Reverse Bias Characteristics Research Articles

Organic doped/undoped interface based diode structure: Distinct mechanisms underlying forward and reverse bias

Abstract Organic semiconductor diodes fabricated using doped/undoped (high-low) homojunction has the potential of providing controlled and high quality current density-voltage (J-V) characteristics based on majority carrier transport. We study mechanisms of transport underlying such characteristics both in forward and reverse bias regimes of typical doped/undoped homojunction organic diodes fabricated using 4,4′,4″-tris( N -3-methylphenyl- N -phenyl-amino) triphenylamine ( m -MTDATA). We study the J-V characteristics over a wide temperature range (200–300 K), and by varying the intrinsic layer thickness between 10 and 100 nm. The forward bias current, before entering into the space charge limited regime, is exponential over several orders of magnitude with the slope being temperature independent for all intrinsic layer thickness down to 10 nm. The reverse bias characteristics, on the other hand, are highly sensitive to the thickness of the intrinsic layer. While the forward bias is controlled by tunneling at the homojunction interface, the reverse bias is controlled by the interface of cathode (aluminum in this case) and the intrinsic layer. We show that the reverse current is due to Fowler-Nordheim tunneling across a barrier height, which is temperature independent but is sensitive to the layer thickness of the intrinsic layer. The origin of the thickness dependence of barrier height (0.45–0.72 eV for 10–20 nm) is attributed to the change in background carrier concentration in the intrinsic layer due to diffusion of carriers from the highly doped side. The results clearly show that the diffusion length of the majority carriers is approximately 1 nm and is comparable to the nearest neighbor jump distance.

Thin SOI lateral IGBT with band-to-band tunneling mechanism

In this paper, a novel 200V lateral IGBT on thin SOI layer with a band-to-band tunneling junction near the anode is proposed. The structure and the operating mechanism of the proposed IGBT are described and discussed. Its main feature is that the novel IGBT structure has a unique abrupt doped p++/n++ tunneling junction in the side of the anode. By utilizing the reverse bias characteristics of the tunneling junction, the proposed IGBT can achieve excellent reverse conducting performance. Numerical simulations suggest that a low reverse conduction voltage drop VR=−1.6V at a current density of 100A/cm2 and a soft factor S=0.63 of the build-in diode are achieved.

Defect-free Ni/GaN Schottky barrier behavior with high temperature stability

In this work, we report on defect-free homogeneous behavior of Ni Schottky contacts patterned on surface treated n-GaN by photolithography with unity ideality factor, high temperature stability, and low reverse leakage. The barrier height (0.7 eV) and ideality factor (1.02) are found to be independent of temperature, indicating a highly homogeneous contact. The contacts are found to be stable with no significant change in ideality factor or leakage current up to an annealing temperature of 600 °C. Temperature dependence of the reverse leakage current shows no evidence for the existence of surface defects that would provide leakage paths, and the behavior was modeled by ATLAS simulations with an ideal homogeneous barrier of 0.7 eV. Consequently, the forward and reverse bias characteristics are explained by a common set of parameters. The surface treatment after the development and prior to metallization included an acid-based surface treatment. X-ray photoelectron spectroscopy (XPS) studies indicate that the hydroxide-based development process during photolithography changes the nitride surface composition by introducing excess C that degrades the ideality factor and introduces barrier inhomogeneity and high leakage currents. XPS studies further demonstrate that the restoration of a stable, Ga-rich surface, similar to as-grown surface, occurs due to the acid-based surface treatment, which is responsible for the observed unity ideality factor, homogeneous barrier, low leakage current, and high temperature stability.

Contact Resistance and Leakage Current of GaN Devices with Annealed Ti/Al/Mo/Au Ohmic Contacts

In recent years, the on-resistance, power loss and cell density of Si power devices have not exhibited significant improvements, and performance is approaching the material limits. GaN is considered an attractive material for future high-power applications because of the wide band-gap, large breakdown field, high electron mobility, high switching speed and low on-resistance. Here we report on the Ohmic contact resistance and reversebias characteristics of AlGaN/GaN Schottky barrier diodes with and without annealing. Annealing in oxygen at 500℃ resulted in an increase in the breakdown voltage from 641 to 1,172 V for devices with an anode-cathode separation of 20 μm. However, these annealing conditions also resulted in an increase in the contact resistance of 0.183 Ω-mm, which is attributed to oxidation of the metal contacts. Auger electron spectroscopy revealed diffusion of oxygen and Au into the AlGaN and GaN layers following annealing. The improved reverse-bias characteristics following annealing in oxygen are attributed to passivation of dangling bonds and plasma damage due to interactions between oxygen and GaN/AlGaN. Thermal annealing is therefore useful during the fabrication of high-voltage GaN devices, but the effects on the Ohmic contact resistance should be considered.

