Reverse Bias Mode Research Articles

Microbial synthesis of semiconductor CdS nanoparticles, their characterization, and their use in the fabrication of an ideal diode

Cadmium sulfide nanoparticles were synthesized intracellularly by a Schizosaccharomyces pombe strain when challenged with 1 mM cadmium in solution. The nanoparticles, a known semiconducting material, exhibited an absorbance maximum at 305 nm. X-ray scattering data showed that the nanoparticles had a Wurtzite (Cd16S20)-type hexagonal lattice structure and most of the nanoparicles were in the size range of 1–1.5 nm. The nanoparticles were used in the fabrication of a heterojunction with poly (p-phenylenevinylene). The diode exhibited ∼75 mA/cm2 current at 10 V when forward biased and the breakdown occurred at ∼15 V in the reverse biased mode. These characteristics are considered ideal for a diode. © 2002 Wiley Periodicals, Inc. Biotechnol Bioeng 78: 583–588, 2002.

Microbial synthesis of semiconductor CdS nanoparticles, their characterization, and their use in the fabrication of an ideal diode.

Cadmium sulfide nanoparticles were synthesized intracellularly by a Schizosaccharomyces pombe strain when challenged with 1 mM cadmium in solution. The nanoparticles, a known semiconducting material, exhibited an absorbance maximum at 305 nm. X-ray scattering data showed that the nanoparticles had a Wurtzite (Cd(16)S(20))-type hexagonal lattice structure and most of the nanoparicles were in the size range of 1-1.5 nm. The nanoparticles were used in the fabrication of a heterojunction with poly (p-phenylenevinylene). The diode exhibited approximately 75 mA/cm(2) current at 10 V when forward biased and the breakdown occurred at approximately 15 V in the reverse biased mode. These characteristics are considered ideal for a diode.

Electronic characteristics of Au/Al xGa 1− xN structures grown with various x values

We have studied the growth characteristics of Al x Ga 1− x N/GaN and electronic properties of Au/Al x Ga 1− x N structures grown by the MOCVD variation of x. The Al x Ga 1− x N layers were grown on undoped GaN/sapphire (0 0 0 1) epitaxial layers in a horizontal MOCVD reactor at a reduced pressure of 300 Torr. Al composition, x in Al x Ga 1− x N/GaN determined by DCXRD are 0.12, 0.15, 0.19 and 0.2, respectively. Nevertheless, enough feeding of TMA (trimethylaluminum) source during the growth at 300 Torr and 1070°C, the mole fraction x in Al x Ga 1− x N did not increase any more above 0.2. The FWHMs of the DCXRD for (0 0 0 2) diffraction from Al 0.12Ga 0.88N, Al 0.15Ga 0.85N, Al 0.19Ga 0.81N and Al 0.2Ga 0.8N are 450, 469, 502 and 474 arcsec, respectively. While the carrier concentration of Al x Ga 1− x N layers increases, the mobility decreases with the increase of Al mole fraction x. After fabricating Au/Al x Ga 1− x N Schottky diodes, electronic properties of the structures were evaluated by I– V measurement as a function of x. The breakdown voltages of Au/Al x Ga 1− x N diodes at reverse bias mode were in the range of 2.5–15 V. And, the turn on voltages at forward bias mode were in the range of 0.5–5.0 V with the variation of x from 0.12 to 0.19. While Au/Al x Ga 1− x N diodes having low mole fraction of Al showed relatively good rectifing behaviors, the samples having high x exhibited very leaky I– V behaviors. The current density–voltage ( J– V) characteristics at forward bias mode were also evaluated as a function of carrier concentrations which are directly related to x. From each curve of ln ( J) vs. V, I yielded the effective barrier heights ( Φ B ) ranging from 0.6 to 0.78 V, and the ideal factor ( η ) is in the range of 2.7–3.5. The barrier heights obtained in this work are close to each other, but the ideal factors are slightly different.

Role of ionic species in determining characteristics of polymer LEDs

Even traces of ionic species in the emissive polymer layer in Al/MEH-PPV/ITO LEDs or in the non-emissive emeraldine base, “EB” layer, in Al/MEH-PPV/EB/ITO LEDs are shown to dramatically change the characteristics of the LEDs including improved quantum yield and/or operation in both forward and reverse bias modes due to reduction of electron/hole injection energies facilitated by ion diffusion phenomenon.

