Threshold voltage control method for amorphous switches

A technique is described whereby the threshold voltage of amorphous memory switches may be controlled by the parameters of the reset pulses used. It is shown that a train of up to 35 low-current reset pulses from a voltage source causes the switch threshold to rise progressively until it equals the generator voltage. Close control of threshold voltage in switch arrays is thus possible, temperature effects can be reduced and high-current pulses avoided. The method also effectively prevents degradation of threshold switches under dc operation.     

Field-Induced Orientation Transition in Ferroelectric Liquid Crystals

Static and dynamic effects of a fast bistable device containing a ferroelectric liquid crystal are investigated theoretically. The switching mechanisms are different at low and high electric fields. There is a critical voltage which separates a bulk switching regime from the regime of domain wall motion. This threshold is determined as a function of physical device parameters.     

Electrical properties of vanadate-glass threshold switches

The electrical properties of thick-film vanadate-glass switches, with both planar and sandwich geometries, have been investigated using a remote double-pulse technique. The effect on delay time of pulse separation, overvoltage and polarity were examined. No forming step was required for any of the devices. Photographic evidence of filament formation confirmed the thermal model of switching suggested by these experiments. Additional evidence, such as the effect of temperature on threshold voltage and electrical conductivity, established the importance of the semiconductor-metal transition in the VO₂ present in the material for the switching action. A computer simulation based on the discontinuous change in electrical conductivity, occurring at 68°C, and due to this transition, was shown to be in broad agreement with the experimental facts.     

On-state behaviour and degradation effects in thin-film amorphous switching devices

The on-state behaviour of thin-film chalcogenide threshold switches was studied with special reference to the influence of electrode materials and the progressive effects of device degradation. For operations during the forming period, the on-state current-voltage characteristics remain identical for both rising and falling current. The on-state minimum voltage is found to be closely proportional to the square root of the electrode thermal conductivity and, in addition, shows some polarity dependence with dissimilar electrode combinations. This behaviour is interpreted in terms of a model in which the voltage drop across the on-state current channel is dominated by constrictions and associated hot spots near the channel/electrode boundaries. After forming and after a further number of operations depending both on electrode material and ambient temperature, one or more stable intermediate steps or “branches” develop in the I–V characteristic of the transition from off-state to on-state. The exact pattern shows some polarity dependence. Branching is identified with the progressive precipitation of tellurium-rich “islands” in the glass as the number of device operations increases and as a result of which the growth of the on-state channel proceeds via a number of “island-hopping” stages.     

双向低触发电压横向晶闸管放电管研究

依据触发电压VS、触发电流IS、维持电流IH及触发电压、维持电流高低温变化率指标要求,利用Silvaco-TCAD半导体器件仿真软件完成了双向低触发电压横向晶闸管(SCR)放电管的设计.详细分析了对触发特性产生显著影响的结构参数(N-衬底区、寄生PNP晶体管P-集电区、寄生NPN晶体管P-基区、N+阴极区、N+触发区、...     

Determination of the contribution of defect creation and charge trapping to the degradation of a-Si:H/SiN TFTs at room temperature and low voltages

In this paper is presented a qualitative investigation upon the mechanisms that cause the shift of the electrical parameters in thin film transistors (TFT). The transistors are made of amorphous hydrogenated silicon (a-Si:H) as active semiconductor layer and substoichiometric silicon nitride (SiN) as gate insulator. The work refers to the degradation at room temperature and low positive and negative gate voltage stresses. The electrical parameters: threshold voltage (V_th), subthreshold swing (S) and flat-band voltage (V_fb) have been determined from the dependence of the drain current on gate voltage (I_d–V_g) and the dependence of the capacitance on the gate voltage (C–V_g) measurements as function of stress time and bias.     

Effect of composition and forming parameters on the conductance of amorphous chalcogenide threshold switches

The effect on the conductance of formed STAG threshold switches of changing both the composition of the virgin films and the ON time of the switching pulse during the forming process has been studied. It was found that the silicon content of the glass has a large effect on the activation energy and conductivity parameters of virgin devices. After forming, the conductance of the device is much higher than in the unformed state; however, silicon no longer controls the conductivity parameters. When switching STAG threshold switches it was found that increasing the ON time of each switching cycle causes a decrease in the conductance of the formed device from its value when formed with very short ON times. For low pulse repetition frequencies the conductance saturates for ON times typically greater than 100 μs. The rate at which the conductance decreases is dependent upon the silicon content of the virgin glass. For high pulse repetition frequencies the conductance initially falls, following the low pulse repetition frequency case, and then rises, finally locking on to a memory state. The observations have been explained in terms of an initial large structural change of the glass, caused through adiabatic heating during switching. Relaxation of this structure back to more stable glass phases is possible during the ON time if the ON state temperature is sufficiently high. For high pulse repetition frequencies the temperature of the device is much higher, allowing for the possibility of crystalline growth. This leads to an increase in conductance and eventual lock on.     

