Large Reverse Current Research Articles

Gate-assisted MoSe2 transistor to detect the streptavidin via supporter molecule engineering

Atomically thin two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductor materials have garnered significant attention due to their remarkable properties surpassing conventional metal-oxide semiconductors. These properties include high carrier mobility, adjustable bandgaps, and exceptional sensitivity, providing them with tremendous potential for various applications. The remarkable characteristics of these 2D semiconductor materials led to fabricating of atomically thin electronic devices including diodes, photodetectors, memristors, and biosensors. Here, we present an atomically thin transistor composed of n-type molybdenum di-selenide (n-MoSe2), exhibiting a large reverse current. The charge transport through the MoSe2 transistor is modulated via back gate voltage, studied at various basing and gate voltages. Finally, the MoSe2 transistor device is utilized as a biosensor to detect the protein (streptavidin) as target analyte at fixed biasing and gate voltages. Using our designed pyrene-based supporter molecule, which is stacked over the MoSe2 surface via π-π stacking, the target protein is identified while avoiding any screening effects like Debye screening. Additionally, the real-time response of the biosensing device is recorded while maintaining a constant biasing voltage of 0.5 V, covering a range of streptavidin concentrations down to the lowest detectable concentration of 1 pM. Such utilization of the MoSe2 device as a biosensor could pave the way for manufacturing cutting-edge electrical devices which can detect a broad range of biomolecules, such as DNA (like ct-DNA) and the COVID-19 spike protein.

AsP/InSe Van der Waals Tunneling Heterojunctions with Ultrahigh Reverse Rectification Ratio and High Photosensitivity

AbstractVan der Waals heterojunctions made of 2D materials offer competitive opportunities in designing and achieving multifunctional and high‐performance electronic and optoelectronic devices. However, due to the significant reverse tunneling current in such thin p–n junctions, a low rectification ratio along with a large reverse current is often inevitable for the heterojunctions. Here, a vertically stacked van der Waals heterojunction (vdWH) tunneling device is reported consisting of black arsenic phosphorus (AsP) and indium selenide (InSe), which shows a record high reverse rectification ratio exceeding 107 along with an unusual ultralow forward current below picoampere and a high current on/off ratio over 108 simultaneously at room temperature under the proper band alignment design of both the Schottky junction and the heterojunction. Therefore, the vdWH tunneling device can function as an ultrasensitive photodetector with an ultrahigh light on/off ratio of 1 × 107, a comparable responsivity of around 1 A W−1, and a high detectivity over 1 × 1012 Jones in the visible wavelength range. Furthermore, the device exhibits a clear photovoltaic effect and shows a spectral detection capability up to 1550 nm. The work sheds light on developing future electronic and optoelectronic multifunctional devices based on the van der Waals integration of 2D materials with designed band alignment.

Growth of large grain-size Cu2ZnSn(SxSe1-x)4 thin films by annealing precursors sputtered from a single quaternary target for solar cells application

A simple and cost effective method of sputtering a single quaternary target following a rapid thermal process (RTP) was applied to prepare Cu2ZnSn(SxSe1-x)4 (CZTSSe) thin films. The different Se supply during thermal annealing was used to adjust the Se contents in films. The structure, morphology, optical and electrical properties of the CZTSSe films were analyzed comprehensively. Polycrystalline kesterite CZTSSe absorbers with large columnar grains were obtained. The grains are densely packed and surfaces are smooth. The evolution of the optical and electrical properties with different Se contents in the CZTSSe films was studied in details. The conversion efficiency of the Se-rich CZTSSe thin film solar cell was determined to be 3.38%, and the corresponding J-V characteristic was analyzed carefully. The severe interface recombination is the key factor for the large ideality factor and reverse saturation current.

A p-i-n junction diode based on locally doped carbon nanotube network.