Characterization of dark noise in CdZnTe spectrometers

Systematic measurements of dark noise spectra of CdZnTe x- and γ-ray spectrometers, correlated with the dc I-V characteristics and detector technology, are reported. The dark noise of two innovative CdZnTe spectrometer configurations are studied: metal-semiconductor-metal (MSM) resistive detectors with three terminals as well as heterostructure PIN detectors with thermally evaporated n+ CdS and p+ ZnTe contacts, which are fabricated on high pressure Bridgman CdZnTe (Zn=10%) crystals. The two innovative CdZnTe spectrometer configurations presented here exhibit very low dark (leakage) current. By reducing the dc value of the dark (leakage) current below 1 nA, shot noise becomes the dominant noise mechanism and the contribution of 1/f noise becomes negligible. The use of non-injecting contacts (evaporated gold) for the MSM detectors and the operation of the PIN detector in the reverse bias mode prevent generation-recombination noise which becomes dominant with injecting contacts (for example MSM detectors with evaporated indium and titanium contacts) or when operating the PIN detector in the forward bias mode. Surface leakage is reduced by applying surface passivation but is eliminated only by using the three terminal MSM configuration which exhibits simple shot noise instead of the suppressed shot noise observed in the two terminal MSM spectrometers. The noise measurements are useful for optimizing detector technology.

An iodine-doped polymer light-emitting diode

The performance of polymer light-emitting diodes (LEDs) having the configuration Al/MEH–PPV/ITO, where MEH–PPV=poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene) has been improved by light iodine doping of the emissive MEH–PPV polymer layer whereby the turn-on voltage is reduced and the external quantum efficiency is increased by an order of magnitude. It differs from nondoped MEH–PPV LEDs in that light emission is observed in both forward and reverse bias modes. It is believed that the mobility of the I3− anions in the (MEH–PPV)x+y(I3)xy− species formed on doping the MEH–PPV compensate for injected charges at the electrodes, thus reducing the electron and hole injection barriers.

Application of thin films of conjugated polymers in electrochemical and conventional light-emitting devices and in related photovoltaic devices

Characteristics of electrochemically-driven light-emitting devices M/MEH-PPV+TBATS/ITO (where M=Al, Cu, Au and TBATS=tetrabutylammonium p-toluenesulfonate) are reported. Light emission is observed in forward and reverse bias modes in each device. Characteristics of “3-layer”, “4-layer” and “5-layer” light-emitting devices having the configurations, Al/MEH-PPV/TTO, Al/MEH-PPV/EB/TTO and Al/EB/MEH-PPV/EB/TTO have been studied. Light emission is observed in forward bias only for the first device but in forward and reverse bias modes for the latter two devices. Operation in AC mode is also observed. Photovoltages are observed for all these devices in the range 0.15–0.87 v when irradiated with Ar + laser or overhead projector lamp

Color-Variable Emission in Multilayer Polymer Electroluminescent Devices Containing Electron-Blocking Layer

Color-variable emission in multilayer polymer electroluminescent (EL) devices has been obtained by the use of poly(N-vinylcarbazole) (PVCz) as an electron-blocking layer. The emissive polymers used were poly ( p-phenylenevinylene), poly(3-dodecylthiophene), and poly(2,5-dinonyloxy- p-phenylenevinylene), whose emission colors were green, red, and orange, respectively. When EL devices consisting of two emissive polymer layers separated by a PVCz layer were driven under either forward or reverse bias mode, emission from the negatively biased polymer was observed. PVCz is assumed to act as an electron-blocking layer due to its small electron affinity (2.3 eV) compared with those of the emissive polymers (2.7–3.5 eV), and to facilitate exciton confinement in the negatively biased polymer layer, where radiative recombination of the charge carriers occurs.

Application of aluminum, copper and gold electrodes in a.c. polymer light-emitting devices

Symmetrically configured a.c. light-emitting (SCALE) ‘5-layer’ devices having the configuration M/EB/P/EB/ITO, where M = Al, Cu or Au, EB = polyaniline (emeraldine base), P = poly(2,5-dihexadecanoxy phenylene vinylene pyridyl vinylene) or PPV-PPyV, and ITO = indium-tin oxide glass, show electroluminescent properties in both forward and reverse bias modes. In the absence of emeraldine base, in the case of aluminum and copper, electroluminescence is observed only in the forward biasmode; in the case of gold no electroluminescence is observed in either forward or reverse bias modes. The electrical properties of the ‘5-layer’ devices (M = Al, Cu) are most surprising since their total resistance at a given applied voltage is significantly less than that of the corresponding devices in which the two polyaniline insulator layers are not present.

