Effect of type-II quantum well of m-MTDATA/α-NPD on the performance of green organic light-emitting diodes

The type-II multiple quantum well (MQW) structure is prepared and introduced into green organic green light-emitting diodes consisting of 4,4′-bis-[N-(naphthyl)-N-phenyl-amino]biphenyl (α-NPD) and tris-(8-hydroxyquinolinato)-aluminum (Alq₃). The quantum well (QW) and wall are fabricated by 4,4′,4″-tris-(3-methylphenylphenylamino)triphenylamine (m-MTDATA) and α-NPD, respectively. The device performance of MQW organic light-emitting diodes (OLEDs) has been improved; the luminous efficiency by 25% and power efficiency by 17% compared with the reference device. The performance improvement can be explained by the increased electron-hole balance in the device due to the hole confinement in the QW structure.     

Theory of switching in p-n-insulator (tunnel)-metal devices: Part I: Punchthrough mode

The four-layered structure (M-I(leaky)-n-p⁺) is found to exhibit a current-controlled negative resistance region in its I-V characteristics. In this paper, a quantitative physical model of the device in the punch-through mode is presented. The negative resistance behaviour is due to a positive feedback mechanism between the tunnel MIS and the n-p⁺ junction parts of the device. The effect of the device parameters on its I-V characteristics is studied.     

A flexible polymer memory device

A flexible polymer memory device is demonstrated in a sandwich structure of polypyrrole/P6FBEu/Au. Conductance switching at a voltage of about 4 V, with an ON/OFF current ratio up to 200, was observed in this flexible memory device. At the low-conductivity state, current density–voltage (J–V) characteristics of the device were dominated by a charge injection current. At the high conductivity state, J–V characteristics were dominated by a space-charge-limited current. Both the ON and OFF states are stable up to 10⁶ read cycles at a read voltage of 1 V. The device can be used as a write-once read-many-times (WORM) memory with good electronic stability.     

High-current,low-forward-drop JBS power rectifiers

The junction-barrier-controlled Schottky (JBS) rectifier is a Schottky rectifier with a p-n junction grid structure integrated into the device structure to improve its reverse blocking characteristics. This paper reports the development of large area (0.5 cm²), 30 V, JBS rectifiers capable of handling over 25 A of forward current while operating at up to 125°C with good reverse blocking characteristics. Trade-off curves between forward voltage drop and reverse leakage current are introduced to allow optimization of these devices.     

Ambipolar field-effect transistor

The results of measurements performed on an amorphous-silicon thin-film transistor structure are presented and interpreted. The device characteristics show a continuous alternation between n-channel and p-channel operation, an “ambipolar” effect that is made possible by the provision of ohmic source and drain contacts.     

Ion-implanted GaAs slow wave monolithic structure

The use of MeV ion-implantation for realization of a GaAs monolithically compatible device is demonstrated. Ion implants up to 6 MeV in energy are used employing Si and S atoms. The fabricated device is an electromagnetic slow wave microstrip-like structure designed for performance into the millimeter wave regime. Phase shift θ and insertion loss L measurements are performed for frequencies 2–18 GHz at room temperature. Comparison of the experimental ion-implanted device results to epitaxial device results indicates comparable electrical performance, with no more than a 30% reduction in θ but with an improvement in loss behavior, namely a L reduction up to 40%. These θ and L differences between the ion-implanted and epitaxial devices are attributed to differences in doping profiles. Theoretical modelling of θ characteristics produces agreement with experimental data to within a few percent.     

Acousto-optic interaction in the lithium niobate crystal upon the surface excitation of the bulk acoustic wave

The interaction of light with the bulk acoustic wave that is excited from the surface of the lithium niobate crystal is experimentally studied. A prototype of the acousto-optic device that employs the XY-cut crystal and the optimized (Y-13°)-cut crystal is presented. Diffraction efficiencies of 1 and 2 %/W are obtained for the first and second prototypes, respectively. It is demonstrated that the polarization characteristics of the device differ from the conventional characteristics by the dependence on the structure of the acoustic beam. An original method for the excitation of the ultrasonic beam can be used in the acousto-optic devices for light control.     