A p-i-n junction diode constructed by the locally doped network of single-walled carbon nanotubes (SWNTs) was investigated. In this diode, the two opposite ends of the SWNT-network channel were selectively doped by triethyloxonium hexachloroantimonate (OA) and polyethylenimine (PEI) to obtain the air-stable p- and n-type SWNTs respectively while the central area of the SWNT-network remained intrinsic state, resulting in the formation of a p-i-n junction with a strong built-in electronic field in the SWNTs. The results showed that the forward current and the rectification ratio of the diode increased as the doping degree increased. The forward current of the device could also be increased by decreasing the channel length. A high-performance p-i-n junction diode with a high rectification ratio (~104), large forward current (~12.2 μA) and low reverse saturated current (~1.8 nA) was achieved with the OA and PEI doping time of 5 h and 18 h for a channel length of ~6 μm.

Electrical properties of pSi/[6,6] phenyl-C61 butyric acid methyl ester/Al hybrid heterojunctions: Experimental and theoretical evaluation of diode operation

In this work, we analyze electrically the Al/p-Si/[6,6] phenyl-C61 butyric acid methyl ester/Al hybrid heterojunction. The barrier height at the p-Si/PCBM interface corresponding to the difference between Si valence band edge and the lowest unoccupied molecular orbital energy level of PCBM is studied with current-voltage (J-V) and capacitance-voltage (C-V) methods and determined to be ≃0.55 eV. This value is in agreement with the onset energy of spectrally resolved photocurrent measurements presented in a previous publication [Matt et al., Adv. Mater. 22, 647 (2010)]. For the J-V characteristics, a thorough model based on an interface generation-recombination current is proposed. All relevant energy levels for this model are obtained experimentally. As origin of the large reverse current, the thermal generation of charge carriers throughout the Si depletion region is identified by the thermal activation measurements.

Capacitance–voltage characteristics of P3HT:PCBM bulk heterojunction solar cells with ohmic contacts and the impact of single walled carbon nanotubes on them

Capacitance–voltage (C–V) characteristics of P3HT:PCBM devices of two different thicknesses are correlated with current density–voltage (J–V) characteristics. The rising portion of the C–V characteristics coincides with the exponential current density below the built-in voltage. The negative capacitance (NC) of these devices is a low frequency phenomenon and it occurs in trap-free space charge limited current (SCLC) regime. The onset frequencies of NC for devices with and without SWNTs also do not follow direct relation with effective mobility. The NC in thin devices has non-monotonic change with voltage for thin devices showing that interface state kinetics can be the reason for its occurrence. The NC of thick devices, on the other hand, increases monotonically with voltage showing that bulk properties dominate in these. Addition of SWNTs to these devices for efficiency improvement does not modify their built-in voltage. Also, the SWNTs do not affect the forward NC behaviour. However, the devices containing SWNTs show NC in reverse bias also which has different frequency dependence with voltage. The reverse bias NC is attributed to the large non-linear reverse current by charge injection into the additional energy levels introduced by SWNTs.

Hybrid films based on silicon nanowires dispersed in a semiconducting polymer for thin film solar cells: Opportunities and new challenges

Hybrid devices based on silicon nanowires dispersed in poly(hexylthiophene: P3HT) thin films have been realized. The different steps, starting from the production of n type silicon nanowire, dispersion of the organic–inorganic components in solution to produce hybrid thin films, which have been integrated in simple photovoltaic diode structures, have been developed. The n type doping of the nanowires is indicated by Raman spectroscopy whereas their crystalline structure is shown by HRTEM. A broadening of the silicon absorption spectrum towards the near infrared is induced by an electronic confinement effect for nanowire diameters of the order of 10nm. Effective dissociation of the photogenerated charge pairs is shown by a partial quenching of P3HT photoluminescence with increasing SiNWs content, whereas an open circuit voltage of 0.68V is deduced from the electrical characteristics under visible light illumination. However photocurrents remain low for the investigated material combinations. The high series resistance appears the most critical point limiting the device performances. The high densities of SiNWs electronic surface states acting as trapping centers can account for the large reverse current. To confirm the high potential of this new type of hybrid films for energy conversion, improvements are needed for a better control of the silicon nanowire/polymer interface, which appears the critical point.