Optimized multilayer indium-free electrodes for organic photovoltaics

Flexible multilayer electrodes that combine high transparency, high conductivity, and efficient charge extraction have been deposited, characterised and used as the anode in organic solar cells. The anode consists of an AZO/Ag/AZO stack plus a very thin oxide interlayer whose ionization potential is fine-tuned by manipulating its gap state density to optimise charge transfer with the bulk heterojunction active layer consisting of poly(n-3-hexylthiophene-2,5-diyl) and phenyl-C61-butyric acid methyl ester (P3HT:BC61BM). The deposition method for the stack was compatible with the low temperatures required for polymer substrates. Optimisation of the electrode stack was achieved by modelling the optical and electrical properties of the device and a power conversion efficiency of 2.9% under AM1.5 illumination compared to 3.0% with an ITO-only anode and 3.5% for an ITO:PEDOT electrode. Dark I–V reverse bias characteristics indicate very low densities of occupied buffer states close to the HOMO level of the hole conductor, despite observed ionization potential being high enough. Their elimination should raise efficiency to that with ITO:PEDOT.

Reverse conducting lateral insulated‐gate bipolar transistors with a non‐local band‐to‐band tunnelling junction

A new reverse conducting lateral insulated-gate bipolar transistor (LIGBT) structure with a non-local band-to-band tunnelling junction is proposed and its operating principle is demonstrated in detail for a power-switching device. By utilising the reverse bias characteristics of the tunnelling junction, the proposed LIGBT can operate successfully in the freewheeling diode mode without an external anti-parallel diode. Analysis has illustrated that the proposed LIGBT can achieve a reverse knee voltage of −1 V and a lower reverse conduction voltage drop of −1.4 V at a current density of 100 A/cm2. It also shows a good temperature effect on the forward and reverse current. Moreover, it exhibits a better soft reverse recovery performance and about two times higher soft factor than the lateral P-i-N diode.

Breakdown mechanisms and reverse current-voltage characteristics of organic bulk heterojunction solar cells and photodetectors

We investigate the reverse current-voltage characteristics and breakdown mechanisms of organic bulk heterojunction solar cells and photodetectors. Dark current and photo current measurements at different temperatures indicate that tunneling is the dominant mechanism at high reverse voltage. A band-to-band tunneling model that accommodates either Gaussian or exponential-parabolic density of states distributions is developed and used for simulations. At high reverse bias, the model explains the observed breakdown, which differs from the bias independent dark current behavior predicted by the Onsager-Braun model. At low reverse voltage, the incorporation of shunt resistance in the model provides good agreement between the measured reverse bias characteristics and simulations.

Signatures of differential Log R–V characteristics for non-uniformity analysis of HgCdTe infrared photodiode arrays

In continuation to our research on the non-uniformity in infrared focal plane arrays of HgCdTe photodiodes, we have examined and found a few important signature parameters in the forward bias region of the electrical characteristics of the photodiodes. These signatures are the peaks of the first and second order derivatives of dynamic resistance–voltage characteristics. They show good correlation with the material and devices parameters and have been investigated to be good signatures for statistical performance variation analysis. In this note, by using theoretical model and statistical analysis of these signatures, we show that the variations in the proposed signatures represent underlying non-uniformity in important material/device parameters. Such signatures may be used along with the other conventional signatures of reverse biased photodiode characteristics to identify the root cause of response non-uniformity in the array with high confidence level.

Designing and Fabrication of the VLD Edge Termination for 3.3 kV SiC SBD

Edge termination guaranteeing high breakdown voltage and robustness in its fabrication are required in SiC power devices. We newly employed the VLD edge termination for 3.3 kV-rated SiC SBDs, which was formed by Al ion implantation using a resist mask having a varying thickness. The breakdown voltage is recorded to be over 96% of the parallel-plane breakdown voltage, and the reverse bias characteristics are well accorded with the result of TCAD simulation.