Application of polyaniline (emeraldine base, EB) in polymer light-emitting devices

We report the fabrication of several multilayer light-emitting-diode (LED) devices based on a novel conjugated polymer, poly(2,5-dihexadecanoxy phenylene vinylene pyridyl vinylene) (PPV-PPy V), involving the use of polyaniline (emeraldine base, EB) as an insulating layer between the emissive polymer layer and the electrodes. In all the above devices with various configurations (‘3-layers’, ‘4-layers’-1’, ‘4-layers-2’ and ‘5-layers’), only the symmetrically configured a.c. light-emitting (SCALE) (‘5-layers’) device shows the capability of operating in both forward and reverse bias modes and in an a.c. mode. The SCALE devices have a typical turn-on voltage of about 4–6 V and work well under both forward and reverse bias modes. It is important to note that the total resistance ( R= V/I+) of the four devices at any given applied potential decreases as the number of insulating polymer layers increases, suggesting that the nature of the electrode/polymer interface plays a critical role in determining the characteristics of the devices.

Spatially resolved photocurrent measurements on a-C:H/c-Si and a-C:H/a-Si:H heterojunction photodiodes

Abstract Heterojunction photodiodes were prepared by r.f. plasma-enhanced chemical vapour deposition of a-C:H films on single crystalline p-type silicon substrates ( N A = 10 14 cm −3 , orientation 〈111〉) and a-Si:H substrates. We studied the quantum efficiency of devices working in photovoltaic and reverse-biased mode in the wavelength region between 350 and 1200 nm. Post-deposition treatment of a-C:H films with hydrogen at high pressures (up to 300 bar) and elevated temperatures (up to 650 K) changed the bulk properties of the a-C:H films significantly. Photoconductivity was observed within the films after the hydrogen processing. The responsivity of the photoconducting a-C:H films, however, is too small to account for the observed quantum efficiencies (~ 20–30%) of the devices. Multiplication of charge carriers is indicated by quantum efficiencies exceeding 100%, which are observed for reverse-biased devices after the hydrogen treatment. According to C-V measurements the formation of the heterojunction leads to an inversion layer on the silicon surface which is essential for properly working devices. In our paper we propose a model for the operation of photodiodes which is supported by optical beam-induced current measurements.

Pulsed and steady-state photon source calibration using windowless Si surface-barrier photodiodes

The pulsed and steady-state photoresponses of silicon surface-barrier photodiodes having sensitive areas of 2.5 cm 2 have been measured at a photon wavelength λ of 850 nm (1.5 eV). The photodiodes studied are either windowless, i.e., have no gold film over the sensitive surface (no-gold photodiode), or have a thin (∼ 10 nm) gold film over the sensitive surface (gold photodiode). Local as well as broad-area photoresponses have been measured with the photodiodes operated in the reverse-bias mode. For comparison, pulsed photoresponses were also measured at λ = 337 nm (3.7 eV). The measured photocurrent pulses from no-gold photodiodes have a long time duration resulting from the large resistance and capacitance of the bare silicon surface. The results of these studies show that the characteristics of the measured photocurrent pulses from no-gold photodiodes are consistent with those expected for a model which treats the bare silicon surface as a two-dimensional RC line consisting of a homogeneously distributed capacitance and resistance and having circular geometry. Further, the results of steady-state photocurrent measurements and pulse amplitude measurements using pulse electronics with time constants in the range 5–1200 μs show that no-gold silicon photodiodes need no initial photon calibration. They serve as photon flux calibration detectors for both pulsed and steady-state photon sources provided the reflectivity of the no-gold photodiode is measured at the time of its use for a photon flux measurement. For the case of a pulsed photon source, the use of pulse electronics having time constants properly chosen to accommodate the slow current pulses from the no-gold photodiode is also required. Gold photodiodes may be calibrated easily by making use of a no-gold photodiode. The gold photodiode is calibrated by measuring photopulse amplitude ratios for pulsed photon sources or photocurrent ratios for steady-state photon sources. The pulse response of no-gold silicon photodiodes to charged particles has also been determined and the results compared with their response to pulsed photon sources and with the charged-particle response measured for gold photodiodes.

Long-term photo and charged particle response of Si surface-barrier photodiodes

The photoresponse and reflectivity of new (∼ one month) and aged (18 years) silicon surface-barrier photodiodes have been measured over the photon wavelength range from 365–700 nm (3.4−1.8 eV). The photodiodes studied have either no gold film over the sensitive surface, or a thin (∼ 1−10 nm) gold film. The photodiodes are operated in the reverse-bias mode at room temperature. The local steady-state photoresponse of both types has been measured over the sensitive area (2.5 cm 2) and the pulse response of the photodiodes to incident α-particles and α-particle induced light pulses from a Csl crystal has measured as a function of gold film thickness. The results of these studies are reported and show that over the wavelength range studied, silicon photodiodes with no gold film need no initial photon calibration and serve as reliable steady-state photon flux calibration detectors for periods at least as long as 18 years, with no photon calibration being required during this time. All that is required is a measurement of the reflectivity of the photodiode (which changes with time) at the time of its use for a photon flux measurement. In addition, these studies show that photodiodes with gold film thickness ⩾ 7.5 nm have a photosensitivity for both steady-state photon fluxes and photon pulses which changes by less than ∼ 5% over a time period of 12 years, and serve as excellent charged particles detectors exhibiting stable charged-particle response over this same time period.