Non-destructive detection of killer defects of diamond Schottky barrier diodes

Reverse characteristics of vertical-type Au/p-diamond (100) Schottky barrier diodes were investigated and characterized with cathodoluminescence, which showed a correlation with crystalline defects. Electrical measurements revealed that most diodes had low reverse current below 0.1 pA in the bias voltage range up to 50 V, although reverse current increases rapidly in some diodes and reaches 0.1 mA at 10 V. A four-fold symmetrical luminescence pattern that aligned crystallographically to the substrate was observed from the Schottky diodes, showing large reverse current. Removal of these defects is a key issue to increase the diode area of diamond-based rectifiers.

Epitaxial GaAs for X-ray imaging

To be used for X-ray imaging, semiconductor materials must exhibit good and uniform electronic properties. Epitaxial layers are therefore better adapted than bulk materials which contain dislocations, precipitates and point defects in variable concentrations depending on the growth mode and the nature of the material. However, they have to be thick enough to absorb photons efficiently. We produced thick epitaxial layers using a proprietary technique and made p/i/n (200–300 μm thick) diodes with this new material. These diodes are characterized by a large reverse current, which can originate from electron emission from deep level defects present in the depleted region or be a leakage current. In order to answer this question, we performed a characterization of the defects present in the material. Here, we describe results obtained from X-ray diffraction, X-ray topography, time resolved photoluminescence and resistivity measurements. We also investigated the possible effect of hydrogen. From these observations, we deduced that defects exhibiting an electrical role are in negligible concentration and concluded that the high reverse current observed is a leakage current.

Analysis and implementation of an active-clamping zero-voltage turn-on switching/zero-current turn-off switching converter

A new isolated dc/dc converter with zero-voltage turn-on switching (ZVS) of power switches and zero-current turn-off switching (ZCS) of diodes is presented. In conventional flyback converters, the main drawbacks are high voltage stresses on power devices, large transformer size and serious reverse recovery current, which resulted in high voltage spikes across output diode. To solve these problems, a buck-boost type of active-clamp circuit is connected in parallel with the primary winding to clamp the voltage spike across the power switch, to recycle the energy stored in the leakage and magnetising inductances of the transformer and to realise the turn-on ZVS by utilising the leakage inductance and output capacitance of the switch. The leakage inductance and the resonant capacitance on the secondary side are resonant to allow the output diodes to turn off at ZCS. Thus, the switching losses and thermal stresses of switches and diodes are reduced, the output diode has no reverse recovery problem and voltage stresses of the output diodes are clamped to the output voltage. Experiments conducted on a laboratory prototype rated at 270 W are provided to verify the effectiveness of the proposed converter.

NOISE CHARACTERISTICS AND RELIABILITY OF LIGHT EMITTING DIODES BASED ON NITRIDES

Optical and electrical noises and correlation factor between optical and electrical fluctuations of nitride-based light emitting diodes (LEDs) have been investigated under forward bias. Their electrical, optical and noise characteristics were compared with ones of LEDs of other materials. LED noise characteristic changes during aging have been measured, too. It is found that optical and electrical noise spectra under forward bias for more reliable LEDs distinguish by lower l/f type fluctuations and Lorentzian type noise at higher frequencies. LEDs with intensive 1/f noise demonstrate shorter lifetime. It is shown that reason of LED degradation is related with defects presence in device structure. These defects can be formed during device fabrication or appear during operation. An analysis of LED current-voltage and electrical noise characteristics under forward and reverse bias has shown that LEDs with intensive 1/f electrical noise, large reverse current (low reverse breakdown voltage) and larger terminal voltage under forward bias distinguish by short lifetime.