A Dynamic Photovoltaic Model Incorporating Capacitive and Reverse-Bias Characteristics

Photovoltaics (PVs) are typically modeled only for their forward-biased dc characteristics, as in the commonly used single-diode model. While this approach accurately models the I-V curve under steady forward bias, it lacks dynamic and reverse-bias characteristics. The dynamic characteristics, primarily parallel capacitance and series inductance, affect operation when a PV cell or string interacts with switching converters or experiences sudden transients. Reverse-bias characteristics are often ignored because PV devices are not intended to operate in the reverse-biased region. However, when partial shading occurs on a string of PVs, the shaded cell can become reverse biased and develop into a hot spot that permanently degrades the cell. To fully examine PV behavior under hot spots and various other faults, reverse-bias characteristics must also be modeled. This study develops a comprehensive mathematical PV model based on circuit components that accounts for forward bias, reverse bias, and dynamic characteristics. Using a series of three experimental tests on an unilluminated PV cell, all required model parameters are determined. The model is implemented in MATLAB Simulink and accurately models the measured data.

Positive MR and Large Temperature–Field Sensitivity in Manganite Based Heterostructures

Studies on the ZnO/La 0.5 Pr 0.2 Sr 0.3 MnO 3 (LPSMO)/SrNb 0.002 Ti 0.998 O 3 (SNTO) heterostructure having varying thickness of p-type LPSMO (100 nm – LP1) and (200 nm – LP2) manganite are carried out. ZnO/LPSMO (n–p) and LPSMO/SNTO (p–n) junctions of both the heterostructures exhibit good rectifying behavior in a wide range of temperature and applied field. Forward and reverse bias characteristics of both the junctions of heterostructures show opposite behavior. The observation of negative magnetoresistance (MR) at 5 K and positive MR at 300 K, in both the heterostructures, has been explained in the context of interface region effects and filling of energy bands of LPSMO manganite. Further, at high temperature, the heterostructures exhibit large temperature (46%K −1 ) and field (40%T −1 ) sensitivities. Dependence of transport, magnetotransport, I – V and sensing properties of the heterostructures, on the temperature, field and film thicknesses have been discussed in this communication.

Characterization of poly-Si thin-film solar cell functions and parameters with IR optical interaction techniques

Three different optical interaction techniques have been employed to characterise the electrical and material parameters of polycrystalline silicon (poly-Si) thin-film solar cells with an interdigitated mesa structure. First, Light Beam Induced Current (LBIC) in the infrared range was used to locally analyse the light collection properties. Second, electroluminescence in forward bias (EL) yielding information on band to band recombination was performed. Third, electroluminescence in reverse bias (ELR) was utilized to gain information on the intraband relaxation. The EL and ELR measurements were performed using cooled Si-CCD (Silicon-based Charge Coupled Device) and InGaAs (Indium Gallium Arsenide) detectors. The high resolution IR-LBIC measurement equipped with a 1,064 nm wavelength laser has been applied to investigate the grain boundary characteristics in the absorber layer. Additionally, the local electrical characteristics of the absorber layer (diffusion length, doping concentration and built-in potential) have been extracted by performing a bias-dependent IR-LBIC measurement based on a simple theoretical model with the assumption of relatively small diffusion length compared to the absorber layer thickness. The local/spatial distribution of the diffusion length in the absorber layer of the thin-film solar cell has been extracted. Furthermore, the temperature dependence of the photocurrent of thin-film solar cells in a temperature range of −25 to +70 °C has been locally investigated using IR-LBIC. Additionally, the temperature dependence of the reverse bias characteristics of the poly-Si thin-film solar cell is analysed and compared with that of monocrystalline Si solar cell. For the EL and ELR measurements a spectral analysis of the emitted light has been performed. From the EL results material properties like diffusion length and process induced defects have been deduced and insights on the quality of production processes like metallization and etching were gained. The complementary information from the ELR measurements provides access to additional types of defects resulting from generation centres, such as lattice disorder, crystal defects and charged coulomb centers.