An inexpensive radiant energy integrator

A battery powered radiant energy integrator is described. The sensor is a miniature silicon photovoltaic cell operated in the reverse bias mode. Current from the cell is integrated on a capacitor and the time integral stored in an electromechanical counter. The instrument has good linearity over five decades of light intensity and good stability both with respect to temperature and battery voltage. Flashlight batteries provide sufficient power to operate the instrument unattended for several months.

Single- and dual-axis lateral photodetectors of rectangular shape

A 2-dimensional diffusion model of rectagular single-and dual-axis, lateral photodiodes is analyzed and the different underlying lateral effects are discussed for the small-signal (unbiased) and fully reverse-biased modes, with either infinite terminating impedances at midlateral point contacts or zero loading impedances at extended lateral contacts. For the single-axis detector with two opposite extended contacts, previous results of the 1-dimensional model appear to remain valid, and the output current difference to sum ratio in the fully reverse-biased mode is a linear function of the position of a narrow incident light beam. For the dual-axis detector with extended contacts at all four sides--except for small gaps at the vertices--an approximately linear relation has been found between the light spot position along an axis and the log ratio of the corresponding output currents. The transient responses of these configurations are discussed, and the utility of operation in the small-signal mode is illustrated with some typical parameter values.

Lateral photodetector operating in the fully reverse-biased mode

A one-dimensional model of a reverse-biased lateral photodetector is analyzed. The effect of background illumination is considered. An ac equivalent circuit for the device is derived. From this equivalent circuit the transient response is calculated and device parameters that determine its characteristics are presented.

Competing detection mechanisms in hot carrier microwave diodes

Polarity reversal with d.c. bias has been obtained in hot carrier microwave diodes. These diodes, characterized by a 1-mil dia. junction of phosphorous-doped gold alloyed into a 4 Ω-cm silicon substrate, behave like Schottky barrier diodes under forward bias, but under small reverse bias the sensitivity goes to zero. With further negative bias the polarity or sense of the detected 1 kc square-wave modulation at 9·3 GHz changes sign and the sensitivity improves until it is within 5–10 dB of the forward bias value. Detection involves majority carriers in both the forward and reverse bias modes. In the latter, mobile carriers localized in the junction region absorb microwave energy and become hotter than carriers in the bulk. The resulting temperature differential gives a thermoelectric voltage sufficiently fast to follow the microwave field down to millimeter wavelengths. (1) Polarity of detected output for this mechanism is opposite to that for a Schottky rectifier.

Detection techniques for nondestructive second-breakdown testing

The development of nondestructive second-breakdown measuring techniques and equipment is described. Such apparatus is essential to the analysis of the "safe area of operation" of transistors for device development and particular circuit application. The present techniques for detecting second breakdown in both the forward-biased and the reverse-biased mode are discussed. When a transistor is operated in the forward-biased mode, hot spots may develop which produce second breakdown if temperature extremes are reached before this condition is recognized. With existing detection methods, the device may already have been damaged before this condition is recognized. Techniques for electronic detection of hot spots before such degradation occurs are illustrated and compared to phosphorus ultraviolet hot spot measurements. When these new detection techniques are used, transistors can be characterized for forward-biased "safe area of operation" for applications such as audio output stages, series regulators, and class AB linear RF power amplifiers. Another form of second breakdown occurs under reverse-biased conditions, when the transistor is in primary breakdown. Because of the small areas involved, second breakdown occurs much more rapidly in this mode than under forward-biased conditions. An accompanying oscillation in the control electrode has proved to be a reliable detection mechanism. This work was performed primarily on multiple-diffused power transistors, in which reverse-biased second breakdown is a major concern. Because a "safe area of operation" in the reverse-biased mode can be established by this test, reliable circuits can be designed for such applications as high-speed inverters, solenoid drivers, servo controls, and UHF class C power amplifiers. Life-test results have proven this test procedure to be a dependable and consistent method for establishing a "safe area of operation" for transistors. Illustrations of the new RCA second-breakdown equipment based on the research described are presented.