A highly UV-selective Schottky-barrier photodiode based on a Ag–GaP contact

A Schottky-barrier photodiode with spectral sensitivity in the 300–340 nm region has been prepared successfully using the transmission window of silver and its barrier effect on GaP. Radio-frequency magnetron sputtering has been used for deposition of the silver film. Silver films with thicknesses in the 10–150 nm range exhibit a transmission window at a wavelength of about 320 nm, which can be used as an ultraviolet (UV) bandpass on a GaP photodiode. Furthermore, the Schottky contact of sputtered silver films on n-type GaP grown by liquid-phase epitaxy shows good characteristics, including large barrier heights (1.2–1.3 eV) and low reverse currents (below 2 pA cm−2 at 5 V). The GaP photodiode with a silver Schottky contact exhibits a sensitivity of 0.02 A W−1 at a wavelength of 320 nm. The simplicity of this structure of a UV sensor opens up the possibility for mass production.

Schottky junctions on semi-insulating LEC gallium arsenide for X- and /spl gamma/-ray spectrometers operated at and below room temperature

This work deals with the study of a Schottky junction used as an X- and /spl gamma/-ray detector in a spectrometer operated in the temperature range from -30/spl deg/C to +22/spl deg/C. The device (7 mm/sup 2/ active area and 100 /spl mu/m thickness), fabricated on liquid encapsulated Czochralski (LEG) semi-insulating Gallium Arsenide, is designed with a noninjecting ohmic contact which allows biasing voltages up to 550 V. At room temperature (22/spl deg/C) the energy resolution is found to be relatively poor (15.5-keV full-width at half-maximum (FWHM) at 59.5 keV) due to the large junction reverse current, whose density (7-37 nA/mm/sup 2/ at V/sub bias/=100-500 V) is within the typical values for Schottky junctions on SI LEC GaAs. By cooling of the detector to -30/spl deg/C, the noise of the reverse current is drastically lowered, thus achieving electronic noise levels around 160-180 rms electrons (1.6-1.8 keV FWHM), At 500-V bias, the /sup 241/Am spectrum has been resolved down to an energy of 4 keV with charge collection efficiency of cce=97% and a resolution of about 2-keV FWHM for the Np L lines and 2.4-keV FWHM for the 59.5-keV /spl gamma/ photons. The linearity of the detector has been measured to be better than /spl plusmn/0.6% within the explored energy range (14-59 keV). From the experimental spectra, it has been analyzed how either the electronic noise or the trapping of the signal charge contribute to the energy resolution of the spectrometer. The result is that despite the high measured cce. The trapping gives a contribution higher than 1.5 keV FWHM for the 59.5-keV spectral line. A comparison between the experimental results and Monte Carlo simulations, based on the Hecht model of charge trapping in detectors, is shown to give a satisfactory justification of the observed phenomena. A total mean drift length of carriers has been experimentally derived, finding an exponential dependence upon the bias voltage applied to the detector.

Performance of large area proximity focused hybrid photo-diodes

We are considering the possibility of using large area proximity focused HPDs to read out the Cherenkov light from aerogel blocks in B-factory experiments. This requires a photon detector with high immunity to magnetic field, large geometrical acceptance and high sensitivity to the single photoelectron. The response of several large area proximity focused HPDs in high magnetic field has been studied. Results showing their substantial immunity to the magnetic field over a wide angular range are discussed. Large area HPDs imply large detector capacitance and reverse current, which are important parameters limiting the signal to noise ratio. In spite of this, excellent single pe sensitivities have been obtained with a commercial charge preamplifier readout.