The correlation between electric field emission phenomenon and Schottky contact reverse bias characteristics in nanostructured systems

Two different morphologies of nanotextured molybdenum oxide were deposited by thermal evaporation. By measuring their field emission (FE) properties, an enhancement factor was extracted. Subsequently, these films were coated with a thin layer of Pt to form Schottky contacts. The current-voltage (I-V) characteristics showed low magnitude reverse breakdown voltages, which we attributed to the localized electric field enhancement. An enhancement factor was obtained from the I-V curves. We will show that the enhancement factor extracted from the I-V curves is in good agreement with the enhancement factor extracted from the FE measurements.

Analysis of surface morphology at leakage current sources of 4H–SiC Schottky barrier diodes

The surface morphologies of leakage current sources of 4H–SiC Schottky barrier diode were analyzed using atomic force microscopy (AFM). Nanosized circular cone shaped pits (nanopits) were observed at the leakage current sources. Leakage currents were generated due to the concentration of electric fields at the nanopits during measurements of the reverse bias characteristics. The positions of the nanopits correspond to the positions of threading dislocations (TDs), which were identified from molten potassium hydroxide (KOH) etching. The most important factor of leakage current generation was determined to be the surface morphology of the TDs rather than the presence of TDs.

Electronic memory effect in a crystalline silicon/polyether heterostructure

AbstractOrganic/inorganic heterojunctions have been fabricated by spin‐coating of an oxadiazole based polyether on top of either n‐type or p‐type crystalline silicon substrates. The prepared heterodiodes showed good rectification. During current‐voltage measurements sharp current switching effects have been observed in the reverse bias characteristics for both types of heterodiodes. In particular a very stable hysteresis in the reverse characteristics of the polyether/n‐type crystalline silicon has been measured at applied voltages below 1V. The application of this heterodiode as electrical memory with an on/off‐ratio of about 2 orders of magnitude has been demonstrated (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrical injection and detection of spin-polarized electrons in silicon through an Fe3Si/Si Schottky tunnel barrier

We demonstrate electrical injection and detection of spin-polarized electrons in silicon (Si) using epitaxially grown Fe3Si/Si Schottky-tunnel-barrier contacts. By an insertion of a δ-doped n+-Si layer (∼1019 cm−3) near the interface between a ferromagnetic Fe3Si contact and a Si channel (∼1015 cm−3), we achieve a marked enhancement in the tunnel conductance for reverse-bias characteristics of the Fe3Si/Si Schottky diodes. Using laterally fabricated four-probe geometries with the modified Fe3Si/Si contacts, we detect nonlocal output signals that originate from the spin accumulation in a Si channel at low temperatures.

Insights in junction photovoltage based sheet resistance measurements for advanced complementary metal-oxide semiconductor

Earlier work has clearly shown that only a very few tools are able to measure reliably sheet resistances on advanced complementary metal-oxide semiconductor (CMOS) structures. One of these promising techniques is the junction photovoltage based technique, which uses a modulated light emitting diode to generate, in a millimeter size area of a single junction isolated structure, excess carriers which are separated by the underlying electrical field, and subsequently outdiffuse laterally. From the lateral variation in junction photovoltage, one can extract in a noncontact way the sheet resistance (Rs) of the top layer and the junction leakage (L) of the junction. First, a simplified theoretical solution of the underlying diffusion equations will be presented. Next, a recently developed simulation framework will be discussed, combining the SCILAB environment with SYNOPSIS∕MEDICI device simulations, allowing for the detailed study of both ideal (in agreement with the theoretical solution) and less ideal advanced CMOS structures. Using this simulation framework, new insights will be proposed relating to the capability of RsL to characterize the impact of the presence of oxide charges on the accuracy of the measurements, and the relevance of the RsL leakage value for the actual diode (device) reverse bias characteristics in the presence of an underlying halo∕well implant.