Investigating the impact of the defect dynamic characteristics on the PBTI in the high-κ gate device

Recent device reliability studies on the metal/high-κ device observed the inter-convertible characteristics of the electron trap levels between the shallow and deep defect states under cyclic positive-bias temperature stressing. Although the oxygen vacancy and oxygen interstitial defect, being two typical types of defects in the high-κ oxide, have been criticized as the culprit for the device reliability issue and investigated in many simulations, all results have indicated that the defect levels induced by them were either too shallow or too deep and failed to explain the above experimental observation. Nevertheless, studies on the static characteristics of vacancy-interstitial (V_O-O_i) model showed scattering distributed electron trap levels within the bandgap, making it as a promising defect type that can account for the above experimental observation. In this work, we investigated the dynamic characteristics of the V_O-O_i defect pair under PBTI stress by tuning the relative position of V_O and O_i. Our simulation results show multiple energy barriers for the structure transformation along the O_i migration path, and the charge trap level of the specific defect pair during the O_i migration is shown to be adjustable within the HfO₂ band gap, depending on the O_i positions. These results depict an atomic picture to help us understand the defect electrical behavior under cyclic positive bias stress condition.     

Impact of interface state trap density on the performance characteristics of different III-V MOSFET architectures

The effect of interface state trap density, D_it, on the current-voltage characteristics of four recently proposed III-V MOSFET architectures: a surface channel device, a flat-band implant-free HEMT-like device with δ-doping below the channel, a buried channel design with δ-doping, and implant-free quantum-well HEMT-like structure with no δ-doping, has been investigated using TCAD simulation tools. We have developed a methodology to include arbitrary energy distributions of interface states into the input simulation decks and analysed their impact on subthreshold characteristics and drive current. The distributions of interface states having high density tails that extend to the conduction band can significantly impact the subthreshold performance in both the surface channel design and the implant-free quantum-well HEMT-like structure with no δ-doping. Furthermore, the same distributions have little or no impact on the performance of both flat-band implant-free and buried channel architectures which operate around the midgap.     

Extraction of electronic parameters of Schottky diode based on an organic Orcein

An Au/Orcein/p-Si/Al device was fabricated and the current-voltage measurements of the devices showed diode characteristics. Then the current-voltage (I-V), capacitance-voltage (C-V) and capacitance-frequency (C-f) characteristics of the device were investigated at room temperature. Some junction parameters of the device such as ideality factor, barrier height, and series resistance were determined from I-V and C-V characteristics. The ideality factor of 2.48 and barrier height of 0.70 eV were calculated using I-V characteristics. It has been seen that the Orcein layer increases the effective barrier height of the structure since this layer creates the physical barrier between the Au and the p-Si. The interface state density N_ss were determined from the I-V plots. The capacitance measurements were determined as a function of voltage and frequency. It was seen that the values of capacitance have modified with bias and frequency.     

New opportunities for SiGe and Ge channel p-FETs

A thin body (fully depleted) strained SGOI device structure (FDSGOI), and a strained SiGe channel layer on SOI, were fabricated using scaled high-κ gate dielectrics and metal gate technology. The uniaxial strain effect and corresponding drive current enhancement reported by Irisawa et al. [1] for narrow-width devices was investigated on these structures. Although the strained FDSGOI device structure exhibited reduced off-state leakage compared to thicker body devices, and long-channel drive current enhancement under uniaxial strain, the loss of drive current enhancement at short channel length led to uncompetitive I_ON-I_OFF characteristics. The SiGe on SOI structure showed the highest long-channel drive current enhancement (nearly 3×) in the narrowest devices, and also showed a significant reduction in off-state current. This trend was maintained down to the shortest channel lengths studied here and resulted in I_ON-I_OFF characteristics that were competitive with contemporary uniaxial strained Si channel devices.     