Properties of filaments in amorphous chalcogenide semiconducting threshold switches

The temperature dependence of the conductivity has been measured for Si Te As Ge amorphous thin-film threshold switches in the unformed, formed and heat-treated states. The results indicate that there are two distinct values of activation energy after forming, each considerably less than the value for the unformed device. It is inferred that each activation energy of the formed device corresponds to a different glass structure; one value corresponds to a filament estimated to be a few microns in diameter, and the other is a consequence of less localised structural changes extending to much larger distances radially. This latter region of structural change has been confirmed experimentally by a radial conductivity probing technique, and has been found to extend out to 100 μm from the filament. The effect of heat treating the glass and the effect that circuit parameters have on the devices will also be discussed.     

Study on the device characteristics of FePc and FePcCl organic thin film Schottky diodes: Influence of oxygen and post deposition annealing

Device characteristics of Al/FePc/Au and Al/FePcCl/Au are performed and found to show rectification properties. The basic diode parameters of the device are determined. The electrical conductivity has been measured both after exposure to oxygen for 20 days and after annealing at temperature up to 473 K. Current density-voltage characteristics under forward bias are found to be due to ohmic conduction at lower voltage regions. At higher voltage regions there is space charge limited conductivity (SCLC) controlled by a discrete trapping level above the valance edge. The electrical parameters of oxygen doped and annealed samples in the ohmic and SCLC region are determined. The reverse bias curves are interpreted in terms of a transition from electrode-limited Schottky emission to the bulk-limited the Poole-Frenkel effect. The Schottky barrier parameters of oxygen doped and annealed structures of FePc and FePcCl are determined from the C²-V characteristics.     

Electrical transport mechanism in boron nitride thin film

Both dc and ac transport characteristics of plasma enhanced chemical vapor deposited (PECVD) boron nitride (BN) thin film was investigated by dc current vs. dc voltage measurement at different temperatures and admittance vs. gate voltage at various frequencies/temperatures, respectively. MIM ≡ metal (Al)-insulator (BN)-metal (Al) or MIS ≡ metal (Al)-BN-semiconductor (p-Silicon) test devices were conventionally produced. Both conductivity anisotropy and dc/ac detailed transport mechanism were analyzed within the frame of a turbostratic structure (t-BN), interfacing the substrate by a thin amorphous layer (a-BN). This defective BN film has been justified by both infrared (IR) analysis and indirectly by the resulting electrical transport behavior. Transport and its variations as a function of temperature/frequency are in agreement with a hopping mechanism across Gauss-like energy distributed localized deep traps.     

Threshold switching and the on-state in non-crystalline chalcogenide semiconductors: An interpretation of threshold-switching research

Experimental research on chalcogenide glasses is reviewed and an interpretation concerning the cause of threshold switching and the basis of the low-impedance on-regime is presented. Observed behavior is consolidated in terms of a single conductivity mechanism and two conducting species governing the off- and on-state electrical characteristics. Phenomenological equations and their consequences are given which are shown to be capable of successfully describing both in functional form and accurate magnitude a very wide variety of experimental data including current — voltage transient characteristics, the Haberland condition, delay time, maximum allowable interruption times, rise and decay characteristics and negative capacitance. Recent research is presented concerning two types of non-thermal radiative emission during the on-regime of a threshold switching event. Various switching models are described and critically discussed in terms of the experimental data and phenomenological equations.     

I–V characteristics and break down studies of Sb2O3 structures

I–V characteristics of sandwiched Al–Sb₂O₃–Al structures have been studied for different thicknesses. The current-voltage curves in general exhibit three regions, ohmic, non ohmic and breakdown regions. The breakdown voltage increases whereas the dielectric strength decreases with increase in Sb₂O₃ film thickness. The electrical breakdown studies have been done for dc and ac voltages and optical photomicrographs of breakdown patterns during different stages of voltage have been taken and the results are explained.     

Percolation phenomena in controlled drug release matrices studied by dielectric spectroscopy and the alternating ionic current method

The combined radial and axial ionic drug release from – as well as the percolating ionic conductivity in – cylindrical tablets was investigated by the alternating ionic current (AIC) method and dielectric spectroscopy (DS), respectively. The binary tablets consisted of mixtures of insulating ethyl cellulose and the poor ionic conductor model drug NaCl at nine different concentrations. We found that the dc conductivity, extracted from DS in a well-defined range of frequencies by a power-law method, could be described by a NaCl volume fraction percolation threshold of ∼0.06 in a 3D conducting network. The low threshold was explained by water-layer-assisted ion conduction in μm-sized ethyl cellulose channels between NaCl grains as probed by Hg porosimetry and SEM. The drug release process, as probed by AIC, could be described by a matrix porosity percolation threshold of 0.22, equivalent to a NaCl volume fraction of ∼0.13. The higher percolation threshold found in the drug release experiments as compared to the DS recordings could be explained by the different probing mechanisms of the analysis methods. The present study should provide valuable knowledge for the analysis of a broad class of ion conducting systems for which the frequency response of the dc ion conductivity is superimposed on other dielectric processes in the dielectric spectrum. It also brings forward knowledge important for the development of controlled drug-delivery vehicles as the presented findings show that the drug release from matrix tablets with unsealed tablet walls substantially differs from earlier investigated release processes for which the drug has only been allowed to escape through one of the flat tablet surfaces.     