Concentration Effects on n-GaN Schottky Diode Current-Voltage (i-v) Characteristics

We focus on the epi layer carrier concentration variation effects to improve the current – voltage (I-V) characteristics of an n-GaN schottky diode. The carrier concentration of 1×10 15cm-3, 1×1016 cm−3, 1×1017 cm−3 were employed. The simulated current was obtained by forward biasing the device of up to 2Volt at room temperature using Pt electrode. The study was conducted by using Atlas/Blaze using various models such as Consrh (Concentration Dependent Shockley Read Hall), Cvt (Lombardi Model), Fermi (Fermi Dirac), Bgn (Bandgap Narrowing), Conmob (Concentration Dependent Mobility), Auger (Auger). We found that as the concentration increases the value of forward current also increase linearly when biased at maximum of 2 volts. The reverse bias characteristics at the same concentration of the simulated diode up to 100Volt were also determined. We found that at low carrier concentration the reverse leakage current is minimum and breakdown voltage is maximum. As the carrier concentration increases there is a linear relationship between reverse leakage current and epi layer doping carrier concentration. By analyzing the forward and reverse characteristics we conclude that in forward bias for low carrier concentration the diode shows schottky rectifying behavior while for higher carrier concentration the diode shows ohmic behavior. For higher carrier concentration there is a linear relationship between carrier concentration (n) and forward current. The reverse leakage current is minimum approaching an ideal value at n≤1×1015cm-3 and breakdown voltage is maximum at these values of concentration. Increasing the concentration from n≤1×1015cm-3 the value of reverse leakage current is approaching to the maximum value as a result breakdown voltage decreases. We conclude that for n-GaN schottky diode the ideal schottky rectifying behavior of I-V characteristics is obtained at carrier concentration of n≤ 1×1015cm-3 for the simulated diodes at different carrier concentration.

Novel HgxCd1−xTe device structure for higher operating temperature detectors

A novel detector structure in which photons are absorbed in narrow-gap material, but the junction lies in wider bandgap material, has been examined. A possible reduction in bulk 1/f noise by a factor of 33 for a 240 K operation was predicted, based on studies that have suggested that the source of 1/f noise is the p-n junction and the large reduction in 1/f noise, which is observed in conventional detectors when the bandgap is increased. The increased bandgap at the junction produces a potential barrier; however, calculations are presented showing that for T > 150 K the thermal energy of the electrons enables them to cross the barrier and reach the p-n junction before recombining in the active region, thus giving high quantum efficiency. Experiments performed on the diodes showed that the quantum efficiency was maintained as expected, but the 1/f noise was not reduced, suggesting that it does not originate at the p-n junction. However, the structure does give excellent reverse-bias characteristics with low breakdown even at 2-V reverse bias.

HgCdTe/CdTe/Si infrared photodetectors grown by MBE for near-room temperature operation

Conventional HgCdTe infrared detectors need significant cooling in order to reduce noise and leakage currents resulting from thermal generation and recombination processes. Although the need for cooling has long been thought to be fundamental and inevitable, it has been recently suggested that Auger recombination and generation rates can be reduced by using the phenomena of exclusion and extraction to produce nonequilibrium carrier distributions. The devices with Auger suppressed operation requires precise control over the composition, and donor and acceptor doping. The successful development of the molecular beam epitaxy (MBE) growth technique for multi-layer HgCdTe makes it possible to grow these device structures. Theoretical calculations suggest that the p n+ layer sequence is preferable for near-room temperature operation due to longer minority carrier lifetime in lightly doped p-HgCdTe absorber layers. However, because the low doping required for absorption and nonequilibrium operation is easier to achieve in n-type materials, and because Shockley-Read centers should be minimized in order to obtain the benefits of Auger suppression, we have focused on p+ n structures. Planar photodiodes were formed on CdTe/Si (211) composite substrates by As implantation followed by a three step annealing sequence. Three inch diameter Si substrates were employed since they are of high quality, low cost, and available in large areas. Due to this development, large area focal plane arrays (FPAs) operated at room temperature are possible in the near future. The structures were characterized by FTIR, x-ray diffraction, temperature dependent Hall measurements, minority carrier lifetimes by photoconductive decay, and in-situ ellipsometry. To study the relative influence of bulk and surface effects, devices with active areas from 1.6 10−5 cm2 to 10−3 cm2 were fabricated. The smaller area devices show better performance in terms of reverse bias characteristics indicating that the bulk quality could be further improved. At 80 K, the zero bias leakage current for a 40 m 40 m diode with 3.2 m cutoff wavelength is 1 pA, the R0A product is 1.1 104-cm2 and the breakdown voltage is in excess of 500 mV. The device shows a responsivity of 1.3 107 V/W and a 80 K detectivity of 1.9 1011 cm-Hz1/2/W. At 200 K, the zero bias leakage current is 5 nA and the R0A product 2.03-cm2, while the breakdown voltage decreases to 40 mV.