Analytical modeling and parameter extraction of top and bottom contact structures of organic thin film transistors

This paper proposes a structure based model of an organic thin film transistor (OTFT) and analyzes its device physics. The analytical model is developed for the top contact structure by mapping the overlap region to the resistance (in the vertical direction) that includes the contact and the bulk sheet resistances. Total device resistance includes the vertical resistance per unit area of the contact region and the sheet resistance of the channel. In addition, the drain and the gate voltages take into account the potential drop across the respective contacts. The gate bias dependent mobility is considered in place of constant mobility, since; it is more realistic and relevant to the organic TFTs. The proposed analytical model is also applied to the bottom contact structure and the current–voltage (I–V) characteristics are obtained. Furthermore, a differential method is employed to extract the parameters, such as, mobility enhancement factor γ, threshold voltage V_T, mobility µ_B, characteristic length L_C, vertical resistance R_V and contact resistance R_C. Finally, the model is validated in terms of electrical characteristics and performance parameters for both top and bottom contact structures. The analytical model results are in close agreement with the experimental results.     

Design and Analysis of InGaN-GaN Modulation Doped Field-Effect Transistors (MODFETs) for Over 60 GHz Operation

A Modulation-Doped Field-Effect Transistor (MODFET) structure realized in InGaN-GaN material system is presented for the first time. An analytical model predicting the transport characteristics of the proposed MODFET structure is given in detail. Electron energy levels inside and outside the quantum well channel of the MODFET are evaluated. The two-dimensional electron gas (2DEG) density in the channel is calculated by self-consistently solving Schrödinger and Poisson's equations simultaneously. Analytical results of the current-voltage and transconductance characteristics are presented. The unity-current gain cutoff frequency (f _T) of the proposed device is computed as a function of the gate voltage V _G . The results are compared well with experimental f _T value of a GaN/AlGaN HFET device. By scaling the gate length down to 0.25 μm the proposed InGaN-GaN MODFET can be operated up to about 80GHz. It is shown in this paper that InGaN-GaN system has small degradation in f _T as the operating temperature is increased from 300°K to 400°K.     

Design and Analysis of InGaN-GaN Modulation-Doped Field-Effect Transistors (MODFETs) for 90 GHz Operations

A Modulation-Doped Field-Effect Transistor (MODFET) structure having quantum wire channel realized in InGaN-GaN material system is presented. This paper presents design and analysis of a novel one-dimensional Modulation-Doped Field-Effect transistor (1D MODFET) in InGaN-GaN material system for microwave and millimeter wave applications. An analytical model predicting the transport characteristics of the proposed MODFET device is also presented. Analytical results of the current-voltage and transconductance characteristics are presented. The unity-current gain cutoff frequency (f _T) of the proposed device is computed as a function of the gate voltage V _G. The results are compared with two-dimensional GaN/AlGaN MODFET and HFET devices. The analytical model also predicts that 0.25 μm channel length devices will extend the use of InGaN-GaN MODFETs to above 90GHz.     

Trapping,emission and generation in MNOS memory devices

This study is concerned with trapping phenomena occuring at the semiconductor-oxide interface and in the nitride layer of variable-threshold metal-nitride-oxide-semiconductor (MNOS) memory devices. The technique consits of biasing the device in such a manner as to charge or discharge either the interface traps or the nitride traps, or both sets of traps simultaneously. The device is then cooled to low temperature with the bias still applied, and at the low temperature the biasing condition is changed, in order to induce the device into a non-steady mode that is quasi-stable at the low temperature. The temperature of the device is then raised at a constant rate, and the resulting current vs temperature (I-T) characteristics is found to be rich in structure. By means of a series of systematic experiments the various portions of the I-T characteristic are identified with emission of electrons from interface states and the nitride traps, and surface generation. From this data the energy distribution of interface states is determied. It is shown that the memory charge in the nitride is distributed throughout the nitride, and temporary memory charge and semi-permanent memory charge are distinguished.     

