis Type="Italic">p</Emphasis>-NiO/<Emphasis Type="Italic">n</Emphasis>-Si Heterostructures Based on Nanostructured Silicon

Silicon nanowires are formed on n-Si substrates by chemical etching. p-NiO/n-Si heterostructures are fabricated by reactive magnetron sputtering. The energy diagram of anisotype p-NiO/n-Si heterostructures is constructed according to the Anderson model. The current–voltage and capacitance–voltage characteristics are measured and analyzed. The main current-transport mechanisms through the p-NiO/n-Si heterojunction under forward and reverse biases are established.     

A collector current model for InAlAs/InGaAsSb/InGaAs double heterojunction bipolar transistors with non-ideal effects

A collector current model incorporating electron diffusion in the base and thermionic emission at the abrupt B–E heterojunction is derived and applied to InGaAsSb heterojunction bipolar transistors (HBTs). Parameters extracted from the model include effective base doping concentration, conduction band discontinuity (ΔE_C) at the base-emitter junction, and emitter resistance. The calculated current from the model is consistent with the measured result. It is found that a high collector current ideality factor is resulted from a nonzero ΔE_C and a low effective base doping concentration. Moreover, a nonzero ΔE_C makes the high-injection effect almost invisible in collector current characteristics, even if it actually exists.