Photoconductivity in Tl4S3 layered single crystals

Photoconductivity measurements are carried out in this work for single crystals of Tl₄S₃ compound by using both pulsed excitation (a.c) and steady state (a.c) methods in order to elucidate the nature of photoconductivity (PC) in this compound. Results are reported in the temperature range from 77 to 300 K, excitation intensity range from 1800 to 3300 Lux, applied voltage range from 8 to 14 V, and wavelength range from 840 to 1450 nm. Both of the ac‐photoconductivity (ac‐PC) and the spectral distribution of the photocurrent are studied at different values of light intensity, applied voltage and temperature. Dependencies of carrier lifetime on light intensity, applied voltage and temperature are also investigated as results of the ac‐PC measurements. By using the results of the dc‐photoconductivity (dc‐PC) measurements, the temperature dependence of the energy gap width is described. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Poole‐Frenkel electrical conduction in europium oxide films deposited on Si(100)

Thin Eu₂O₃ films were prepared on Si (P) substrates to form MOS devices. The oxide crystal structure was determined by X‐ray diffraction (XRD). The electrical transport properties of the devices with amorphous and crystalline Eu oxide were investigated. The current‐voltage and current‐temperature characteristics suggest a Poole‐Frenkel (PF) type mechanism of carrier transport through the device when the applied field is more than 105 V/cm. A deviation from PF leakage current course was found and attributed to the current carrier trapping. We have also observed that, the dielectric spectra of MOS structure are different when the insulator is an amorphous or crystalline thin film. From which we calculate the relaxation time (τ) of the interface (insulator/semiconductor) dipoles. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mechanisms of dc-current transfer in tris(acetylacetonato)iron(III) films

Thin tris(acetylacetonato)iron(III) films were prepared by sublimation in vacuum on p-Si substrates for electrical investigation. The prepared films were characterized by X-ray diffraction (XRD) which shows the material to exhibit polycrystalline orthorhombic structure. Thin films of the complex were prepared as a gate-insulator for metal/insulator/Si (MIS) device. The capacitance-gate voltage, C(V_g) characteristics of the constructed MIS device were measured, from which the relative permittivity and the density of the charges in the sample were extracted. The dc-electrical conduction in the complex film was studied at room temperature and in the temperature range of (293–353 K). It was found that the data follow a Poole–Frenkel (PF) mechanism for low voltages and a trap-charge-limited space-charge-limited conductivity (TSCLC) mechanism for higher voltages. The switching observed between the two mechanisms was explained. The characteristic parameters both mechanisms were also determined. It was concluded that the dc-conduction can be described by hopping between structural defects that form trap levels in localized states near the bottom of the mobility band. Therefore, the density of structural defects in the film, which are controllable by the method of preparation, is critical in determining the mechanism of current transfer.     

Calculation of threshold voltage for phase-change memory device

Calculations of threshold voltage, which starts up transformation of active region in memory device from amorphous to crystalline state in phase-change random access memory, are reported. The calculations are based on the assumption that the emission of holes from the traps stimulates switching by forming conductive channel between electrodes. The results obtained by calculation correlate with the experimental data and the data reported in literature.     

DC breakdown studies on CeO2 films

The electrical breakdown studies were made on Al—CeO₂—Al thin film capacitors using dc voltage. The optical photomicrographs of the breakdown patterns were obtained at several stages of breakdown using universal optical microscope. Breakdown patterns seems to be originated at some inhomogeneity which may cause to form single-hole breakdown and is followed by propagating mechanism. The results were discussed.     

Influence of crystalline volume fraction on the performance of high mobility microcrystalline silicon thin-film transistors

The influence of the crystalline volume fraction of hydrogenated microcrystalline silicon on the device performance of thin-film transistors fabricated at temperatures below 200 °C was investigated. Transistors employing microcrystalline silicon channel material prepared close to the transition to amorphous growth regime exhibit the highest charge carrier mobilities exceeding 50 cm²/V s. The device parameters like the charge carrier mobility, the threshold voltage and the subthreshold slope will be discussed with respect to the crystalline volume fraction of the intrinsic microcrystalline silicon material.     

Instrinsic instability and current channelling in thermally controlled,two-terminal switching devices

Electrical threshold switching characteristics are reported for a bulk, two-terminal chalcogenide device. In addition to a simple load line instability, a more abrupt switching transition is observed, which is identified as an intrinsic instability, associated with the formation of a current channel. Thermally controlled current channelling is discussed in relation to bulk and thin-film devices and emphasis is given to some consequences of channel constriction at the electrode interface in thin-film devices. A simple analysis of the constriction yields a minimum holding voltage of the same order as is observed in thin-film devices.