Comparison of 3C–SiC, 6H–SiC and 4H–SiC MESFETs performances

A comparative analysis of the main DC and microwave performances of MESFETs made of the commercially available silicon carbide polytypes 3C–SiC, 6H–SiC and 4H–SiC is presented. In this purpose, we have developed an analytical model that takes into account the basic material properties such as field dependent mobility, critical electric field, ionization grade of impurities, and saturation of the charge carrier velocity. For a better precision in appreciating device characteristics in the case of a short gate device, the influences of the gate length and parasitic elements of the structure, e.g. source and drain resistances, are considered too. Cut-off frequency f_T, the corresponding output power P_m and the thermal stability are also evaluated and compared with the available experimental data, revealing the specific electrical performances of MESFETs, when any of the three polytypes is used in device fabrication.     

Optical characteristics of a superlattice avalanche photodiode

Avalanche photodiodes using III–V materials are suitable for use in long distance fiber optic communication systems due to their faster speed of response and high gain. The superlattice APD is expected to be far more attractive than the conventional APD for their better noise performance. Theoretical studies have been carried out on the photoresponse characteristics of an Al_xGa_1−xAs/GaAs superlattice p⁺−i−n⁺ structure. It is observed that for a particular d.c. multiplication factor the normalised gain of the device remains constant with frequency and falls steadily after a certain frequency. The band width of the response curve increases with decrease in d.c. multiplication factor. Furthermore, the output current of the superlattice structure increases with the increase in optical power. The device also shows a good percentage of quantum efficiency.     

Geometrical and crystal structures,optical absorption and device characterization of N-(5-{[antipyrinyl-hydrazono]-cyanomethyl}-[1,3,4]thiadiazol-2-yl)-benzamide

The molecular structure of the N-(5-{[antipyrinyl-hydrazono]-cyanomethyl}-[1,3,4]thiadiazol-2-yl)-benzamide (ACTB) is optimized theoretically in which the energies of highest occupied molecular orbital and lowest unoccupied molecular orbital are calculated. ACTB crystalizes in triclinic structure with a space group, P2. ACTB thin films were prepared by using thermal evaporation technique onto quartz and n-Si single crystal substrates. The optical properties of the films are investigated in terms of the spectrophotometric measurements of the transmittance and reflectance. The current–voltage (I–V) characteristics of the fabricated In/ACTB/n-Si/Au diode are studied in temperature range 298–398 K. The device showed rectification behavior. At low forward voltage, the thermionic theory is applied for determining the ideality factor and barrier height as a function of temperature. The series resistance of the device is found to decrease with increasing temperature. At relatively high forward voltage, the space charge limited current dominated by exponential distribution of traps is found to be the operating mechanism in which the trapping parameters and charge carriers mobility are estimated.     

Influence of gate-to-source and gate-to-drain recesses on GaAs camel-like gate field-effect transistors

In this article, the characteristics of the GaAs homojunction camel-like gate field-effect transistors with and without the gate-to-source and gate-to-drain recesses structures are first investigated and compared. As to the device without the recesses structure, a second channel within the n ⁺-GaAs cap layer is formed at large gate bias, which could enhance the drain output current and transconductance. Furthermore, a two-stage relationship between drain current (and transconductance) versus gate voltage is observed in the recesses structure. The simulated results exhibit a maximum drain saturation current of 447 (351 mA/mm) and a maximum transconductance of 525 (148 mS/mm) in the studied device without (with) the recesses structure. Consequentially, the demonstration and comparison of the variable structures provide a promise for design in circuit applications.     

Influence of insulator thickness nonuniformity on the switching of the Al/SiO2/n-Si tunnel MOS structure at reverse bias

Current-voltage characteristics of the reversely biased Al/SiO₂/n-Si MOS structure are calculated taking into account the nonuniformity of oxide thickness distribution over an area at a nominal thickness of 1–3 nm. It is known that the characteristics are S-shaped in a certain range of average SiO₂ thickness, which suggests that a device is bistable. Holding and threshold voltage shifts, caused by statistical thickness variations, were predicted. In response to electrical stress, the root-mean-square deviation of the SiO₂ thickness increases, which results in a shift of the threshold voltage to higher values. The calculations are complemented by experimental data.