On the High-Speed Turn-Off of a Power Transistor by a Small Saturable Core

An effective method for reducing the size and weight of the switching dc-dc converter is to increase the switching frequency. To accomplish this with no deterioration in efficiency, it is necessary to suppress the switching loss per cycle by shortening the switching time. An efficient dc-dc converter operating at the megahertz region is presented. The turn-off time is shortened by using the current feedback and the core saturation. The turn-off mechanism is analyzed with the equivalent circuits. As a result, it is found that the excess carrier in the base region is discharged quickly by the large reverse base current, which flows when the energy stored in the base-emitter capacitance of the power transistor is transferred to the saturated inductance of the core with high frequency oscillation.

Real and apparent effects of strong electric fields on the electron emission from midgap levels EL2 and EL0 in GaAs

The effects of strong electric fields, up to 4×105 V/cm, on the electron emission from the GaAs dominant midgap levels EL2 and EL0 were investigated by employing differential capacitance transients on GaAs-Au Schottky diodes. It was found that, in diodes with normal reverse bias characteristics, both levels exhibited a small field enhancement of electron emission, well within the range of the Poole–Frenkel effect. In contrast, very pronounced ‘‘apparent’’ electric field effects were observed in diodes with large reverse bias current. Thus, the conflicting reported results on the magnitude of the field enhancement of the electron emission from EL2 must be attributed to the characteristics of the Schottky diodes employed rather than to those of EL2.

On the theory of transient process after reversal of a p–i–n diode current from forward to reverse direction (I)

en

Design of Neutron Radiation Tolerant High Effitciency Microwave Avalanche Diode Sources (Trapatt Oscillators)

The effects of permanent neutron damage on the operation of high efficiency avalanche microwave sources are explained. The resulting design criteria for increasing the neutron radiation tolerance of these devices are verified by comparing experimental results for two different device doping profiles. In terms of RF failure, the device with the narrower space-charge region is more radiation resistant. This is because RF failure is related to recombination of carriers as they transit the space-charge region. A theoretical curve predicting neutron damage failure versus frequency of operation is presented and shown to agree with the experimental data taken from the two different TRAPATT oscillators. The DC failure of the devices takes the form of a negative temperature coefficient at large reverse breakdown currents which is related to carrier removal at the edges of the space-charge region. In terms of this DC failure mechanism the device with the more abrupt doping profile is more radiation tolerant.

Preparation and some Properties of Mg2Ge Single Crystals and of Mg2Ge p-n Junctions

A technique was developed for the preparation of high-quality single crystals of Mg2Ge with controlled dopings. It utilizes the reaction of stoichiometric amounts of the constituents in a graphite crucible inside a hermetically sealed tantalum bomb, followed by the solidification of the molten compound into a single crystal by lowering the bomb through a temperature gradient (Bridgman technique). Several crystals without intentionally added dopants were grown as well as crystals to which the donors antimony, arsenic, boron, the acceptor sodium, or the apparently insoluble element uranium, had been added. A technique was also developed for the preparation of p-n junctions on n-type Mg2Ge, utilizing the alloying to and diffusing into the Mg2Ge of a thin evaporated gold film. p-n junctions prepared in this way exhibit undesirably large reverse currents, but high breakdown field strength and very pronounced surface passivation properties, which might make Mg2Ge a desirable material for MOS (metal-oxide-semiconductor) devices.

Channels and Excess Reverse Current in Grown Germanium p-n Junction Diodes

Measurements have been made on grown germanium p-n junction diodes in an attempt to correlate the excess reverse current that occurs under exposure to water vapor with the presence of an n-type surface-conductivity layer, or channel, on the p-side of the junction. Using an optical method to measure the length of this surface layer, it has been found that (1) no channel is formed on a freshly CP4-etched surface, and that (2) for well-oxidized surfaces, large excess reverse currents occur which are directly proportional to the length of the channel for all measured values of applied voltage and humidity. By combining this observation with the known behavior of the channel conductivity with humidity and voltage, a formula for the excess current as a function of the applied bias is derived. In spite of several oversimplifications in the model, the predicted values agree reasonably well with the experimental results for biases greater than one volt.